Ben-Gai, T., A. Bitan, A. Manes, P. Alpert and Y. Kushnir, 2001: Temperature and surface pressure anomalies in Israel and the North Atlantic Oscillation. Theoretical and Applied Climatology, 69(3-4): 171-177.
Teleconnections associated with changing patterns of temperature and pressure anomalies over Israel during the second half of the 20th century are investigated. Relatively high, statistically significant, correlation coefficients of -0.8 and +0.9 were found between the North Atlantic Oscillation (NAO) Index anomalies and smoothed (5 year running mean) cool season temperature and surface pressure anomalies in Israel, respectively.
Cane, M. A. and P. Molnar, 2001: Closing of the Indonesian seaway as a precursor to east African aridification around 3-4 million years ago. Nature, 411(6834): 157-162.
Global climate change around 3-4 Myr ago is thought to have influenced the evolution of hominids, via the aridification of Africa, and may have been the precursor to Pleistocene glaciation about 2.75 Myr ago. Most explanations of these climatic events involve changes in circulation of the North Atlantic Ocean due to the closing of the Isthmus of Panama. Here we suggest, instead, that closure of the Indonesian seaway 3-4 Myr ago could be responsible for these climate changes, in particular the aridification of Africa. We use simple theory and results from an ocean circulation model to show that the northward displacement of New Guinea, about 5 Myr ago, may have switched the source of flow through Indonesia-from warm South Pacific to relatively cold North Pacific waters. This would have decreased sea surface temperatures in the Indian Ocean, leading to reduced rainfall over eastern Africa. We further suggest that the changes in the equatorial Pacific may have reduced atmospheric heat transport from the tropics to higher latitudes, stimulating global cooling and the eventual growth of ice sheets.
Canizares, R., A. Kaplan, M. A. Cane, D. Chen and S. E. Zebiak, 2001: Use of data assimilation via linear low-order models for the initialization of El Niño Southern Oscillation predictions. Journal of Geophysical Research-Oceans, 106(C12): 30947-30959.
The utility of a Kalman filter (KF) for initialization of an intermediate nonlinear coupled model for El Niño - Southern Oscillation prediction is studied via an approximation of the nonlinear coupled model by a system of seasonally dependent linear models. The low-dimensional nature of such an approximation allows one to determine a sequence of "perfect" initial states that start a trajectory segment best fitting the observed data. Defining these perfect initial conditions as "true" states of the model, we compute a priori parameters of the KF and test its ability to produce an estimate of the "truth" superior to the less theoretically sound estimates. We find that in this application such a KF does not produce an estimate outperforming a pure observational projection as an initial condition for the coupled model forecast. The violation of standard KF assumptions on temporal whiteness of observational errors and system noise is identified as the reason for this failure.
Chen, D., 2001: Application of altimeter observation to El Niño prediction. International Journal of Remote Sensing, 22(13): 2621-2626.
A new version of the Lamont forecast model is used to assess the impact of TOPEX/POSEIDON altimeter data on predicting short-term climate change. with emphasis on the 1997/98 El Niño and subsequent La Niña. As compared to forecasts initialized with only wind data, the model's predictive accuracy was improved when the altimeter sea level data are used for model initialization. This is due to the effectiveness of sea level data in correcting the model ocean state. For this particular application, the effect of altimeter sea level observations is comparable to that of tide gauge measurements.
Chen, D., 2001: Applying satellite remote sensing to predicting 1999-2000 La Niña. Remote Sensing of Environment, 77(3): 275-278.
The usability of altimeter sea level data (TOPEX/POSEIDON) and scatterometer wind data (QuikSCAT) in El Niño and the Southern Oscillation (ENSO) prediction is investigated with the latest version of the Lamont forecast model. The emphasis of this study is on the effectiveness of these data sets in initializing the model to forecast the 1999-2000 La Niña conditions. Both the altimeter and scatterometer observations helped to improve the model, with the former being more effective for this period. It is possible and extremely useful to apply these data to real-time ENSO forecasting. In principle, it is advisable to assimilate multiple data sets so that they can complement one another in providing the correct initial conditions for the model. (C) 2001 Elsevier Science Inc. All rights reserved.
Chiang, J. C. H., S. E. Zebiak and M. A. Cane, 2001: Relative roles of elevated heating and surface temperature gradients in driving anomalous surface winds over tropical oceans. Journal of the Atmospheric Sciences, 58(11): 1371-1394.
Elevated heating by cumulus convection and sea surface temperature gradients are both thought to contribute to surface winds over tropical oceans. The relative strength and role of each mechanism is examined by imposing forcing derived from data on a linear primitive equation model with idealized parameterizations for the two forcings, and comparing the response with observed surface winds. Two test cases are studied: one related to the El Niño-Southern Oscillation, and the other related to the "dipole" mode in the tropical Atlantic. It is found that in both cases, elevated heating dominates the surface zonal wind response, and contributes significantly to the meridional wind response, especially in the subtropics and the South Pacific and South Atlantic convergence zone regions. Surface temperature gradients dominate the meridional wind forcing in regions near the equator with strong meridional temperature gradients.
Clement, A. C., M. A. Cane and R. Seager, 2001: An orbitally driven tropical source for abrupt climate change. Journal of Climate, 14(11): 2369-2375.
Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean-atmosphere model show that, under certain orbital configurations of the past, variability associated with El Niño-Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Niña. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.
Curchitser, E. N., D. B. Haidvogel and M. Iskandarani, 2001: Transient adjustment of circulation in a midlatitude abyssal ocean basin with realistic geometry and bathymetry. Journal of Physical Oceanography, 31(3): 725-745.
The early stages in the adjustment of the circulation in a midlatitude abyssal basin with realistic geometry and bathymetry are studied using an inverted 1 1/2-layer model of the Eastern Mediterranean Sea as a natural test basin. The model is forced with a localized sidewall mass source and a compensating distributed uniform mass sink. The basin is spun up by topographic waves with planetary vorticity gradients playing a minor role. A shallow sill in the middle of the basin dynamically separates the abyssal Eastern Mediterranean Sea into two regions. Despite a constant mass flux into the basin, the resulting energy input is time dependent and is correlated with nonlocal dynamics. A positive feedback develops between the source region and the interior circulation during the early adjustment; for a higher interface height level in the source region, the amount of energy injected by the mass source increases.
Dery, S. J. and M. K. Yau, 2001: Simulation of an Arctic ground blizzard using a coupled blowing snow-atmosphere model. Journal of Hydrometeorology, 2(6): 579-598.
A ground blizzard occurred from 16 to 18 November 1996 in the northern sectors of the Mackenzie River basin of Canada and the adjacent Beaufort Sea. This hazardous event, accompanied by a low-level jet with wind speeds approaching 20 m s(-1) and extensive blowing snow near the surface (but clear sky aloft), is forced by a strong sea level pressure gradient that forms between a rapidly intensifying anticyclone over the Nunavut and Northwest Territories of Canada and an intense depression over the frozen Arctic Ocean. The event is first simulated at a horizontal grid size of 18 km using the uncoupled Canadian Mesoscale Compressible Community (MC2) model. This experiment is shown to capture the rapid anticyclogenesis event within 2 hPa of its central sea level pressure and the blizzard conditions near the Canadian Arctic coastline and the Beaufort Sea. Meteorological conditions observed at Trail Valley Creek (TVC), a small Arctic tundra watershed in which ground blizzard conditions were experienced during the event, are also accurately reproduced by the uncoupled simulation with the notable exception of the blowing snow process. Thus, the mesoscale model is then coupled to the "PIEKTUK" blowing snow model, and a second simulation is conducted. This additional experiment reveals the presence of extensive blowing snow associated with a strong low-level jet over TVC and the adjacent frozen Beaufort Sea. Over the 2-day event, blowing snow sublimation and transport combined to erode 1.6 mm snow water equivalent from the surface mass balance of TVC. The concurrent moistening and cooling of near-surface air due to blowing snow sublimation emerge during the blizzard but to a lesser extent than in an idealized modeling framework, as a consequence of entrainment and advective processes. Therefore, blowing snow sublimation rates are evaluated to be 1.8 times larger than in the stand-alone application of the PIEKTUK model to the same data.
Dery, S. J. and M. K. Yau, 2001: Simulation of blowing snow in the Canadian Arctic using a double-moment model. Boundary-Layer Meteorology, 99(2): 297-316.
We describe in this paper the development of a double-moment model of blowing snow and its application to the Canadian Arctic. We first outline the formulation of the numerical model, which solves a prognostic equation for both the blowing snow mixing ratio and total particle numbers, both moments of particles that are gamma-distributed. Under idealized simulations, the model yields realistic evolutions of the blowing snow particle distributions, transport and sublimation rates as well as the thermodynamic fields at low computational costs. A parametrization of the blowing snow sublimation rate is subsequently derived. The model and parametrization are then applied to a Canadian Arctic tundra site prone to frequent blowing snow events. Over a period of 210 days during the winter of 1996/1997, the near-surface relative humidity consistently approaches saturation with respect to ice. These conditions limit snowpack erosion by blowing snow sublimation to approximate to3 mm snow water equivalent (swe) with surface sublimation removing an additional 7 mm swe. We find that our results are highly sensitive to the proper assimilation of the humidity measurements and the evolving thermodynamic fields in the atmospheric boundary layer during blowing snow. These factors may explain the lower values of blowing snow sublimation reported in this paper than previously published for the region.
Evans, M. N., M. A. Cane, D. P. Schrag, A. Kaplan, B. K. Linsley, R. Villalba and G. M. Wellington, 2001: Support for tropically-driven Pacific decadal variability based on paleoproxy evidence. Geophysical Research Letters, 28(19): 3689-3692.
Two independent proxy reconstructions of sea surface temperature reflect a common pattern of Pacific decadal sea surface temperature variability over the past two centuries. Since the pattern extends to both the northern and southern hemispheres, this result supports the idea that Pacific decadal variability is a basin-wide phenomenon originating in the tropics.
Giannini, A., J. C. H. Chiang, M. A. Cane, Y. Kushnir and R. Seager, 2001: The ENSO teleconnection to the tropical Atlantic Ocean: Contributions of the remote and local SSTs to rainfall variability in the tropical Americas. Journal of Climate, 14(24): 4530-4544.
Recent developments in Tropical Atlantic Variability (TAV) identify the El Niño-Southern Oscillation (ENSO) as one of the leading factors in the interannual climate variability of the basin. An ENSO event results in Tropic-wide anomalies in the atmospheric circulation that have a direct effect on precipitation variability, as well as an indirect effect, that is, one mediated by sea surface temperature (SST) anomalies generated in the remote ocean basins. In order to separate the relative contributions of the atmospheric and oceanic components of the ENSO teleconnection to the tropical Atlantic Ocean, results from two ensembles of atmospheric general circulation model (AGCM) experiments, differing in oceanic boundary conditions, are compared. AGCM integrations performed with the Community Climate Model version 3 (CCM3), forced by global, observed SST during 1950-94 reproduce the observed ENSO-related rainfall anomalies over the tropical Americas and adjacent Atlantic. A parallel ensemble of integrations, forced with observed SST in the tropical Atlantic only, and climatology elsewhere, is used to separate the effect of the direct atmospheric teleconnection from the atmosphere's response to the ENSO-forced SST anomalies in the Atlantic basin.
Giannini, A., M. A. Cane and Y. Kushnir, 2001: Interdecadal changes in the ENSO teleconnection to the Caribbean region and the North Atlantic oscillation. Journal of Climate, 14(13): 2867-2879.
The El Nino-Southern Oscillation (ENSO) phenomenon and variability in the subtropical North Atlantic high sea level pressure (SLP) are known to affect rainfall in the Caribbean region. An El Niño event is associated with drier-than-average conditions during the boreal summer of year (0), and wetter-than-average conditions during the spring of year (+1). Dry conditions during the summer of year (0) of an El Niño are associated with the locally divergent surface circulation engendered by the eastward shift of deep convection in the Pacific Ocean. Wet conditions during the spring of year (+1) of an El Niño are associated with the lagged warming of the tropical North Atlantic Ocean. Variability in the strength of the North Atlantic high is governed mainly by the North Atlantic oscillation (NAO) with a positive NAO phase implying a stronger than normal high and vice versa. The NAO is negatively correlated with Caribbean rainfall indirectly via anomalous sea surface temperatures (SST) associated with anomalies in the surface wind speed at tropical latitudes and directly via anomalous subsidence. The combined effect of the two phenomena is found to be strongest when the two signals interfere constructively:
Giannini, A., Y. Kushnir and M. A. Cane, 2001: Seasonality in the impact of ENSO and the North Atlantic high on Caribbean rainfall. Physics and Chemistry of the Earth Part B-Hydrology Oceans and Atmosphere, 26(2): 143-147.
Caribbean rainfall is affected by climate variability of Pacific and Atlantic origin, e.g. the El Niño-Southern Oscillation (ENSO) phenomenon, and variability in the North Atlantic High sea level pressure (SLP) center, respectively. During the lifetime of an ENSO cycle, the basin experiences dry and wet extremes. In the case of a warm event, the dry extreme precedes the mature ENSO phase, and can be explained in terms of a direct response to the atmospheric anomaly generated by the warm sea surface temperatures (SST) in the eastern equatorial Pacific. The wet extreme follows the mature phase, and is consistent with the lagged warming effect of ENSO on tropical North Atlantic SSTs. The wintertime state of the North Atlantic High is hypothesized to affect Caribbean rainfall through its effect on tropical SST. A strong North Atlantic High SLP center during the early months of the calendar year strengthens the trade winds, hence cooling SSTs in the tropical latitudes of the North Atlantic. The effect lingers on most noticeably until the start of the Caribbean rainy season, in May-June, when cool SSTs are associated with deficient rainfall in the basin. (C) 2000 Elsevier Science Ltd. All rights reserved.
Goddard, L., S. J. Mason, S. E. Zebiak, C. F. Ropelewski, R. Basher and M. A. Cane, 2001: Current approaches to seasonal-to-interannual climate predictions. International Journal of Climatology, 21(9): 1111-1152.
This review paper presents an assessment of the current state of knowledge and capability in seasonal climate prediction at the end of the 20th century. The discussion covers the full range of issues involved in climate forecasting, including (1) the theory and empirical evidence for predictability; (2) predictions of surface boundary conditions, such as sea surface temperatures (SSTs) that drive the predictable part of the climate; (3) predictions of the climate; and (4) a brief consideration of the application of climate forecasts. Within this context, the research of the coming decades that seeks to address shortcomings in each area is described. Copyright (C) 2001 Royal Meteorological Society.
Gordon, A. L., M. Visbeck and B. A. Huber, 2001: Export of Weddell Sea Deep and Bottom Water. Journal of Geophysical Research-Oceans, 106(C5): 9005-9017.
An extensive set of conductivity-temperature-depth (CTD)/lowered acoustic Doppler current profiler (LADCP) data obtained within the northwestern Weddell Sea in August 1997 characterizes the dense water outflow from the Weddell Sea and overflow into the Scotia Sea. Along the outer rim of the Weddell Gyre, there is a stream of relatively low salinity, high oxygen Weddell Sea Deep Water (defined as water between 0 degrees and -0.7 degreesC), constituting a more ventilated form of this water mass than that found farther within the gyre. Its enhanced ventilation is due to injection of relatively low salinity shelf water found near the northern extreme of Antarctic Peninsula's Weddell Sea shelf, shelf water too buoyant to descend to the deep-sea floor. The more ventilated form of Weddell Sea Deep Water flows northward along the eastern side of the South Orkney Plateau, passing into the Scotia Sea rather than continuing along an eastward path in the northern Weddell Sea. Weddell Sea Bottom Water also exhibits two forms: a low-salinity, better oxygenated component confined to the outer rim of the Weddell Gyre, and a more saline, less oxygenated component observed farther into the gyre. The more saline Weddell Sea Bottom Water is derived from the southwestern Weddell Sea, where high-salinity shelf water is abundant. The less saline Weddell Sea Bottom Water, like the more ventilated Weddell Sea Deep Water, is derived from lower-salinity shelf water at a point farther north along the Antarctic Peninsula. Transports of Weddell Sea Deep and Bottom Water masses crossing 44 degreesW estimated from one LADCP survey are 25 x 10(6) and 5 x 10(6) m(3) s(-1), respectively. The low-salinity, better ventilated forms of Weddell Sea Deep and Bottom Water flowing along the outer rim of the Weddell Gyre have the position and depth range that would lead to overflow of the topographic confines of the Weddell Basin, whereas the more saline forms may be forced to recirculate within the Weddell Gyre.
Harnik, N. and R. S. Lindzen, 2001: The effect of reflecting surfaces on the vertical structure and variability of stratospheric planetary waves. Journal of the Atmospheric Sciences, 58(19): 2872-2894.
The effects of an upper-stratospheric reflecting surface on the vertical structure of stratospheric planetary waves are considered. A diagnostic of the basic-state wave propagation characteristics, which is particularly useful for determining the existence and location of turning surfaces for meridional and vertical propagation, is developed. The diagnostic used is a more accurate indicator of wave propagation regions than the index of refraction because it diagnoses meridional and vertical propagation separately.
Hazeleger, W., M. Visbeck, M. A. Cane, A. R. Karspeck and N. H. Naik(Henderson), 2001: Decadal upper ocean temperature variability in the tropical Pacific. Journal of Geophysical Research-Oceans, 106(C5): 8971-8988.
Decadal variability in upper ocean temperature in the Pacific is studied by using observations and results from model experiments. Especially propagation of upper ocean thermal anomalies from the midlatitudes to the tropics is studied as a possible source for decadal equatorial thermocline variability. In the observations, propagation along the subtropical gyre of the North Pacific is clear. However, no propagation into the equatorial region is found. Model experiments with an ocean model forced with observed monthly wind and wind stress anomalies are performed to study the apparent propagation. Distinct propagation of thermal anomalies in the subtropics is found in the model, although the amplitude of the anomalies is small. The anomalies clearly propagate into the tropics, but they do not reach the equatorial region. The small response at the equator to extratropical variability consists of a change in the mean depth of the thermocline. It appears that most variability in the subtropics and tropics is generated by local wind stress anomalies. The results are discussed by using results from a linear shallow water model in which similar features are found.
Hazeleger, W., R. Seager, M. Visbeck, N. H. Naik(Henderson) and K. Rodgers, 2001: Impact of the midlatitude storm track on the upper Pacific Ocean. Journal of Physical Oceanography, 31(2): 616-636.
Transient eddies in the atmosphere induce a poleward transport of heat and moisture. A moist static energy budget of the surface layer is determined from the NCEP reanalysis data to evaluate the impact of the storm track. It is found that the transient eddies induce a cooling and drying of the surface layer with a monthly mean maximum of 60 W m(-2). The cooling in the midlatitudes extends zonally over the entire basin. The impact of this cooling and drying on surface heat fluxes, sea surface temperature (SST), water mass transformation, and vertical structure of the Pacific is investigated using an ocean model coupled to an atmospheric mixed layer model. The cooling by atmospheric storms is represented by adding an eddy-induced transfer velocity to the mean velocity in an atmospheric mixed layer model. This is based on a parameterization of tracer transport by eddies in the ocean. When the atmospheric mixed layer model is coupled to an ocean model, realistic SSTs are simulated. The SST is up to 3 K lower due to the cooling by storms. The additional cooling leads to enhanced transformation rates of water masses in the midlatitudes. The enhanced shallow overturning cells affect even tropical regions. Together with realistic SST and deep winter mixed layer depths, this leads to formation of homogeneous water masses in the upper North Pacific, in accordance to observations.
Honda, M., H. Nakamura, J. Ukita, I. Kousaka and K. Takeuchi, 2001: Interannual seesaw between the Aleutian and Icelandic lows. Part I: Seasonal dependence and life cycle. Journal of Climate, 14(6): 1029-1042.
The seasonal dependence and life cycle of the well-known interannual seesawlike oscillation between the intensities of the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated, based on the National Meteorological Center operational analyses for the period from 1973 to 1994. It is found that the correlation between the AL and IL intensities is significantly negative only from February to mid-March. It is also found that the seesaw exhibits an equivalent barotropic structure within the troposphere. For this late-winter period an index is defined that measures the intensity difference between the two lows. A linear lag regression analysis between this index and circulation anomalies averaged in each of the nine 45-day periods from early winter to midspring reveals that the stationary AL and IL anomalies constituting the seesaw do not start developing simultaneously over the respective ocean basins in the course of a particular winter season. Rather, the seesaw formation is initiated by the amplification of the AL anomalies with wave-activity accumulation in early through midwinter. In midwinter, part of the wave activity accumulated over the North Pacific propagates across North America in the form of a stationary Rossby wave train, which appears to trigger the formation of stationary anomalies over the North Atlantic. The IL anomalies thus initiated amplify and then become matured by late winter through the persistent feedback forcing from migratory eddies around the Atlantic storm track, while the AL anomalies remain strong until late winter through the continual feedback forcing from the Pacific storm track. It is suggested that interannual variability in the IL intensity for late winter tends to be strongly influenced by the AL anomalies that develop over the North Pacific in early through midwinter. The AL-IL seesaw is robust in a sense that it is apparent even after the influence of El Nino-Southern Oscillation is statistically removed from the data, suggestive of the importance of midlatitude processes in the seesaw formation.
Houghton, R. W. and C. Ho, 2001: Diapycnal flow through the Georges Bank tidal front: A dye tracer study. Geophysical Research Letters, 28(1): 33-36.
A fluorescent dye tracer, Fluorescein, injected into the bottom mixed layer at the seaward edge of the tidal front on Georges Bank has provided the first quantitative measurement of an on-bank diapycnal Lagrangian flow through the front. From the warming of the dye patch, 2.5x10(-6) degreesC/s and 7.8x10(-6) degreesC/s on the south flank and northeast peak respectively, as it passed through the frontal temperature gradient we infer an on-bank flow of 1.9 cm/s on the south flank and 3.2 cm/s on the northeast peak. The heat flux required for this warming is predominantly due to vertical mixing within the tidal front. These observations further demonstrate the utility of direct Lagrangian measurements and provide quantitative estimates of the cross-frontal exchange on Georges Bank, the focus of the U.S GLOBEC Northwest Atlantic/Georges Bank Phase III program.
Houghton, R. W. and R. G. Fairbanks, 2001: Water sources for Georges Bank. Deep-Sea Research Part II-Topical Studies in Oceanography, 48(1-3): 95-114.
The origins of Georges Bank source waters can be quantified by the oxygen isotope - salinity tracer. Regionally, water is "tagged" by freshwater of different H-2 O-18/H-2 O-16 ratios, and these waters can subsequently be identified after mixing downstream. This conservative Lagrangian tracer of source waters is useful for tracking passive plankton and larvae that may be important to sustain the Georges Bank ecosystem. The origin of Georges Bank freshwater sources is primarily in the Gulf of St. Lawrence consisting of Labrador Shelf Water, which enters via the Strait of Belie Isle, and St. Lawrence River Water. These two mix with the underlying Slope Water in the Gulf of St. Lawrence and along the Scotian Shelf. At the Northeast Channel, the input to the Gulf of Maine, there is a single freshwater endmember consisting of 5.6% St. Lawrence River Water anal 94.6% Labrador Shelf Water by volume. These proportions remained constant for the years 1995-1997. The water on central Georges Bank consists of a mixture of water entering the Northeast Channel with Gulf of Maine River outflow. There is considerable interannual variation in the Maine River Water proportion, ranging from 3.3% in 1982 to 0.23% by volume in 1996. During 1995-1998 there was no correlation between Georges Bank salinity, which decreased monotonically, and the proportion of Maine River Water on the Bank. The interannual fluctuations of salinity on Georges Bank are driven by variations in upstream Scotian Shelf sources and its mixing onto the Bank and not by changes in local river outflow. (C) 2000 Published by Elsevier Science Ltd.
Huang, H. P., K. M. Weickmann and C. J. Hsu, 2001: Trend in atmospheric angular momentum in a transient climate change simulation with greenhouse gas and aerosol forcing. Journal of Climate, 14(7): 1525-1534.
The authors investigate the change of atmospheric angular momentum (AAM) in long, transient, coupled atmosphere-ocean model simulations with increasing atmospheric greenhouse gas concentration and sulfate aerosol loading. A significant increase of global AAM, on the order of 4 x 10(25) kg m(2) s(-1) for 3 x CO2-1 x CO2, was simulated by the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled model. The increase was mainly contributed by the relative component of total AAM in the form of an acceleration of zonal mean zonal wind in the tropical-subtropical upper troposphere. Thus, under strong global warming, a super-rotational state emerged in the tropical upper troposphere. The trend in zonal mean zonal wind in the meridional plane was characterized by 1) a tropical-subtropical pattern with two maxima near 30 degrees in the upper troposphere, and 2) a tripole pattern in the Southern Hemisphere extending through the entire troposphere and having a positive maximum at 60 degreesS. The implication of the projected increase of global AAM for future changes of the length of day is discussed.
Khatiwala, S., B. E. Shaw and M. A. Cane, 2001: Enhanced sensitivity of persistent events to weak forcing in dynamical and stochastic systems: Implications for climate change. Geophysical Research Letters, 28(13): 2633-2636.
Low-dimensional models can give insight into the climate system, in particular its response to externally imposed forcing such as the anthropogenic emission of greenhouse gases. Here, we use the Lorenz system, a chaotic dynamical system characterized by two "regimes", to examine the effect of a weak imposed forcing. We show that the probability density functions (PDF's) of time-spent in the two regimes are exponential, and that the most dramatic response to forcing is a change in the frequency of occurrence of extremely persistent events, rather than the weaker change in the mean persistence time. This enhanced sensitivity of the "tails" of the PDF's to forcing is quantitatively explained by changes in the stability of the regimes. We demonstrate similar behavior in a stochastically forced double well system. Our results suggest that the most significant effect of anthropogenic forcing may be to change the frequency of occurrence of persistent climate events, such as droughts, rather than the mean.
Khatiwala, S., M. Visbeck and P. Schlosser, 2001: Age tracers in an ocean GCM. Deep-Sea Research Part I-Oceanographic Research Papers, 48(6): 1423-1441.
Observations of transient tracers such as tritium and helium-3 (He-3) are frequently combined to construct "age-like" quantities generally interpreted to represent time elapsed since a fluid parcel was last at the surface. In a turbulent ("diffusive") environment such as the ocean, we must regard the fluid parcel as being composed of material fluid elements that have spent different lengths of time since their last contact with the surface. Hence, they are characterized by an age spectrum or distribution of transit times. In this study we explore the concepts of tracer-derived "ages" and the transit-time probability density function (PDF) with the aim of improving our understanding of their interpretation. Using an ocean general circulation model, we illustrate the effect of mixing on tracer-derived "ages" within the Atlantic Ocean. The mixing biases such ages towards younger values with respect to the ideal or mean age of a water parcel. In the North Atlantic, this bias is particularly pronounced in the thermocline because of large vertical gradients in tracer concentration, and in the deep ocean, where the penetration of recently ventilated water creates large gradients along the isopycnal surfaces. In contrast, the effect of mixing appears to be relatively small in the subtropical subduction region. Calculations of the transit-time PDF in the ocean model show, however, that the mean age can potentially be very large because of contributions from long transit-time pathways, in spite of the fact that such pathways make up a small fraction of the fluid parcel. These results illustrate the key idea that tracer-derived ages are weighted towards the leading part of the transit-time distribution, while the ideal age is more sensitive to its "tail". These tracers are thus sensitive to and help constrain different time scales. We also find that the ideal age converges much more rapidly to the mean age compared with the first moment of the age spectrum, an important consideration in numerical studies. (C) 2001 Elsevier Science Ltd. All rights reserved.
Khodri, M., Y. Leclainche, G. Ramstein, P. Braconnot, O. Marti and E. Cortijo, 2001: Simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation. Nature, 410(6828): 570-574.
According to Milankovitch theory, the lower summer insolation at high latitudes about 115,000 years ago allowed winter snow to persist throughout summer, leading to ice-sheet build-up and glaciation(1), But attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only(2-5). These results point towards the importance of feedback effects-for example, through changes in vegetation or the ocean circulation-for the amplification of solar forcing(6-9). Here we present a fully coupled ocean-atmosphere model of the last glaciation that produces a build-up of perennial snow cover at known locations of ice sheets during this period. We show that ocean feedbacks lead to a cooling of the high northern latitudes, along with an increase in atmospheric moisture transport from the Equator to the poles. These changes agree with available geological data(10-15) and, together, they lead to an increased delivery of snow to high northern latitudes, The mechanism we present explains the onset of glaciation-which would be amplified by changes in vegetation-in response to weak orbital forcing.
Krahmann, G., M. Visbeck and G. Reverdin, 2001: Formation and propagation of temperature anomalies along the North Atlantic Current. Journal of Physical Oceanography, 31(5): 1287-1303.
A general circulation ocean model has been used to study the formation and propagation mechanisms of North Atlantic Oscillation (NAO)-generated temperature anomalies along the pathway of the North Atlantic Current (NAC). The NAO-like wind forcing generates temperature anomalies in the upper 440 m that propagate along the pathway of the NAC in general agreement with the observations. The analysis of individual components of the ocean heat budget reveals that the anomalies are primarily generated by the wind stress anomaly-induced oceanic heat transport divergence. After their generation they are advected with the mean current. Surface heat flux anomalies account for only one-third of the total temperature changes. Along the pathway of the NAC temperature anomalies of opposite signs are formed in the first and second halves of the pathway, a pattern called here the North Atlantic dipole (NAD). The response of the ocean depends fundamentally on R-t = (L/upsilon)/tau, the ratio between the time it takes for anomalies to propagate along the NAC [(L/upsilon) similar to 10 years] compared to the forcing period tau. The authors find that for NAO periods shorter than 4 years (R-t > 1) the response in the subpolar region is mainly determined by the local forcing. For NAO periods longer than 32 years (R-t < 1); however, the SST anomalies in the northeastern part of the NAD become controlled by ocean advection. In the subpolar region maximal amplitudes of the temperature response are found for intermediate (decadal) periods (R-t <similar to> 1) where the propagation of temperature anomalies constructively interferes with the local forcing. A comparison of the NAO-generated propagating temperature anomalies with those found in observations will be discussed.
Latif, M., K. Sperber, J. Arblaster, P. Braconnot, D. Chen, A. Colman, U. Cubasch, C. Cooper, P. Delecluse, D. DeWitt, L. Fairhead, G. Flato, T. Hogan, M. Ji, M. Kimoto, A. Kitoh, T. R. Knutson, H. Le Treut, T. Li, S. Manabe, O. Marti, C. Mechoso, G. Meehl, S. Power, E. Roeckner, J. Sirven, L. Terray, A. Vintzileos, R. Voss, B. Wang, W. Washington, I. Yoshikawa, J. Yu and S. Zebiak, 2001: ENSIP: the El Niño simulation intercomparison project. Climate Dynamics, 18(3-4): 255-276.
An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Nino/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.
Marshall, J., Y. Kushnir, D. Battisti, P. Chang, A. Czaja, R. Dickson, J. Hurrell, M. McCartney, R. Saravanan and M. Visbeck, 2001: North Atlantic climate variability: Phenomena, impacts and mechanisms. International Journal of Climatology, 21(15): 1863-1898.
Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena. their climatic impacts and our present state of understanding of their underlying cause. Copyright (C) 2001 Royal Meteorological Society.
Martinson, D. G. and M. Steele, 2001: Future of the Arctic sea ice cover: Implications of an Antarctic analog. Geophysical Research Letters, 28(2): 307-310.
Recent observations reveal a significant change in the upper ocean characteristics of the eastern Arctic in 1995. The change is manifested through the loss of a near-surface layer known as the cold halocline layer (CHL). Without the CHL, the Arctic water column looks and behaves like the Antarctic water column. The expected local impact is the appearance of significant winter ocean heat fluxes (15 - 20 W/m(2)) and reduction of winter ice growth by 70 - 80% relative to years in which the CHL was present. Preliminary results suggest a partial recovery of the CHL in the late 1990's, tracking the weakening of the Arctic Oscillation.
Muench, R. D., J. H. Morison, L. Padman, D. Martinson, P. Schlosser, B. A. Huber and R. Hohmann, 2001: Maud Rise revisited. Journal of Geophysical Research-Oceans, 106(C2): 2423-2440.
An oceanographic field program called the Antarctic Zone Flux experiment was carried out in the eastern Weddell Sea during austral winter (July-September) 1994. Data from a drift buoy array were used in concert with shipboard observations to provide exceptionally high horizontal resolution of upper ocean hydrographic parameters near Maud Rise. Chemical and tracer data were obtained from the ship. We identify a "warm pool" southwest of the rise as a dynamically necessary region of positive (cyclonic) vorticity that is associated with a Taylor column over the rise. Both a warm "halo" surrounding the Taylor column and the warm pool are associated with thermocline shoaling that is a necessary condition for high upward heat fluxes to occur. These features extend the influence of Maud Rise bottom topography on upper ocean heat flux over a region that is larger, by a factor of at least 2, than the area directly overlying the rise. Areal mean upward heat fluxes of about 25 W m(-2) are derived using both upper ocean T ("instantaneous") values and tracer data ("integrated") values. Fluxes derived over the warm halo and pool regions using only upper ocean T exceeded 100 W m(-2) at specific sites. Elsewhere in the region, the T-derived heat fluxes varied widely from <10 to >50 W m(-2), whereas the tracer-derived heat fluxes showed a considerably more uniform distribution. Our mean values are similar to those that have been previously reported. Historical ice cover data have shown that the geographical region encompassed by Maud Rise and the warm pool area to the southwest is a preferred site for polynya formation, consistent with these findings. Time series analyses of the historical upper ocean data set suggest that conditions conducive to polynya formation are correlated with climate processes remote from the Southern Ocean.
Orsi, A. H., S. S. Jacobs, A. L. Gordon and M. Visbeck, 2001: Cooling and ventilating the abyssal ocean. Geophysical Research Letters, 28, 2923-2926 pp.
The abyssal ocean is filled with cold, dense waters that sink along the Antarctic continental slope and overflow sills that lie south of the Nordic Seas. Recent integrations of chlorofluorocarbon-11 (CFC) measurements are similar in Antarctic Bottom Water (AABW) and in lower North Atlantic Deep Water (NADW), but Antarctic inputs are approximate to 2 degreesC colder than their northern counterparts. This indicates comparable ventilation rates from both polar regions, and accounts for the Southern Ocean dominance over abyssal cooling. The decadal CFC-based estimates of recent ventilation are consistent with other hydrographic observations and with longer-term radiocarbon data, but not with hypotheses of a 20(th) -century slowdown in the rate of AABW formation. Significant variability is not precluded by the available ocean measurements, however, and interannual to decadal changes are increasingly evident at high latitudes.
Orton, P. M. and G. C. Kineke, 2001: Comparing calculated and observed vertical suspended sediment distributions from a Hudson River Estuary turbidity maximum. Estuarine, Coastal and Shelf Science, 52(3): 401-410.
Suspended-sediment concentrations calculated using a vertical suspended-sediment distribution equation were compared to observations from a field study of the lower turbidity maximum of the Hudson River Estuary. At four stations, an instrumented tripod measured vertical profiles of suspended-sediment concentration, current velocity, salinity and temperature through a tidal cycle. Bed and suspended-sediment samples were also analysed to determine inorganic sediment size distributions. Velocities were as high as 1·3 m/s, with suspended-sediment concentrations up to 2000 mg/L. When a well-defined pycnocline existed, cross-isopycnal mixing was strongly damped (based on the gradient Richardson number). Suspended-sediment profiles were calculated with a stratification-modified Rouse equation, using (1) reference concentrations measured at 20 cm above the bed, (2) estimates of shear velocity based on the quadratic stress law, and (3) a constant sediment settling velocity of 0·22 cm/s. Differences between mean calculated and observed total suspended load for each station were ~17, 7, 14 and 58%, respectively. An uncertainty analysis revealed that the two parameterizations most likely to account for differences of this magnitude were those used for settling velocity and stratification. Best results were found when substituting a power law relationship for settling velocity based on suspended-sediment concentration. This demonstrates the improvement which a power law formulation can provide over the commonly used constant ws parameterization in fine sediment environments.
Ou, H. W., 2001: Possible bounds on the Earth's surface temperature: From the perspective of a conceptual global-mean model. Journal of Climate, 14(13): 2976-2988.
A global-mean model is used here to elucidate possible bounds on the surface temperature of a simplified ocean-atmosphere system. Extending previous one-dimensional models, it has included as internal variables the low-level and high-level cloud covers and the turbulent wind at the surface. The main hypothesis for the model closure is that the conversion rate from the solar to the kinetic energy- or, equivalently, the rate of internal entropy production- is maximized, which has been applied with considerable success in past latitudinal models. From the model derivation, it is found that the surface temperature is narrowly bounded below by the onset of the greenhouse effect and above by the rapid increase of the saturation vapor pressure. Because both are largely intrinsic properties of water, the resulting surface temperature is mostly insensitive to detailed balances or changing external conditions. Even with a 50% change of the solar constant from its present-day value, the model temperature has varied by only about 10 K. The reason that the heat balances can be maintained is an internal adjustment of the low cloud cover, which offsets the solar effect. The model offers a plausible explanation of an equable climate in the geological past so long as there is a substantial ocean.
Parkinson, C. L., D. Rind, R. J. Healy and D. G. Martinson, 2001: The impact of sea ice concentration accuracies on climate model simulations with the GISS GCM. Journal of Climate, 14(12): 2606-2623.
The Goddard Institute for Space Studies global climate model (GISS GCM) is used to examine the sensitivity of the simulated climate to sea ice concentration specifications in the type of simulation done in the Atmospheric Model Intercomparison Project (AMIP), with specified oceanic boundary conditions. Results show that sea ice concentration uncertainties of +/-7% can affect simulated regional temperatures by more than 6 degreesC, and biases in sea ice concentrations of +7% and -7% alter simulated annually averaged global surface air temperatures by -0.10 degrees and +0.17 degreesC, respectively, over those in the control simulation. The resulting 0.27 degreesC difference in simulated annual global surface air temperatures is reduced by a third, to 0.18 degreesC, when considering instead biases of +4% and -4%. More broadly, least squares fits through the temperature results of 17 simulations with ice concentration input changes ranging from increases of 50% versus the control simulation to decreases of 50% yield a yearly average global impact of 0.0107 degreesC warming for every 1% ice concentration decrease, that is, 1.07 degreesC warming for the full +50% to -50% range. Regionally and on a monthly average basis, the differences can be far greater, especially in the polar regions, where wintertime contrasts between the +50% and -50% cases can exceed 30 degreesC. However, few statistically significant effects are found outside the polar latitudes, and temperature effects over the nonpolar oceans tend to be under 1 degreesC, due in part to the specification of an unvarying annual cycle of sea surface temperatures. The +/-7% and +/-4% results provide bounds on the impact (on GISS GCM simulations making use of satellite data) of satellite-derived ice concentration inaccuracies, +/-7% being the current estimated average accuracy of satellite retrievals and +/-4% being the anticipated improved average accuracy for upcoming satellite instruments. Results show that the impact on simulated temperatures of imposed ice concentration changes is least in summer, encouragingly the same season in which the satellite accuracies are thought to be worst. Hence, the impact of satellite inaccuracies is probably less than the use of an annually averaged satellite inaccuracy would suggest.
Rind, D., M. Chandler, J. Lerner, D. G. Martinson and X. Yuan, 2001: Climate response to basin-specific changes in latitudinal temperature gradients and implications for sea ice variability. Journal of Geophysical Research-Atmospheres, 106(D17): 20161-20173.
We use experiments with the GISS general circulation model to investigate how changes in latitudinal temperature gradients affect atmospheric circulation in different ocean basins, with particular attention paid to the implications for high-latitude sea ice. The results are relevant to both estimated past climate changes, current climate gradient changes (e.g., El Nino-Southern Oscillation events), and proposed future climate responses to greenhouse gases. Sea surface temperature gradients are increased/decreased in all ocean basins, and in the Pacific and Atlantic separately, without changing sea ice or global average temperature. Additional experiments prescribe sea ice growth/reduction with global cooling/warming. As expected, increased gradients strengthen the subtropical jet stream and deepen the subpolar lows in each hemisphere, but results in the Northern and Southern Hemispheres differ in fundamental ways. In the Northern Hemisphere, increased storm intensities occur in the ocean basin with the increased gradient; in the Southern Hemisphere the deeper storms occur in the ocean basin with the decreased gradient. Alterations of the gradient in one ocean basin change longitudinal temperature gradients; an increased gradient in one basin from tropical heating results in subsidence in the tropics in the other basin, mimicking the effect of a decreased gradient in that basin. The subtropical jet is therefore strengthened over the basin with the increased gradient and decreased over the other ocean basin. Hence in many respects, regional effects, such as the strength of subpolar lows in an individual basin, are amplified when the gradient changes are of opposite sign in the two ocean basins. The Southern Hemisphere response occurs because gradient increases in one ocean basin, by strengthening the subtropical jet, shift storm tracks equatorward and away from the potential energy source associated with cold air advection from Antarctica. At the same time, with a weaker subtropical jet in the other basin, storms move poleward and strengthen. This latter effect may explain observed sea ice variations that are out of phase in the Atlantic and Pacific Ocean basins in the Southern Hemisphere (referred to as the Antarctic dipole) as well as upper ocean variability in the Weddell gyre. Gradient changes produce little effect on sea level pressure in the Arctic, unless sea ice is changed. With Arctic sea ice reductions, the sea ice response acts as a positive feedback, inducing cyclonic circulation changes that would enhance its removal, as may be occurring due to the current high phase of the North Atlantic Oscillation.
Robertson, R., 2001: Internal tides and baroclinicity in the southern Weddell Sea. Part I: Model description. Journal of Geophysical Research-Oceans, 106(C11): 27001-27016.
Near the continental shelf break in the southern Weddell Sea, Warm Deep Water and Western Shelf Water meet. Mixing mechanisms, such as internal tides, have the potential to mix these water masses and form Antarctic Bottom Water. A modified version of the Princeton Ocean Model was utilized to investigate the internal tidal fields generated by the interaction of the M-2 barotropic tide with topography for transects across the continental shelf and slope in the southern Weddell Sea. Internal tides were generated over the upper continental slope as predicted by linear internal wave theory. Although the essentially two-dimensional domain resulted in differences between the model elevations and the observations exceeding the observational uncertainties, the cross-slope velocities agreed well with differences less than the uncertainties for 78% of the existing observations.
Robertson, R., 2001: Internal tides and baroclinicity in the southern Weddell Sea. Part II: Effects of the critical latitude and stratification. J. Geophys. Res., 106: 27,017-27,034.
Tidal dynamics affect Antarctic Bottom Water and Ice Shelf Water production in the southern Weddell Sea and the general circulation and mixing. In this region, tidal dynamics were found to be extremely sensitive both to the proximity of the continental shelf-slope break to the critical latitude ycrit and to the existence of a shelf break front. If the continental slope was not in the vicinity of ycrit or a shelf break front was absent, generation of internal tides was negligible. Generation of internal tides was greatest when both a shelf break front was present and the continental slope was near and equatorward of ycrit. When a shelf break front was present and the continental slope was near but poleward of ycrit, a strong benthic response occurred. This response included development of benthic inertial oscillations at the continental shelf-slope break, increased benthic shear, a thicker benthic boundary layer, and increased benthic dissipation. A shelf break front with even a slight horizontal density gradient was found to significantly enhance internal tide generation and/or the benthic effects.
Robertson, R., L. Padman and M. D. Levine, 2001: A correction to the baroclinic pressure gradient term in the Princeton Ocean Model. Journal of Atmospheric and Oceanic Technology, 18(6): 1068-1075.
An error in the calculation of the baroclinic pressure gradient term in the Princeton Ocean Model (POM) was identified while modeling the M-2 tidal current near its critical latitude in the southern Weddell Sea. The error arises from the present calculation of density, which involves the subtraction of a background density profile from the density field calculated at each internal time step. The small displacement of sigma surface depths relative to the surface, as surface elevation changes, causes a slight error in the calculation of the vertical and horizontal gradients of potential density. The error is largest at the seabed over rapidly changing bathymetry such as the continental slope. The baroclinic pressure gradient error is typically much smaller than the Coriolis term in the momentum equations and, therefore, usually unimportant. Close to the critical latitude, however, near-resonance between the error and Coriolis terms can cause an energetic and spatially complex spurious inertial mode to develop. The error is significant when modeling tides near their critical latitudes, and will contribute to the error in the baroclinic pressure gradient in other simulations. Two methods were suggested for fixing this problem. The preferred method was tested by applying the new form of POM to the southern Weddell Sea. The new results are consistent with both current meter data and predictions of linear internal wave theory.
Rogers, A. N., D. H. Bromwich, E. N. Sinclair and R. I. Cullather, 2001: The atmospheric hydrologic cycle over the Arctic Basin from reanalyses. Part II: Interannual variability. Journal of Climate, 14(11): 2414-2429.
Previously, the atmospheric moisture budgets over the Arctic Basin as represented by reanalysis data from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis and from the European Centre for Medium-Range Weather Forecasts reanalysis were evaluated for the overlap period of 1979-93 and found to be very similar to each other and to the available observations. Here emphasis is on the 50 yr of the NCEP-NCAR reanalysis (January 1949-May 1999) to depict the interannual variability of the atmospheric moisture fluxes across 70 degreesN and their convergence farther north.
Seager, R., Y. Kushnir, N. H. Naik(Henderson), M. A. Cane and J. Miller(Nakamura), 2001: Wind-driven shifts in the latitude of the Kuroshio-Oyashio Extension and generation of SST anomalies on decadal timescales. Journal of Climate, 14(22): 4249-4265.
The causes of decadal variations of North Pacific sea surface temperatures (SSTs) are examined using a hindcast performed with an ocean general circulation model thermodynamically coupled to an atmospheric mixed layer model (OGCM-AML model) and forced by the time history of observed surface winds. The "shift'' in North Pacific Ocean climate that occurred around 1976/77 is focused on since this is the best observed example available. After the 1976/77 shift the Aleutian low deepened and moved to the southeast of its previous position. This placed anomalous cyclonic flow over the North Pacific. The SST response, as simulated by the ocean model, consisted of two components: a fast and local part and a delayed and remote part. In the central Pacific stronger westerlies cool the ocean by increased equatorward Ekman drift. Here the dynamical cooling is sufficiently large that the surface fluxes damp the SST anomaly. This Ekman response is fast and local and cools the SSTs beginning in 1977 and persisting through 1988. In the early 1980s cool SSTs emerge in the latitude of the Kuroshio-Oyashio Extension east of Japan and persist until 1989. It is shown that this region of cooling is associated with a southward displacement of the latitude of the confluence between the subpolar and subtropical gyres. This is consistent with the southward shift in the zero wind stress curl line. The timescale for the gyre adjustment is no more than 4 yr. These results compare favorably with observations that also first show the central Pacific cooling and, later, cooling east of Japan. Observations show the cooling in the Kursohio-Oyashio Extension region to be damped by surface fluxes, implying an oceanic origin. The timescale of adjustment is also supported by analyses of observations. The delayed response of the ocean to the varying winds therefore creates SST anomalies as the latitude of the gyre confluence varies.
Seager, R., Y. Kushnir, P. Chang, N. H. Naik(Henderson), J. Miller(Nakamura) and W. Hazeleger, 2001: Looking for the role of the ocean in tropical Atlantic decadal climate variability. Journal of Climate, 14(5): 638-655.
Ocean models are used to investigate how variations in surface heat fluxes and ocean heat transports contribute to variations of tropical Atlantic SSTs on decadal timescales. The observed patterns of variability, deduced from reanalyses of the National Centers for Environmental Prediction (NCEP), are found to involve the ocean's response to variations in the strength of the northeast and southeast trades. Stronger trade winds are associated with anomalously cool surface temperatures. The trade winds and surface temperatures in each hemisphere appear to behave independently but each is associated with anomalous cross-equatorial flow. A numerical model is used in an attempt to simulate this variability. The model is an ocean general circulation model coupled to a simple model of the atmospheric mixed layer and is forced by NCEP winds from 1958 to 1998. The model reasonably reproduces the observed variability. Analysis of the ocean model's mixed layer energy budget shows that, on decadal timescales, the surface temperature variability is forced by the changes in surface fluxes and is damped by changes in the ocean heat transport. The changes in ocean heat transport are dominated by the horizontal advection of anomalous temperatures by the mean meridional currents. If advection of the mean SST field by anomalous currents is neglected, then the history of observed surface temperatures can still be adequately represented. If advection of the anomalous SSTs by the mean circulation is also neglected, then the model significantly overestimates the surface temperature anomalies but reproduces their temporal evolution. In the more complete models, between 15 degreesN and 15 degreesS, the changes in ocean heat transport are largely in phase with the changes in surface heat fluxes and SST. Evidence for ocean heat transport either leading or lagging development of surface temperature anomalies is weak in the deep Tropics but appears more persuasive in the northern subtropics. Consistent with these findings, SST anomalies are largely stationary in the deep Tropics but appear to propagate poleward in the northern subtropics. Nonetheless these results suggest that the role of the ocean in tropical Atlantic decadal climate variability is largely passive and damping. Differences with other models that show a more critical role for the ocean, and relevance to reality, are discussed.
Smith, P. C., R. W. Houghton, R. G. Fairbanks and D. G. Mountain, 2001: Interannual variability of boundary fluxes and water mass properties in the Gulf of Maine and on Georges Bank: 1993-1997. Deep-Sea Research Part II-Topical Studies in Oceanography, 48(1-3): 37-70.
Analysis of three years (October 1993-September 1996) of monthly mean current, temperature and salinity observations from moorings in the major Gulf of Maine (GOM) inflows off southwest Nova Scotia (C2) and in Northeast Channel (NECE) reveals some new features of the annual NECE cycles, including (1) a peak in near-surface ( < 75 m) inflow ill spring (vs. late summer at depth), suggesting that dynamic control of shallow and deep layers may be different, (2) a maximum near-surface cross-channel flow toward Georges Bank (GB) in late winter, suggesting a climatological tendency for Scotian Shelf "cross-overs" in that season, and (3) the absence of a significant salinity cycle over most Of the water column. Deviations from the annual cycle indicate that the first part of the observation period was characterized by enhanced warm, saline, deep ( > 75 m) NECE inflow, followed by later episodes of enhanced cold, fresh inflow at C2 and shallow NECE. Generally, the flow rates at C2 and deep NECE were out of phase, with increased inflow at C2 (deep NECE) associated with reduced inflow at deep NECE (C2) and cooler, fresher (warmer,saltier) conditions at both sites. Freshwater transport anomalies to the Gulf are maximum in the surface layers and largely negative(positive) over the first(last) half of the measurements. The timing of these freshwater inflow variations is consistent with observed fluctuations in hydrographic measurements in the GOM and GB, which reached peak salinities in late 1994, then declined through 1995-1996. Oxygen isotope analysis suggests that almost all of the fresh water present on the central cap of GB in 1996 and early 1997 is of northern (Scotian Shelf) origin as opposed to 1994 and 1995 when Maine River Waters contributed 38 and 26%, respectively, to the freshwater (relative to 34.8) on the cap. A simple box model driven by observed changes in the boundary fluxes indicates that over the last half of the measurement period (April 1995-September 1996), the volumetric flow rate through the COM increased by 10(5) m(3) s(-1) (roughly 17% of the total transport, 5.83 x 10(5) m(3) s(-1)), and that increased freshwater fluxes in the surface layers at C2. and NECE produced a net decrease of 0.73 in the salinity of the outflow waters. Average volumetric transports at C2(NECE) were roughly twice(half) those observed in the late 1970s, but the total is consistent with climtological estimates. The net change in the freshwater fluxes exceeds the total climatological mean estimate. Examination of possible local and remote sources confirms that the origin of the 1996-1997 freshwater anomaly is in the northern Labrador Sea/Baffin Bay and results from exceptionally cold winters in the early 1990s. Analysis of a similar event in the early 1980s suggests their occurrence is part of a quasi-decadal climate signal which follows the North Atlantic Oscillation (NAO). Crown copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.
Stieglitz, M., A. Ducharne, R. D. Koster and M. Suarez, 2001: The impact of detailed snow physics on the simulation of snow cover and subsurface thermodynamics at continental scales. Journal of Hydrometeorology, 2(3): 228-242.
The three-layer snow model of Lynch-Stieglitz is coupled to the global catchment-based land surface model of the National Aeronautics and Space Administration's Seasonal to Interannual Prediction Project, and the combined models are used to simulate the growth and ablation of snow cover over the North American continent for the period of 1987-88. The various snow processes included in the three-layer model, such as snow melting and refreezing, dynamic changes in snow density, and snow insulating properties, are shown (through a comparison with the corresponding simulation using a much simpler snow model) to lead to an improved simulation of ground thermodynamics on the continental scale. This comparison indicates that the three-layer model, originally developed and validated at small experimental catchments, does indeed capture the important snow processes that control the growth and the ablation of continental-scale snowpack and its snow insulation capabilities.
Susanto, R. D., A. L. Gordon and Q. N. Zheng, 2001: Upwelling along the coasts of Java and Sumatra sand its relation to ENSO. Geophysical Research Letters, 28(8): 1599-1602.
Upwelling along the Java-Sumatra Indian Ocean coasts is a response to regional winds associated with the monsoon climate. The upwelling center with low sea surface temperature migrates westward and toward the equator during the southeast monsoon (June to October). The migration path depends on the seasonal evolution of alongshore winds and latitudinal changes in the Coriolis parameter. Upwelling is eventually terminated due to the reversal of winds associated with the onset of the northwest monsoon and impingement of Indian Ocean equatorial Kelvin waves. Significant interannual variability of the Java-Sumatra upwelling is linked to ENSO through the Indonesian throughflow (ITF) and by anomalous easterly wind. During El Niño episodes, the Java-Sumatra upwelling extends in both time (into November) and space (closer to the equator). During El Niño (La Niña), the ITF carries colder (warmer) water shallowing (deepening) thermocline depth and enhancing (reducing) upwelling strength.
Thurnherr, A. M. and K. J. Richards, 2001: Hydrography and high-temperature heat flux of the Rainbow hydrothermal site (36 degrees 14 ' N, Mid-Atlantic Ridge). Journal of Geophysical Research-Oceans, 106(C5): 9411-9426.
On the basis of an extensive set of conductivity-temperature-depth, lowered acoustic Doppler current profiler (LADCP), and nephelometry profiles and tow-yes the hydrography, flow field, and particle plume of the Rainbow hydrothermal site on the Mid-Atlantic Ridge are analyzed. In the rift valley the water column is less dense and stratified than both eastern and western off-ridge water, with T/S characteristics consistent with inflow across a sill from the east. The buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m, The bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m. Large-amplitude internal waves consistent with tidal forcing are observed near the sill, but LADCP measurements suggest that the tidal signal is not strong enough to lead to Row reversal at plume depth. Above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream. Density-averaged light-scattering profiles show the hydrothermal particle plume to be Gaussian in depth, even in the near field, where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest. The temperature anomalies associated with the mean near-source particle plume are of order -5 x 10(-3) degreesC; that is, the plume is cold/fresh as expected from the background hydrography of the deep Atlantic. Using the flow field, light-scattering, and hydrographic anomaly observations, the heat flux associated with the hydrothermal particle plume at Rainbow is estimated to lie between 1 and 5 GW.
Ting, M. F., H. L. Wang and L. H. Yu, 2001: Nonlinear stationary wave maintenance and seasonal cycle in the GFDL R30 GCM. Journal of the Atmospheric Sciences, 58(16): 2331-2354.
In this study, the climatological stationary wave maintenance is examined from nonlinear perspective using the GFDL R30 GCM outputs, a fully nonlinear stationary wave model, and a linear stationary wave model. The primary focus of the study is on the nature of the stationary nonlinearity and relative contribution to the total nonlinearity by various factors, such as heating, orography, and the interaction between flows forced by heating and orography. It is found that both the nonlinear effect of the diabatic heating and the nonlinear interaction between flows forced by orography and diabatic heating are important contributors toward the total stationary nonlinearity in northern winter and summer. Some regional features, such as the anticyclone off the northwest coast of North America in winter and the southwestern U.S. summer anticyclone, are entirely due to the nonlinear interaction between flows forced by heating and orography.
Tourre, Y. M., B. Rajagopalan, Y. Kushnir, M. Barlow and W. B. White, 2001: Patterns of coherent decadal and interdecadal climate signals in the Pacific Basin during the 20th century. Geophysical Research Letters, 28(10): 2069-2072.
Two distinct low-frequency fluctuations are suggested from a joint frequency domain analysis of the Pacific Ocean (30S-60 degreesN) sea surface temperature (SST) and sea level pressure (SLP). The lowest frequency signal reveals a spatially coherent interdecadal evolution. In-phase SST and SLP anomalies are found along the subarctic frontal zone (SAFZ). It is symmetric about the equator. with tropical SST anomalies peaking near 15 degrees latitudes in the eastern Pacific. The other low-frequency signal reveals a spatially coherent decadal evolution. It is primarily a low-latitude phenomenon. Tropical SST anomalies peak in the central equatorial ocean with evidence of atmospheric teleconnections. These interdecadal and decadal signals join the ENSO and quasibiennial signals in determining dominant patterns of Pacific Ocean natural climate variability. Relative phasing and location of the SST and SLP anomalies for the decadal. ENSO, and the quasi-biennial signals, are similar to one another but significantly different from that of the interdecadal signal.
Tremblay, L. B., 2001: Can we consider the Arctic Oscillation independently from the Barents Oscillation? Geophysical Research Letters, 28(22): 4227-4230.
An EOF analysis of a constructed time series mimicking the Northern Hemisphere SLP variability of the last 50 years shows that the Barents Oscillation (BO) appears as a means to represent the sudden eastward shift of the northern center of action associated with the Arctic Oscillation (AO) observed in the mid-seventies. This sudden shift (non-stationarity) appears in an EOF analysis as a step change in the relative phase between the principal components associated with the EOFs of the AO and BO. The results also show that an EOF analysis of a constant amplitude signal can produce artificial trends and/or amplitude changes in the principal component associated with a given mode (eg. AO) when such non-stationarities are present in the signal. In this case, different modes of variability represented by EOF's cannot be considered independently from one another. In the example presented, although the principal components are completely uncorrelated from one another, perfect correlation and anti-correlation axe present in the first and second parts of the time series respectively.
Ukita, J. and D. G. Martinson, 2001: An efficient adjustable-layering thermodynamic sea-ice model formulation for high-frequency forcing. Annals of Glaciology, 33: 253-260.
Recent observations suggest that high-frequency forcing events have profound influence oil the air-sea-ice interactions in the polar region. Studying these events with sea-ice models requires close examination of the model sensitivity that may arise from the high-frequency variability of the forcing. We show that the maximum layer thickness is dictated by the time-scale of the forcing variability and that the computation of the surface temperature develops enhanced sensitivity at high-frequency forcing. We resolve these constrains by developing all "adjustable-layering" thermodynamic formulation for ice and snow that re-computes the number of layers required each time-step to satisfy this maximum thickness, which preserves the total enthalpy and general internal thermal gradients. The conservation equations form a tri-diagonal system ideal for a Fast and efficient implicit solution. Furthermore, we resolve the issue of the high sensitivity of the surface flux balance by solving the linearized version of the flux boundary condition simultaneously with the overall conservation system. In this paper we develop the analyses specifying the model requirements, describe the model system and test its algorithmic implementation.
Wang, Zhiren , Z. Wang and D. Wu, 2001: The Spatio-Temporal Distribution of the Global Tropical Storms. Dynamics of Atmospheric and Oceanic Circulations and Climate-Celebration of the 80th Birthday of Prof. ZHU Baozhen(China Meteorological Press): 326-337.
Statistical analysis to the global tropical storms shows that there are some significant statistical characteristics in the their spatio-temporal distribution. In every oceanic area, the storm-occurring frequency is of seasonal concentration, with the peaks of frequency of the tropical storms corresponding to the peaks of SST. In the Pacific Ocean and the Indian Ocean, the areas of the storm-dense months go geographically counter-clockwise a year. The total occurrence frequency of the global tropical storms is the highest in August and September, and the lowest in April and May. The occurring density and extending extent of storms are larger in the Northern Hemisphere than in the Southern one. The largest density is located in the North-East and-West Pacific Ocean. The storm-occurring frequency in each oceanic area is of interannual and decadal oscillation, existing distinctly the quasi-periods of two to three years, six to seven years, eleven to twelve years and twenty-one to twenty-two years. It is quite often that the stronger the storms, the longer the life-spans of the storms. Among months, the stronger and longer-lasting storms occur more often in the months with higher storm frequency than in the months with lower storm frequency; yet, among years, the stronger and longer-lasting storms usually occur in the years with less storms. In the large-scale background, the storm moves along the anticyclonic path.
Wilson, P. R., D. G. Ainley, N. Nur, S. S. Jacobs, K. J. Barton, G. Ballard and J. C. Comiso, 2001: Adelie penguin population change in the pacific sector of Antarctica: relation to sea-ice extent and the Antarctic Circumpolar Current. Marine Ecology-Progress Series, 213: 301-309.
One of the longest continuing data sets involving a marine organism in the Antarctic is that of annual estimates of breeding population size of Adelie penguins Pygoscelis adeliae at colonies on Ross Island, Ross Sea, 1959 to 1997. The sizes of these colonies have displayed significant interannual variability during the 29-yr period. We hypothesized that changes are related to natural environmental factors; and used path analysis to analyze annual variation in population growth in relation to physical environmental factors during that part of the record with comparable sea-ice satellite imagery from 1973 to 1997. The Ross Sea sector of the Southern Ocean lying north of Ross Island, from 150 degreesE to 1300 W, comprised our study area. Annual population growth measured during summer was explained best, and inversely, by the extent of sea-ice in the study area 5 winters earlier, and in some way related to the Southern Oscillation. Analysis of a subset of the sea-ice data from 1979 to 1997 indicated strong correlations to ice conditions in the eastern portion of the study area (174 to 130 degreesW), and virtually no correlations to the western half (150 degreesE to 175 degreesW). This result supported other indirect evidence that the Ross Island penguins winter in the eastern Ross Sea/western Amundsen Sea. A demographic model indicated that variation in survival of juveniles and subadults might account for the observed population variation, and would also explain the 5-yr lag as 5 yr is the average age of recruitment to the summer breeding population. Extensive sea-ice during winter appears to reduce subadult survival, expressed subsequently when these cohorts reach maturation. We hypothesize that extensive (more northerly) sea-ice Limits access of penguins to productive waters known to occur south of the southern boundary of the Antarctic Circumpolar Current, with starvation or increased predation disproportionately affecting less-experienced birds. The observed patterns of penguin population change, including those preceding the satellite era, imply that sea-ice extent has changed significantly over recent decades.
Yuan, X. J. and D. G. Martinson, 2001: The Antarctic Dipole and its predictability. Geophysical Research Letters, 28(18): 3609-3612.
This study investigates the nature of interannual variability of Antarctic sea ice and its relationship with the tropical climate. We find that the dominant interannual variance structure in the sea ice edge and surface air temperature fields is organized as a quasi-stationary wave which we call the "Antarctic Dipole" (ADP). It is characterized by an out-of-phase relationship between the ice and temperature anomalies in the central/eastern Pacific and Atlantic sectors of the Antarctic. The dipole consists of a strong standing mode and a weaker propagating motion within each basin's ice field. It has the same wavelength as the Antarctic Circumpolar Wave (ACW) and dominates the ACW variance. The dipole is clearly associated with tropical ENSO events; it can be predicted with moderate skill using linear regression involving surface temperature two to four months ahead. The prediction performs better in extreme warm/cold years, and best in La Niña years.
Zappa, C. J., W. E. Asher and A. T. Jessup, 2001: Microscale wave breaking and air-water gas transfer. Journal of Geophysical Research-Oceans, 106(C5): 9385-9391.
Laboratory results showing that the air-water gas transfer velocity k is correlated with mean square wave slope have been cited as evidence that a wave-related mechanism regulates k at low to moderate wind speeds [Jahne et al,, 1987; Beck et al., 1999]. Csanady  has modeled the effect of microscale wave breaking on air-water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and Ag, the fractional area coverage of microscale breaking waves. Simultaneous, colocated infrared (IR) and wave slope imagery is used to verify that Ag detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and Ag made at the University of Washington wind-wave tank at wind speeds from 4.6 to 10.7 m s(-1), we show that k is linearly correlated with Ag, regardless of the presence of surfactants. This result is consistent with Csanady's  model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.
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