Black, D. E., L. C. Peterson, J. T. Overpeck, A. Kaplan, M. N. Evans and M. Kashgarian, 1999: Eight centuries of North Atlantic Ocean atmosphere variability. Science, 286(5445): 1709-1713.
Climate in the tropical North Atlantic is controlled Largely by variations in the strength of the trade winds, the position of the Intertropical Convergence Zone, and sea surface temperatures. A high-resolution study of Caribbean sediments provides a subdecadally resolved record of tropical upwelling and trade wind Variability spanning the past 825 years. These results confirm the importance of a decadal (12- to 13-year) mode of Atlantic variability believed to be driven by coupled tropical ocean-atmosphere dynamics. Although a well-defined interdecadal mode of variability does not appear to be characteristic of the tropical Atlantic, there is evidence that century-scale variability is substantial. The tropical Atlantic may also have been involved in a major shift in Northern Hemisphere climate variability that took place about 700 years ago.
Bromwich, D. H., Q. S. Chen, Y. F. Li and R. I. Cullather, 1999: Precipitation over Greenland and its relation to the North Atlantic Oscillation. Journal of Geophysical Research-Atmospheres, 104(D18): 22103-22115.
The omega equation method based on an equivalent isobaric geopotential height in sigma coordinates has been used to retrieve the precipitation over Greenland. This approach is designed to accurately represent the topographic effects of the Greenland Ice Sheet on atmospheric motion and precipitation. The 11 year mean precipitation from 1985 to 1996 over all of Greenland is 376 mm yr(-1), which is close to the long-term mean precipitation of 346 mm yr(-1) estimated from glaciological data. The precipitation over all of Greenland shows that the largest value in 1986 is 472 mm yr(-1) and the smallest value in 1995 is 309 mm yr(-1). The major interannual variability of the atmospheric circulation in the North Atlantic can be represented by the variation of the North Atlantic Oscillation (NAO) index, which is most pronounced during winter. It is found that if the NAO index increases, the total precipitation over Greenland decreases, and vice versa. The correlation coefficient between these two series for 1985-1995 is -0.75. The mean precipitation over southern Greenland, where the majority of precipitation falls, is more closely related to the NAO index in winter, and their correlation coefficient is -0.80. This relationship can be understood from the composite maps of sea level pressure and Greenland precipitation for the high and low index months. During months of high NAO index values, the Icelandic Low is strong. During months of low NAO index values, the monthly mean low is located to the southwest of Greenland over the Labrador Sea. Precipitation amounts over the southeast coast of Greenland are about 100 mm larger during the low NAO index months than the high NAO index months. Precipitation over all of Greenland during the low NAO index months is higher. There are significant downward trends in annual precipitation from 1985-1995 for all of Greenland and its southern and central west coastal regions, amounting to about 3% per year.
Bromwich, D. H., R. I. Cullather and R. W. Grumbine, 1999: An assessment of the NCEP operational global spectral model forecasts and analyses for Antarctica during FROST. Weather and Forecasting, 14(6): 835-850.
Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50 degrees S during the July 1994 special observing period of the Antarctic First Regional Observing Study of the Troposphere project. Over the Antarctic plateau, the poor representation of the continent's terrain creates ambiguity in assessing the quality of surface variables. An examination of the vertical temperature profile, however, finds the near-surface temperature inversion strength to be substantially smaller than the observed climatology at the zero forecast hour. This arises, from surface temperatures that are warmer than expected. Significant adjustment occurs in a variety of fields over the first few days of the medium-range forecast, which likely results from the initial hour's suspect temperature profile. A spatially oscillating series of forecast anomalies in the zonally averaged temperature cross section stretches to middle latitudes by day 3. Near-surface and upper-troposphere values are found actually to improve at the South Pole with forecast time, although some fields continue to adjust through day 7. Although the examination presented here does nor give a complete diagnosis. differences between observations and analyses Suggest deficiencies with the model initial fields have a major role in producing the substantial model drift found. Atmospheric moisture over the continental interior does not change significantly with forecast hour, although the distinct contrast between nearshore and interior conditions lessens with forecast time. A spurious high-latitude wave pattern is found for a variety of variables. The pattern of this distortion remains constant with forecast hour. Over the ocean, large forecast pressure and height differences with analyses are associated with blocking conditions. However, it is unclear whether this results from deficiencies in the forecast model or the meager observational network over the Southern Ocean.
Chen, D. K., M. A. Cane and S. E. Zebiak, 1999: The impact of NSCAT winds on predicting the 1997/1998 El Niño: A case study with the Lamont-Doherty Earth Observatory model. Journal of Geophysical Research-Oceans, 104(C5): 11321-11327.
Using the NASA scatterometer (NSCAT) winds for initialization has greatly improved the Lamont-Doherty Earth Observatory model forecasts of the 1997/1998 El Niño. The improvement is mostly attributed to the better resolved wind field in the southeast tropical Pacific. Because of the simplicity of the model and the short record of the NSCAT data, our model results should be taken as indicative rather than conclusive. Nevertheless, it is crucial to assimilate accurate information into the initial model state to predict the development of El Niño. Satellite-derived wind products certainly have the potential to provide such information for real-time forecasting.
Chen, D. K., W. T. Liu, S. E. Zebiak, M. A. Cane, Y. Kushnir and D. Witter, 1999: Sensitivity of the tropical Pacific Ocean simulation to the temporal and spatial resolution of wind forcing. Journal of Geophysical Research-Oceans, 104(C5): 11261-11271.
The effects of temporal and spatial smoothing of wind forcing were evaluated in a model simulation of the tropical Pacific Ocean variability during the onset phase of the 1997/1998 El Niño. A total of 16 experiments were performed using the NASA scatterometer wind data smoothed at time intervals from 1 to 30 days and on spatial scales from 1 degrees to 10 degrees. A major effect of the temporal smoothing of winds is to warm sea surface temperature (SST) by reducing the energy input for vertical turbulent mixing. When the daily wind forcing was replaced by the monthly average, the mean SST increased by 0.5 degrees to 1 degrees over most of the tropical Pacific. The spatial smoothing of winds is not as effective as the temporal smoothing in causing SST warming, but it has a more severe influence on dynamical ocean response for smoothing scales above 5 degrees. The onset of the 1997/1998 El Niño can be successfully simulated using the wind forcing averaged to monthly intervals and 2 degrees squares. For climate models the spatial smoothing of wind forcing on scales larger than the width of the equatorial waveguide is a more serious limitation than the temporal smoothing on scales up to 1 month.
Clement, A. C., R. Seager and M. A. Cane, 1999: Orbital controls on the El Niño/Southern Oscillation and the tropical climate. Paleoceanography, 14(4): 441-456.
The global synchroneity of glacial-interglacial events is one of the major problems in understanding the link between Milankovitch forcing and the climate of the late Quaternary. In this study we isolate a part of the climate system, the tropical Pacific, and test its sensitivity to changes in solar forcing associated with changes in the Earth's orbital parameters. We use a simplified coupled ocean-atmosphere model that is run for the past 150,000 years and forced with Milankovitch changes in the solar insolation. This system responds primarily to the presessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant. The mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the El Niño/Southern Oscillation (ENSO). Changes in the ENSO behavior result in a mean tropical climate change. The hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to Milankovitch forcing.
Clement, A. and R. Seager, 1999: Climate and the tropical oceans. Journal of Climate, 12(12): 3383-3401.
An attempt is made to determine the role of the ocean in establishing the mean tropical climate and its sensitivity to radiative perturbations. A simple two-box energy balance model is developed that includes ocean heat transports as an interactive component of the tropical climate system. It is found that changes in the zonal mean ocean heat transport can have a considerable affect on the mean tropical sea surface temperature (SST) through their effect on the properties of subtropical marine stratus clouds or on the water vapor greenhouse effect of the tropical atmosphere. The way that the tropical climate adjusts to changes in the ocean heat transport is primarily through the atmospheric heat transport, without changing the net top of the atmosphere radiative balance. Thus, the total amount of low-latitude poleward heat transport is invariant with respect to changes in ocean circulation in this model. These results are compared with analogous experiments with general circulation models.
Clement, A., M. A. Cane and R. Seager, 1999: Patterns and mechanisms of twentieth century climate change. World Resources Rev., 10: 161-185.
The problem of assessing the impacts of industrialization on climate hinges on the ability of scientists to define what kind of changes are outside the range of the natural variability of the climate system, and to identify the mechanisms that could lead to such changes. In this report, we present a new analysis of historical sea surface temperature (SST) changes over the past century, and point out that amidst a mean global warming, there have been significant regions of cooling. A mean warming of the climate has been anticipated based on the radiative effects of increasing greenhouse gases in the atmosphere. Climate models that have been used to simulate this effect, however, produce a larger warming than observed over the 20th century. In addition, the spatial pattern of temperature change produced in these increased CO sub(2) simulations do not bear any particular resemblance to the pattern of warming and cooling in the observations. These discrepancies suggest that there are other mechanisms in the climate system that have been operating in the 20th century that are not represented in the climate models. Increased levels of sulfate aerosols have been proposed as a missing forcing that could offset the warming due to increased greenhouse gases. However, our understanding of the radiative effects of these particles is insufficient to explain all of the discrepancy between the model results and the observations. In this report we propose a two-part mechanism whereby, initially, tropical ocean dynamics act to minimize the surface temperature change in response to a uniform heating of the tropical oceans. The consequence is a change in the distribution of SSTs in the tropical Pacific which appears to be consistent with the observations of SST change over the 20th century. This redistribution of SSTs then alters the atmospheric circulation in much the same way as the well-documented effects of the El Niño/Southern Oscillation. The change in atmospheric circulation results in regions of both warming and cooling over the much of the globe, some of which are consistent with the observed changes. The first step is illustrated in a simple coupled ocean-atmosphere model. The second step is investigated with an atmospheric general circulation model forced by imposed tropical Pacific SST anomalies. This mechanism does not seem to be operative in the state of the art coupled ocean-atmosphere models used in climate change studies, and it proposed here as a possible source of the discrepancy between the simulated and observed changes in climate over the 20th century.
Curchitser, E. N., D. B. Haidvogel and M. Iskandarani, 1999: On the transient adjustment of a mid-latitude abyssal ocean basin with realistic geometry: the constant depth limit. Dynamics of Atmospheres and Oceans, 29(2-4): 147-188.
The early stages in the adjustment of a mid-latitude abyssal basin with realistic geometry are studied using an inverted one and one-half 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 mass sink. A flat bottom basin is investigated for comparison with existing theories on abyssal gyral spin-up. and as a precursor to a study with realistic topography. As in existing theories, the early adjustment is dominated by sub-inertial Kelvin and Rossby waves. Obstacles and the varying coastal geometry do not impede the passage of the Kelvin wave, though the circuit time of the main Kelvin wave signal is reduced by an aggregate 6% for the abyssal Eastern Mediterranean basin. The scattering of the Kelvin wave due to small-scale variations in the coastline is also shown not to be significant to the adjustment. The relatively short period of time needed to reach a statistical steady state is attributed to western boundary current formation in response to local Kelvin wave dynamics. Upon cessation of the sidewall forcing, sub-inertial motion controls the spin-down adjustment with basin-scale Rossby waves becoming the most pronounced feature of the flow. Two dynamical issues of particular interest emerge in these simulations: the retardation of Kelvin wave propagation around the abyssal basin and the roles of detrainment and sidewall forcing in the interior vorticity balance. An idealized simulation using an elliptical basin is used to illustrate that the mechanism for Kelvin wave retardation is a geometrically induced dispersion due to large-scale variations in the coastline. A dynamical analysis of the interior circulation shows that detrainment alone does not develop a Sverdrup response. Both the localized sidewall injection and the detrainment are needed to describe the interior dynamics, with both poleward and equatorward flows developing during the adjustment. (C) 1999 Elsevier Science B.V. All rights reserved.
Dery, S. J. and M. K. Yau, 1999: A bulk blowing snow model. Boundary-Layer Meteorology, 93(2): 237-251.
We present in this paper a simple and computationally efficient numerical model that depicts a column of sublimating, blowing snow. This bulk model predicts the mixing ratio of suspended snow by solving an equation that considers the diffusion, settling and sublimation of blowing snow in a time-dependent mode. The bulk model results compare very well with those of a previous spectral version of the model, while increasing its computational efficiency by a factor of about one hundred. This will allow the use of the model to estimate the effects of blowing snow upon the atmospheric boundary layer and to the mass balance of such regions as the Mackenzie River Basin of Canada.
Dery, S. J. and M. K. Yau, 1999: A climatology of adverse winter-type weather events. Journal of Geophysical Research-Atmospheres, 104(D14): 16657-16672.
Using the European Centre for Medium-Range Weather Forecasts Re-Analysis gridded data, a global climatology of blowing snow, blizzard, and high-windchill events is conducted for the period 1979-1993. The results show that these phenomena, occur primarily over flat;, open surfaces with long seasonal or perennial snow covers such as the Greenland and Antarctic ice fields as well as the Arctic tundra. On a regional scale, emphasis is given to the Mackenzie River Basin (MRB) of Canada, where fewer events take place within the boreal forest as opposed to the Arctic tundra. Interannual and monthly variabilities in the number of events are also evident and are due primarily to 10-m wind speed anomalies at high latitudes for blowing snow and blizzard events, while high-windchill events are more sensitive to air temperatures near the surface. We also find that high-windchill episodes are the more frequent events, since they occur at 9.3% of all possible grid points and times on a yearly basis, while blowing snow at 6.5% and blizzards at 1.4% are less common events. Compositing of principal meteorological fields show that anticyclones and lee cyclones are prominent features associated with blowing snow events in some sections of the MRB.
Gordon, A. L. and J. L. McClean, 1999: Thermohaline stratification of the Indonesian Seas: Model and observations. Journal of Physical Oceanography, 29(2): 198-216.
The Indonesian Throughflow, weaving through complex topography. drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 drg ton average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 3 degrees C warmer than the observed temperature. Within the 5 degrees-28 degrees C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile. is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found dawn to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flares Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Scram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timer. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timer Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv = 10(6) m(3) s(-1)) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However. during the prolonged EI Niño of the early 1990s the throughflow is suspected to be lower than average and, indeed, the model transports for the non-El Niño months of August 1988 and February 1989 are larger It is likely that aspects of the model bathymetry, particularly that of the Torres Strait, which is too open to the South Pacific, and the Makassar Strait, which is too restrictive, may be the cause of the discrepancies between observations and model.
Gordon, A. L., R. D. Susanto and A. Ffield, 1999: Throughflow within Makassar Strait. Geophysical Research Letters, 26(21): 3325-3328.
From late November 1996 to early July 1998 velocity measurements were made at two moorings within a constriction in Makassar Strait near 3 degrees S. The 1997 average throughflow is 9.3 Sv, with an uncertainty off about +/- 2.5 Sv depending on how the surface flow is taken into account. The results show that throughflow within Makassar Strait can account for all of the Pacific to Indian interocean transport. The correlation of transport to ENSO may be as high as 0.73, though the time series is too short to say this with assurance. Most of the remaining variance is explained by the annual cycle, with a June maximum and December minimum. A strong intra-seasonal event occurs from late May to July, 1997.
Hines, K. M., R. W. Grumbine, D. H. Bromwich and R. I. Cullather, 1999: Surface energy balance of the NCEP MRF and NCEP-NCAR reanalysis in Antarctic latitudes during FROST. Weather and Forecasting, 14(6): 851-866.
The surface energy budget in Antarctic latitudes is evaluated for the medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction (NCEP) and for the NCEP-National Center for Atmospheric Research reanalysis project during the winter, spring. and summer special observing periods (SOPs) of the Antarctic First Regional Observing Study of Troposphere project. A significant change in the energy balance resulted from an extensive model update beginning with the forecasts initialized on 11 January 1995 during the summer SOP. Both the forecasts and the reanalysis include significant errors in the surface energy balance over Antarctica. The errors often tend to cancel and thus produce reasonable surface temperature fields. General errors include downward longwave radiation about 30-50 W m(-2) too small. Lower than observed cloudiness contributes to this error and to excessive downward shortwave radiation at the surface. The model albedo over Antarctica, about 75%, is lower than that derived from observations, about 81%. During the polar day, errors in net longwave and net shortwave radiation tend to cancel. The energy balance over Antarctica in the reanalysis is, in general, degraded from that of the forecasts.
Huang, H. P., 1999: Scale-dependent properties of optimal perturbations on a zonally varying barotropic flow. Journal of the Atmospheric Sciences, 56(9): 1238-1247.
The scale-dependent characteristics of the optimal perturbations in a zonally asymmetric barotropic model are examined. The dependence of the optimal energy growth on the initial scale is investigated through the calculations of spectrally constrained optimal perturbations. Considering an optimization time of tau = 3 days and a basic state containing an idealized Asian jet, the optimal amplification factor generally increases with the decrease of the imposed initial scale. In the absence of diffusion, the most amplifying scale becomes the smallest scale in the model. An energetics analysis shows that the energy conversion in the optimal excitation process is dominated by the shear straining term, with a sharp increase in the scale of the perturbation accompanying the explosive energy growth. These results show the similarity between the optimally growing process in the zonally asymmetric system and the shear straining process in a parallel shear flow. Except when a small tau is considered or a sufficiently strong diffusion is used in the system, the optimal energy growth for small-scale disturbances sensitively depends on the zonally varying feature of the basic state. With tau = 3 days, the optimal amplification factors for small-scale disturbances are reduced significantly when the idealized Asian jet is shortened by only one-fifth. At the same time, those for medium- and large-scale disturbances are almost unaffected by the change of the basic state. The reasons for this contrast of the sensitivity property between the small and large scales are discussed.
Huang, H. P., P. D. Sardeshmukh and K. M. Weickmann, 1999: The balance of global angular momentum in a long-term atmospheric data set. Journal of Geophysical Research-Atmospheres, 104(D2): 2031-2040.
The balance of global atmospheric angular momentum is examined in a long time series of "reanalysis" data generated at the National Centers for Environmental Prediction (NCEP). A systematic negative bias of about -10 Hadleys (10(18) kg m(2) s(-2)) is obtained in the total torque in all seasons. The sum of the frictional (T-F) and mountain (T-M) torques contributes about half of this negative bias in the northern summer and autumn. The torque due to the parameterized gravity wave drag, T-G, contributes the other half of the negative bias in summer but contributes the whole of it in winter. In the annual mean the projected angular momentum imbalance due to T-G exceeds that due to T-M + T-F. Consistent with the budget analysis of the assimilated data, losses of angular momentum are also found in an extensive set of medium-range forecasts made with the NCEP reanalysis model. The average rates of loss of relative angular momentum in these forecasts, -7 Hadleys in January and -6 in July, are comparable to the bias of the total torque in the angular momentum budget. Further investigations are suggested to determine the model error, especially that associated with the parameterized gravity wave drag, related to the budget imbalance found in this study.
Khatiwala, S. P., R. G. Fairbanks and R. W. Houghton, 1999: Freshwater sources to the coastal ocean off northeastern North America: Evidence from (H2O)-O-18/(H2O)-O-16. Journal of Geophysical Research-Oceans, 104(C8): 18241-18255.
Oxygen isotope ((H2O)-O-18/(H2O)-O-16)/salinity data identify freshwater sources along the northeastern North American continental margin. The oxygen isotope (delta(18)O)-salinity (S) properties of various water types are distinguished. Sea ice formation on the Labrador Shelf is shown to influence delta(18)O-S values. It is estimated that 2-3 m of freshwater is extracted from the water column to form sea ice. It is hypothesized that waters on the Scotian Shelf, Gulf of Maine, and the Middle Atlantic Eight are composed of slope water diluted by an upstream low-salinity source. This upstream source is a mixture of brine-enriched Labrador Shelf Water and St. Lawrence River water. The delta(18)O value of the apparent freshwater component (delta(18)O(S=0)) of waters on the Scotian Shelf and farther south is approximate to-20%, which has been used to suggest a sole high-latitude freshwater source. We show instead that the St. Lawrence River contributes approximate to 35% of the freshwater on the Scotian Shelf, in agreement with the physical oceanographic evidence. The remaining freshwater is supplied by high-latitude rivers dominated by Arctic runoff. Finally, the isotope evidence identifies Baffin Bay as an important pathway by which freshwater from the Arctic Ocean can reach the Labrador Sea.
Kumar, K. K., B. Rajagopalan and M. A. Cane, 1999: On the weakening relationship between the Indian monsoon and ENSO. Science, 284(5423): 2156-2159.
Analysis of the 140-year historical record suggests that the inverse relationship between the El Niño-Southern Oscillation (ENSO) and the Indian summer monsoon (weak monsoon arising from warm ENSO event) has broken down in recent decades. Two possible reasons emerge from the analyses. A southeastward shift in the Walker circulation anomalies associated with ENSO events may Lead to a reduced subsidence over the Indian region, thus favoring normal monsoon conditions. Additionally, increased surface temperatures over Eurasia in winter and spring, which area parr. of the midlatitude continental warming trend, may favor the enhanced land-ocean thermal gradient conducive to a strong monsoon. These observations raise the possibility that the Eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal Level despite strong ENSO events.
Kumar, K. K., R. Kleeman, M. A. Cane and B. Rajagopalan, 1999: Epochal changes in Indian monsoon-ENSO precursors. Geophysical Research Letters, 26(1): 75-78.
Precursors for the Indian monsoon are known to be highly epoch specific in their skills in predicting the monsoon on seasonal time scales. We show that the various precursors are correlated with the monsoon, only when they are correlated with ENSO, which happened in the recent period 1951 - 1990, but not in the 1990's and the period 1911-50. This accounts for the skill in monsoon prediction during 1951-90. We find that ENSO and its precursors tend toward higher amplitude and 3 - 5 year periods in the 1951 - 1990 epoch, and toward decreased amplitude and 5 - 7 year periods in the 1911 - 1950 epoch. We argue that the shift to lower frequency and amplitude in the earlier epoch diminished the association between the monsoon precursors and ENSO, leading to diminished skill in predicting the, monsoon in that epoch. However, the simultaneous relationship between the monsoon and ENSO has been stable over the past 140 years, suggesting that the monsoon - ENSO teleconnections are robust once ENSO is established. Changes in the frequency of ENSO have implications for statistical prediction schemes for ENSO and the monsoon.
Levy, M., M. Visbeck and N. H. Naik(Henderson), 1999: Sensitivity of primary production to different eddy parameterizations: A case study of the spring bloom development in the northwestern Mediterranean Sea. Journal of Marine Research, 57(3): 427-448.
The abilities of the Gent and McWilliams (1990) (GM) and Horizontal Diffusion (HD) eddy-parameterizations to represent the mesoscale effects relevant for primary production are compared and analyzed. Following Levy et al. (1999a), this is done in the case study of the spring bloom that follows the formation of a dense water patch in the northwestern Mediterranean Sea. It is shown that, unlike HD, the use of the GM parameterization can capture many aspects of the primary production enhancement associated with the restratifying action of mesoscale eddies. However, predicted primary production, when using the GM parameterization, is sensitive to the GM's parameter set, and particularly to the maximum value of the lateral mixing coefficient, k(max).
Lilly, J. M., P. B. Rhines, M. Visbeck, R. Davis, J. Lazier, F. Schott and D. Farmer, 1999: Observing deep convection in the Labrador sea during winter 1994/95. Journal of Physical Oceanography, 29(8): 2065-2098.
A 12-month mooring record (May 1994-June 1995), together with accompanying PALACE float data, is used to describe an annual cycle of deep convection and restratification in the Labrador Sea. The mooring is located at 56.75 degrees N, 52.5 degrees W, near the former site of Ocean Weather Station Brave, in water of similar to 3500 m depth. This is a pilot experiment for climate monitoring, and also for studies of deep-convection dynamics. Mooring measurements include temperature (T), salinity (S), horizontal and vertical velocity, and acoustic measurement. of surface winds. The floats made weekly temperature-salinity profiles between their drift level (near 1500 m) and the surface.
Ou, H. W., 1999: A model of tidal rectification by potential vorticity mixing. Part I: Homogeneous ocean. Journal of Physical Oceanography, 29(4): 821-827.
In previous studies of tidal generation of mean flow over varying topography, the rectification mechanism has generally invoked bottom friction as a source of tidal flux of momentum and vorticity (hence referred as "friction" mechanism). The author proposes a different mechanism based on horizontal mixing of potential vorticity. Drawing analogy from tidal dispersion of passive tracers, this mixing is parameterized through a diffusivity (hence called "diffusivity" mechanism) that is quadratic in the tidal amplitude. In this, Part 1, the mean along-isobath flow near a shelf break is determined for a homogeneous ocean and contrasted with that induced by friction mechanism. In Part 2, the effect of a front will be considered.
Rhein, M., U. Send, B. Klein and G. Krahmann, 1999: Interbasin deep water exchange in the western Mediterranean. Journal of Geophysical Research-Oceans, 104(C10): 23495-23508.
Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins . There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.
Rodgers, K. B., M. A. Cane, N. H. Naik(Henderson) and D. P. Schrag, 1999: The role of the Indonesian Throughflow in equatorial Pacific thermocline ventilation. Journal of Geophysical Research-Oceans, 104(C9): 20551-20570.
The role of the Indonesian Throughflow (ITF) in the thermocline circulation of the low-latitude Pacific Ocean is explored using a high-resolution primitive equation ocean circulation model. Seasonally forced runs for a domain with an open Indonesian passage are compared with seasonally forced runs for a closed Pacific domain. Three cases are considered: one with no throughflow, one with 10 Sv of imposed ITF transport, and one with 20 Sv of ITF transport. Two idealized tracers, one that tags northern component subtropical water and another that tags southern component subtropical water, are used to diagnose the mixing ratio of northern and southern component waters in the equatorial thermocline. It is found that the mixing ratio of north/south component waters in the equatorial thermocline is highly sensitive to whether the model accounts for an ITF. Without an ITF, the source of equatorial undercurrent water is primarily of North Pacific origin, with the ratio of northern to southern component water being approximately 2.75 to 1. The ratio of northern to southern component water in the Equatorial Undercurrent with 10 Sv of ITF is approximately 1.4 to 1, and the ratio with 20 Sv of imposed ITF is 1 to 1.25. Estimates from data suggest a mean mixing ratio of northern to southern component water of less than 1 to 1. Assuming that the mixing ratio changes approximately linearly as the ITF transport varies between 10 and 20 Sv, an approximate balance between northern and southern component water is reached when the ITF transport is approximately 16 Sv. It is also shown that for the isopycnal surfaces within the core of the equatorial undercurrent, a 2 degrees C temperature front exists across the equator in the western equatorial Pacific, beneath the warm pool. The implications of the model results and the temperature data for the heat budget of the equatorial Pacific are considered.
Schlosser, P., R. Bayer, G. Bonisch, L. W. Cooper, B. Ekwurzel, W. J. Jenkins, S. Khatiwala, S. Pfirman and W. M. Smethie, 1999: Pathways and mean residence times of dissolved pollutants in the ocean derived from transient tracers and stable isotopes. Science of the Total Environment, 238: 15-30.
During the past decades, a variety of transient tracers have been used to derive information on pathways and mean residence times of oceanic water masses. Here, we discuss how information obtained in such studies can be applied to studying the spreading of dissolved pollutants in the ocean. The discussion focuses on the transient tracers tritium/He-3 and the (H2O)-O-18/(H2O)-O-16 ratio of water. These tracers are used in combination with CFCs and C-14 in a case study of Arctic Ocean contaminant transport to: (1) separate the freshwater components contained in the near-surface waters; (2) infer mean pathways of freshwater and associated contaminants from the (H2O)-O-18/(H2O)-O-16 distribution in the surface waters; and (3) determine mean residence times of the surface, intermediate, deep and bottom waters. (C) 1999 Elsevier Science B.V. All rights reserved.
Schmidt, D. S., R. A. Schmidt and J. D. Dent, 1999: Electrostatic force in blowing snow. Boundary-Layer Meteorology, 93(1): 29-45.
Separation of electrostatic charge during the transport of particles by wind adds a force to the gravitational and fluid forces that determine trajectories of particles moving by saltation. Evaluating this electrostatic force requires the electric field strength very near the saltation surface, and charge-to-mass ratios for the moving particles. Field mill readings 4 cm above the surface in a moderate blizzard showed electric field strength as high as +30 kV m(-1). Another experiment gave charge-to-mass ratios of individual saltation particles in low-level drifting that ranged between +72 mu C kg(-1) and 208 mu C kg(-1). From these measurements, we estimated electrostatic forces as large as the gravitational force on some saltating particles. Including forces of this magnitude in the equations of motion significantly alters predicted saltation trajectories from those for uncharged particles. Predictions appear reasonable that for some saltating particles, the electrostatic force prevents new surface impacts. These results should help improve models of energy transfer in the planetary boundary layer during blizzards and sandstorms.
Send, U., J. Font, G. Krahmann, C. Millot, M. Rhein and J. Tintore, 1999: Recent advances in observing the physical oceanography of the western Mediterranean Sea. Progress in Oceanography, 44(1-3): 37-64.
The Mediterranean Sea has been in investigated intensively since the early nineties, using modern techniques and collaborative approaches, This overview summarizes some of the resulting advances that were made concerning the physical oceanography of the western Mediterranean. The water mass formation processes are now much better understood and have been quantified to a large extent. The boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy, thus enabling future investigation of interannual variability, The dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses. The deep circulation is being investigated with Lagrangian techniques (tracers acid floats). First results show a large component of the deep water originating from the Tyrrhenian Sea and intense cyclonic and anticyclonic eddy flows, (C) 1999 Elsevier Science Ltd. All rights reserved.
Stieglitz, M., J. Hobbie, A. Giblin and G. Kling, 1999: Hydrologic modeling of an arctic tundra watershed: Toward Pan-Arctic predictions. Journal of Geophysical Research-Atmospheres, 104(D22): 27507-27518.
A simple land surface model is used to explore the dynamics of the hydrologic cycle operating in arctic tundra regions. The model accounts for the topographic control of surface hydrology, ground thermal processes, and snow physics. The approach described relies only on the statistics of the topography rather than the details of the topography and is therefore computationally inexpensive and compatible with the large spatial scales of today's climate models. As such, the model can easily be applied on an arctic-wide basis to explore issues ranging from the delivery of seasonal melt water to the Arctic Ocean to impacts of climate change on the hydrologic cycle.
Tourre, Y. M., B. Rajagopalan and Y. Kushnir, 1999: Dominant patterns of climate variability in the Atlantic Ocean during the last 136 years. Journal of Climate, 12(8): 2285-2299.
Dominant spatiotemporal patterns of joint sea surface temperature (SST) and sea level pressure (SLP) variability in the Atlantic Ocean are identified using a multivariate frequency domain analysis. Five significant frequency bands are isolated ranging from the quasi biennial to the quasi decadal. Two quasi-biennial bands are centered around 2.2- and 2.7-yr periods: two interannual bands are centered around 3.5- and 4.4-yr periods; the fifth band at the quasi-decadal frequency is centered around 11.4-yr period. Between 1920 and 1955, the quasi-decadal band is less prominent compared to the quasi-biennial bands. This happens to be the period when SLP gradually increased over the Greenland-Iceland regions. The spatial pattern at the quasi-decadal frequency displays an out-of-phase relationship in the SLP in the vicinity of the subtropical anticyclones in both hemispheres (indicative of an out-of-phase quasi-decadal variability in the North and South Atlantic Hadley circulation). The quasi-decadal frequency also displays an out-of-phase relationship in the SSTs north and south of the mean position of the intertropical convergence zone (ITCZ). This short-lived structure, lasting for approximately two years, supports the argument that a tropical SST dipole pattern is one of the characteristics of the quasi-decadal signal. All five frequency bands represent to some extent fluctuations of the North Atlantic oscillation and are associated with tropical Atlantic Ocean warming (cooling) with different spatial evolution. The two interannual bands show opposite SST evolution to the south of the ITCZ, that is, southeastward evolution from the western tropical Atlantic for the 3.5-yr period and westward spreading from the eastern tropical Atlantic for the 4.4-yr period. Moreover, a significant coherence (with a 1-yr phase lag) is found between the SST time series along the equatorial Atlantic obtained from the 3.5-yr period, and the SST time series in the NINO3 area in the Pacific. It is cautiously argued that the 3.5-yr period is largely associated with the global El Nino-Southern Oscillation phenomenon. while the evolution of the 4.4-yr period depends more upon Atlantic local conditions.
Tourre, Y. M., Y. Kushnir and W. B. White, 1999: Evolution of interdecadal variability in sea level pressure, sea surface temperature, and upper ocean temperature over the Pacific Ocean. Journal of Physical Oceanography, 29(7): 1528-1541.
Interdecadal variability in sea level pressure (SLP) and sea surface temperature (SST) anomalies in the Pacific Ocean was "quasiperiodic" from 1900-91. The coherent variability of this phenomenon is investigated using gridded observational data from the turn of the century (SST and SLP) and of upper ocean heat content (HS) from the recent two and a half decades. The nominal cycle in atmosphere-ocean variables is roughly two decades long, but growth and decay can happen on a shorter timescale (e.g., half a cycle or so). The authors divide the full cycle into four phases: An onset phase, during which a weak SLP anomaly pattern off Japan takes approximately 2-4 yr to expand eastward, leads to large SLP anomalies in the region of the Aleutian low. A quasistationary growth phase, with the midlatitude SLP anomaly pattern in the eastern ocean, intensifies over a 2-4 yr period. The persistent SLP anomalies evolve in concert with large SST (and HS anomalies) of the same polarity located to the south-southwest along the subarctic frontal zone (SAFZ). During the growth phase, SST anomalies with opposite polarity develop to the east, associated with anomalous atmospheric circulation along the North American coastline. Near the end of the growth phase a narrow tongue of enhanced SST anomalies is found along the subtropical front near Hawaii, slightly to the west of the subduction region associated with the subtropical gyre. Following is a decay phase during which the midocean SST and SLP anomalies weaken, while the HS anomalies persist in the southern part of the subtropical gyre. Concomitantly, a weak anomalous east-west SLP gradient is established in the tropical Pacific and could contribute to the interdecadal variability of the southern oscillation index. Finally, a return phase occurs (identical to the onset phase but with opposite polarity), during which SST anomalies move from the Alaskan gyre and from the center of the subtropical gyre to merge onto the SAFZ. During the evolution of the interdecadal phenomenon, the overall structure of IIS highlights the notion that the subtropical gyre integrates the thermal and dynamical forcing induced by the persistent surface circulation anomaly. Since only the oceanic anomalies retain their character throughout the seasonal cycle, the interdecadal phenomenon may owe its existence to coupled ocean-atmosphere interaction in which ocean surface anomalies feed back on the atmosphere providing the necessary links between consecutive winter seasons. It is suggested that this can occur through interactions in either midlatitudes (delayed-negative feedback) and/or the Tropics.
Toyota, T., J. Ukita, K. I. Ohshima, M. Wakatsuchi and K. Muramoto, 1999: A measurement of sea ice albedo over the southwestern Okhotsk Sea. Journal of the Meteorological Society of Japan, 77(1): 117-133.
In order to estimate sea ice albedo around the marginal sea ice zone of the southwestern Okhotsk Sea, we conducted the measurement of albedo aboard the ice breaker Soya in early February of 1996 and 1997. Using upward and downward looking pyranometers mounted at the bow of the ship, we obtained albedo data. We also measured ice concentration and thickness quantitatively by a video analysis. The observations show a good correlation between albedo and ice concentration. From a linear regression, sea ice albedo (ice concentration = 100 %) is estimated to be 0.64 +/- 0.03 at the 95 % confidence level. The developed snow grains on sea ice due to sea water and/or solar radiation may be responsible for this somewhat lower value, compared with that over the snow-covered land fast ice in the polar region. Deviations of the observed values from this regression have a statistically significant correlation with solar zenith cosine at the 99 % level, and with ice thickness at the 95 % level. The linear regression formula which predicts albedo is also derived as the variables of ice concentration and solar zenith cosine. Although the regression coefficients are both statistically significant, the coefficient of ice concentration is much more significant in this formula than that of solar zenith cosine. The deviation of the observed albedo from this regression seems to be mainly caused by ice surface conditions rather than by ice thickness or cloud amount. All these results suggest that snow cover on sea ice plays an important role in determining the surface albedo.
Ukita, J. and M. Ikeda, 1999: The frontier research system for global change - The International Arctic Research Center (Frontier-IARC): Its origin and tentative science plan. Marine Technology Society Journal, 33(1): 81-84.
The Frontier Research System for Global Change-the International Arctic Research Center (Frontier-IARC) is a research program funded by the Frontier Research System for Global Change. The program is jointly run under a cooperative agreement between the Frontier Research System for Global Change and the University of Alaska Fairbanks. The aim of the program is to understand the role of the Arctic region in global climate change. The program concentrates its research effort initially on the areas of air-sea-ice interactions, bio-geochemical processes and the ecosystem. To understand the arctic climate system in the context of global climate change, we focus on mechanisms controlling arctic-subarctic interactions, and identify three key components: the freshwater balance, the energy balance, and the large-scab atmospheric processes. Knowledge of details of these components and their interactions will be gained through Long-term monitoring, process studies, and modeling; our focus will be on the Latter two categories.
Wang, H. L. and M. F. Ting, 1999: Seasonal cycle of the climatological stationary waves in the NCEP-NCAR reanalysis. Journal of the Atmospheric Sciences, 56(22): 3892-3919.
The maintenance mechanisms of the climatological stationary waves and their seasonal cycle are investigated with a linear stationary wave model and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1985 to 1993, The stationary wave model is linearized about the zonal-mean flow and subjected to the zonally asymmetric stationary wave forcings. It has rhomboidal wavenumber 30 truncation and 14 vertical sigma levels. The forcings for the linear model include diabetic heating, orography, stationary nonlinearity, and transient vorticity and heat Aux convergences. The NCEP-NCAR reanalysis provides a high quality global dataset for this study.
Wang, H., M. F. Ting and M. Ji, 1999: Prediction of seasonal mean United States precipitation based on El Niño sea surface temperatures. Geophysical Research Letters, 26(9): 1341-1344.
A method for seasonal predictions of U.S. precipitation based on tropical Pacific sea surface temperature (SST) anomalies is developed in this letter with the singular value decomposition(SVD). This method was applied to the 1997/98 El Niño. The 1997 summer and 1997/98 winter precipitation over the United States were predicted using forecast SST from the National Centers for Environmental Prediction (NCEP) coupled ocean-atmosphere model. A cross validation, based on the percentages of hit between the hindcast and observed precipitation for the past El Niño and La Niña events, indicates a certain degree of predictability in both winter and summer seasons. During El Niño, significant predictability of summer precipitation is found over the Northern Plains and Atlantic States, while during La Niña it is in the Midwest. Above normal precipitation in the Northern Plains and the Midwest and below normal precipitation in the Atlantic States is associated with a warm phase of the El Niño-Southern Oscillation (ENSO) in summer. For winter precipitation, significant predictability is detected over the Gulf Coast States, the Southern Plains and California. Over these regions, wetter conditions are generally associated with the warm phase of ENSO.
Witter, D. L. and A. L. Gordon, 1999: Interannual variability of South Atlantic circulation from 4 years of TOPEX/POSEIDON satellite altimeter observations. Journal of Geophysical Research-Oceans, 104(C9): 20927-20948.
Variability of large-scale and regional South Atlantic circulation is investigated using TOPEX/POSEIDON sea level observations. Interannual variations are identified from empirical orthogonal functions of gridded sea level fields, year-to-year fluctuations of root-mean-square sea level variability, and variability of Agulhas eddies evaluated from the along-track data. Two modes of variability are identified. A basin-scale mode indicates that sea level in the eastern South Atlantic underwent a transition from a state of high sea level and enhanced gyre-scale geostrophic circulation in 1993 and 1994, to a state of lower sea level and more sluggish circulation in 1996. The dominant mode of basin-scale zonal wind has the same temporal signature, suggesting a link between the observed variation of gyre-scale circulation and the regional wind forcing. Time variations of this mode also coincide with a transition from a broad Agulhas eddy corridor observed in 1993 and 1994 to a narrower corridor observed in 1996. The input of salt and vorticity to the South Atlantic subtropical gyre via Agulhas eddies may therefore be partially controlled by interannual variations of the wind-forced, large-scale circulation. A second mode isolates interannual variations in the Brazil-Malvinas Confluence region. During 1993, eddy variability along the Brazil Current extension was relatively strong and variability along the continental slope was weak. The opposite pattern was observed in 1995. These variations may be related to interannual variations of the latitude of the confluence. While variations associated with both modes are smaller than those observed on seasonal timescales, these interannual variations contribute significantly to the total South Atlantic variability.
Yuan, X. J., D. G. Martinson and W. T. Liu, 1999: Effect of air-sea-ice interaction on winter 1996 Southern Ocean subpolar storm distribution. Journal of Geophysical Research-Atmospheres, 104(D2): 1991-2007.
Air-sea-ice interaction processes in the Southern Ocean are investigated utilizing space-observed surface winds, sea ice concentration, and sea surface temperature (SST) from September through December, 1996. The sea ice edge (SIE) shows three ice-extent maxima around the Antarctic during September and October when sea ice coverage is maximum. They are located in the central Indian Ocean, east of the Ross Sea, and in the eastern Weddell Gyre. During September and October, most of the strong and long lasting storms initiate northeast of the three sea ice maxima. Such spatial distributions of storms and sea ice reflect coupling processes of the air-sea-ice interaction. A relatively stable, wave number 3 atmospheric circulation pattern that is believed to be fixed by the land-ocean distribution prevails during the ice maximum season. The ice-extent maxima coincide with strong southerlies and divergent wind fields associated with this pattern, which suggests that the mean atmospheric circulation determines the ice distribution. The ice-extent maxima can enhance the regional meridional surface pressure gradient and therefore strengthen the westerly winds north of the ice edge. The decreasing ice extent east of the ice maxima creates a local zonal thermal gradient which enhances local southerlies. This positive feedback between the wave pattern in the mean atmospheric circulation and ice distribution partially causes the eastward propagation of the ice maxima and also provides a favorable condition for cyclogenesis northeast of the ice-extent maxima. The mechanism of the cyclogenesis is the baroclinic instability caused by the cold air blown from the ice pack to the warm open-ocean waters. Where the SST is warmest off the SIE and the southerlies are the strongest, the potential for cyclogenesis is most likely. This is consistent with the observations.
The database was updated today.