Blanchet, I., C. Frankignoul and M. A. Cane, 1997: A comparison of adaptive Kalman filters for a tropical Pacific Ocean model. Monthly Weather Review, 125(1): 40-58.
The Kalman filter is the optimal linear assimilation scheme only if tile first- and second-order statistics of the observational and system noise are correctly specified. If not, optimality can be reached in principle by using all adaptive filter that estimates both the stale vector and the system error statistics. In this study, the authors compare the ability of three adaptive assimilation schemes at estimating ail unbiased, stationary system noise. The adaptive algorithms at implemented in a reduced space linear model for thr tropical Pacific, Using a twin experiment approach, the algorithms are compared by assimilating sea level data at fixed locations mimicking the tropical Pacific tide gauges network. It is shown that the description of the system error covariance matrix requires too many parameters for the adaptive problem to be well posed. However, the adaptive procedures are efficient if the number of noise parameters is dramatically reduced and their performance is shown to be closed ttl optimal, that is, based on the true system noise covariance, The link between those procedures is elucidated, and the question of their applicability and respective computational cost is discussed.
Cane, M. A., A. C. Clement, A. Kaplan, Y. Kushnir, D. Pozdnyakov, R. Seager, S. E. Zebiak and R. Murtugudde, 1997: Twentieth-century sea surface temperature trends. Science, 275(5302): 957-960.
An analysis of historical sea surface temperatures provides evidence for global warming since 1900, in line with land-based analyses of global temperature trends, and also shows that over the same period, the eastern equatorial Pacific cooled and the zonal sea surface temperature gradient strengthened. Recent theoretical studies have predicted such a pattern as a response of the coupled ocean-atmosphere system to an exogenous heating of the tropical atmosphere. This pattern, however, is not reproduced by the complex ocean-atmosphere circulation models currently used to simulate the climatic response to increased greenhouse gases. Its presence is likely to lessen the mean 20th-century global temperature change in model simulations.
Chen, D. K., S. E. Zebiak, M. A. Cane and A. J. Busalacchi, 1997: Initialization and predictability of a coupled ENSO forecast model. Monthly Weather Review, 125(5): 773-788.
The skill of a coupled ocean-atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean-atmosphere coupling and the associated reduction of ''initialization shock'' and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were nor successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.
Cullather, R. I., D. H. Bromwich and R. W. Grumbine, 1997: Validation of operational numerical analyses in Antarctic latitudes. Journal of Geophysical Research-Atmospheres, 102(D12): 13761-13784.
Available rawinsonde, automatic weather station (AWS), ship, and synthesized long-term observations are used to evaluate the Antarctic numerical analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) from 1985 to 1994. Twice-daily variations in the ECMWF surface pressure analyses compare closely with AWS units of the U.S. Antarctic Program and ship observations. The NCEP analyses lover the same period show substantial improvement, particularly during the period 1985-1990. Surface air temperatures and winds do not agree so closely, which may result from analyses error, the localized nature of the fields, or a combination. Validation of the analyses standard pressure level fields using available rawinsonde data reveal a general long-term decrease in RMS errors with time for both analyses. RMS errors in NCEP 200 hPa geopotential heights of over 200 geopotential meters (gpm) for central plateau stations are evident only prior to May 1986. However, a significant upward trend from 1989 to 1993 in geopotential height RMS differences is apparent at several levels. The ECMWF analyses are generally found to be superior and offer a reasonable depiction of the broadscale atmospheric circulation; however, deficiencies in midtropospheric temperatures and lower tropospheric winds are evident. Comparisons of ship data from individual cruises of the S.A. Agulhas and the RN Nathaniel B. Palmer to the numerical analyses reveal substantial agreement for pressure and temperature variables. Observations from the Nathaniel B. Palmer in the Amundsen and Bellingshausen Seas were not available to the weather forecasting centers. Results presented here indicate that a large amount of the available data is being incorporated and that large deficiencies identified in previous studies are being addressed, although areas of concern remain. Deficiencies in comparisons to specific stations are common to both analyses, implying continued communications problems. In particular, grid values corresponding to individual stations including the now-closed Leningradskaya base and Mirnyy are found to be conspicuously deficient at the 200 hPa level for both analyses.
Cullather, R. I., Harshvardhan and K. A. Campana, 1997: Climatology of cloud and radiation fields in a numerical weather prediction model. Theoretical and Applied Climatology, 57(1-2): 11-33.
Satellite-derived datasets are used to verify the cloud cover and radiation field generated by a T62 (horizontal resolution) version of the operational global model at the National Meteorological Centre (NMC). An ensemble of five day forecasts for July 1985 is used, as well as 30 day climatological forecasts for July 1985, October 1985, January 1986, and April 1986.
Eshel, G. and N. H. Naik(Henderson), 1997: Climatological coastal jet collision, intermediate water formation, and the general circulation of the Red Sea. Journal of Physical Oceanography, 27(7): 1233-1257.
The authors present climatologies of a numerical model of the Red Sea, focusing on the dynamics of winter intermediate water formation. Northward flowing boundary currents are identified as the major dynamical elements. At the northern boundary, the eastern current follows the geometry, eventually turning back to the south. At similar to 26 degrees N and the western wall the two boundary currents collide. At the collision site, the denser eastern current subducts under the western boundary current. The subduction forces the western boundary current eastward into the interior. Convection communicates the surface fluxes to the downwelled plume and intermediate water forms. The estimated rate, 0.11 Sv (Sv = 10(6) m(3) s(-1)), agrees with previous estimates. The authors identify basin-scale sea-surface tilt to the north due to variable thermohaline forcings as the key dynamical variable. The resultant geostrophic eastward cross-channel flow interacts with the boundaries and creates upwelling and surface topography spatial patterns that drive the coastal jets. Upwelling-induced vortex stretching dominates the vorticity balance and governs the separation of the western boundary current from the western wall. The process ceases in the summer.
Gordon, A. L., S. B. Ma, D. B. Olson, P. Hacker, A. Ffield, L. D. Talley, D. Wilson and M. Baringer, 1997: Advection and diffusion of Indonesian throughflow water within the Indian Ocean South Equatorial Current. Geophysical Research Letters, 24(21): 2573-2576.
Warm, low salinity Pacific water weaves through the Indonesian Seas into the eastern boundary of the Indian Ocean. The Indonesian Throughflow Water (ITW) adds freshwater into the Indian Ocean as it spreads by the advection and diffusion within the Indian Ocean's South Equatorial Current (SEC). The low salinity throughflow trace, centered along 12 degrees S, stretches across the Indian Ocean, separating the monsoon dominated regime of the northern Indian Ocean from the more typical subtropical stratification to the south. ITW is well represented within the SEC thermocline, extending with concentrations above 80% of initial characteristics from the sea surface to 300-m within the eastern half of the Indian Ocean, with 60% concentration reaching well into the western Indian Ocean. The ITW transport within the SEC varies from 4 to 12 x 10(6) m(3)sec(-1), partly in response to variations of the injection rate at the eastern boundary and to the likelihood of a zonally elongated recirculation cell between the Equatorial Counter Current and the SEC within the Indian Ocean. Lateral mixing disperses the ITW plume meridionally with an effective isopycnal mixing coefficient of 1.1 to 1.6 x 10(4) m(2)sec(-1).
Houghton, R. W., 1997: Lagrangian flow at the foot of a shelfbreak front using a dye tracer injected into the bottom boundary layer. Geophysical Research Letters, 24(16): 2035-2038.
Convergent flow at the foot of the shelfbreak front in the Middle Atlantic Eight has been detected using a dye tracer, Rhodamine-WT, injected into the bottom boundary layer. The observations substantiate model simulations by Chapman and Lentz (1994) of convergent flow in the vicinity of the front which would be very difficult if not impossible to detect with conventional moored current measurements. By following the dispersal of the dye patch over a 4 day period Lagrangian velocities of the order of 0.015 mis with respect to the front were resolved even as the frontal boundary was displaced similar to 12 km onshore. The water-following properties of the dye tracer provides a useful technique for studying the small-scale circulation and mixing at the frontal boundary.
Jacobs, S. S. and J. C. Comiso, 1997: Climate variability in the Amundsen and Bellingshausen Seas. Journal of Climate, 10(4): 697-709.
Satellite data reveal a 20% decline in sea ice extent in the Amundsen and Bellingshausen Seas in the two decades following 1973. This change is negatively correlated with surface air temperatures on the west side of the Antarctic Peninsula, which have increased similar to 0.5 degrees C decade(-1) since the mid-1940s. The recession was strongest during summer, when monthly average minima in 1991-92 removed much of the incipient multiyear ice over the continental shelf. This would have lowered the regional-mean ice thickness, impacting snow ice formation, brine production, and vertical heat flux. The northern ice edge contracted by similar to 1 degrees of latitude in all seasons from 1973-79 to 1987-93, returning toward mean conditions in 1993-95. The decline included multiyear cycles of several years in length, superimposed on high interannual variability. A review of atmospheric forcing shows winds consistent with mean and extreme ice extents, and suggests links to larger-scale circulation changes in the South Pacific. Historical ocean measurements are sparse in this sector, but mixed-layer depths and upper pycnoclines beneath the sea ice resemble those in the Weddell Sea. Weaker surface currents or changes in the upwelling of Circumpolar Deep water on the continental shelf could have contributed to the anomaly persistence.
Jessup, A. T., C. J. Zappa and H. Yeh, 1997: Defining and quantifying microscale wave breaking with infrared imagery. Journal of Geophysical Research-Oceans, 102(C10): 23145-23153.
Breaking without air entrainment of very short wind-forced waves, or microscale wave breaking, is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air-sea interface. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. In this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air-water interface, producing signatures that can be detected with infrared imagery. Simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process. The infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage, which is substantial under the moderate wind speed conditions investigated. The results imply that "bursting" phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts. Oceanic measurements show the ability to quantify microscale wave breaking in the field. Our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air-sea heat and gas fluxes, as well as unprecedented details on the origin and evolution of microscale wave breaking.
Jessup, A. T., C. J. Zappa, M. R. Loewen and V. Hesany, 1997: Infrared remote sensing of breaking waves. Nature, 385(6611): 52-55.
ENERGY dissipation due to deep-water wave breaking plays a critical role in the development and evolution of the ocean surface wave field. Furthermore, the energy lost by the wave held via the breaking process is a source for turbulent mixing and air entrainment, which enhance ah-sea heat and gas transfer(1-3). But the current lack of reliable methods for measuring energy dissipation associated with wave breaking inhibits the quantitative study of processes occurring at ocean surfaces, and represents a major impediment to the improvement of global wave-prediction models(4). Here we present a method for remotely quantifying wave-breaking dynamics which uses an infrared imager to measure the temperature changes associated with the disruption and recovery of the surface thermal boundary layer (skin layer). Although our present results focus on quantifying energy dissipation-in particular, we show that the recovery rate of the skin layer in the wakes of breaking waves is correlated with the energy dissipation rate-future applications of this technique should help to elucidate the nature of important small-scale surface processes contributing to air-sea heat(5) and gas(6) flux, and lead to a fuller understanding of general of ocean-atmosphere interactions.
Kaplan, A., Y. Kushnir, M. A. Cane and M. B. Blumenthal, 1997: Reduced space optimal analysis for historical data sets: 136 years of Atlantic sea surface temperatures. Journal of Geophysical Research-Oceans, 102(C13): 27835-27860.
A computationally efficient method for analyzing meteorological and oceanographic historical data sets has been developed. The method combines data reduction and least squares optimal estimation. The data reduction involves computing empirical orthogonal functions (EOFs) of the data based on their recent, high-quality portion and using a leading; EOF subset as a basis for the analyzed solution and for fitting a first-order linear model of time transitions. We then formulate optimal estimation problems in terms of the EOF projection of the analyzed field to obtain reduced space analogues of the optimal smoother, the Kalman filter, and optimal interpolation techniques. All reduced space algorithms are far cheaper computationally than their full grid prototypes, while their solutions are not necessarily inferior since the sparsity and error in available data often make estimation of small-scale features meaningless. Where covariance patterns can be estimated from the available data, the analysis methods fill gaps, correct sampling errors, and produce spatially and temporally coherent analyzed data sets. ils with classical least squares estimation, the reduced space versions also provide theoretical error estimates for analyzed values. The methods are demonstrated on Atlantic monthly sea surface temperature (SST) anomalies for 1856-1991 from the United Kingdom Meteorological Office historical sea surface temperature data set (version MOHSST5). Choice of a reduced space dimension of 30 is shown to be adequate. The analyses are tested by withholding a significant part of the data and prove to be robust and in agreement with their own error estimates; they are also consistent with a partially independent optimal interpolation (OI) analysis by Reynolds and Smith  produced in the National Centers for Environmental Prediction (NCEP) (known as the NCEP OI analysis). A simple statistical model is used to depict the month-to-month SST evolution in the optimal smoother algorithm. Results are somewhat superior to both the Kalman filter, which relies less on the model, and the optimal interpolation, which does not use it at all. The method generalizes a few recent works on using a reduced space for data set analyses. Difficulties of methods which simply fit EOF patterns to observed data are pointed out, and the more complete analysis procedures developed here are suggested as a remedy.
Krupitsky, A. and M. A. Cane, 1997: A two-layer wind-driven ocean model in a multiply connected domain with bottom topography. Journal of Physical Oceanography, 27(11): 2395-2404.
The behavior of the solution to a two-layer wind-driven model in a multiply connected domain with bottom topography imitating the Southern Ocean is described. The abyssal layer of the model is forced by interfacial friction, crudely simulating the effect of eddies. The analysis of the low friction regime is based on the method of characteristics. It is found that characteristics in the upper layer are closed around Antarctica, while those in the lower layer are blocked by solid boundaries. The momentum input from wind in the upper layer is balanced by lateral and interfacial friction and by interfacial pressure drag. In the lower layer the momentum input from interfacial friction and interfacial pressure drag is balanced by topographic pressure drag. Thus, the total momentum input by the wind is balanced by upper-layer lateral friction and by topographic pressure drag.
Kushnir, Y., V. J. Cardone, J. G. Greenwood and M. A. Cane, 1997: The recent increase in North Atlantic wave heights. Journal of Climate, 10(8): 2107-2113.
The nature and causes of the recent increase in North Atlantic wave heights are explored by combining a numerical hindcast with a statistical analysis. The numerical hindcast incorporates a IO-yr history (1980-89) of North Atlantic, twice daily wind analyses to generate a monthly averaged significant wave height (SWH) history. The hindcast compares favorably with published monthly averaged SWH observations. The link between model-generated wintertime monthly SWH and monthly averaged sea lever pressure (SLP) data is determined by means of a canonical correlation analysis (CCA). Within the analysis domain, most of the variance in SWH and SLP is captured by two pairs of joint patterns. The leading pair consists of a SLP dipole resembling the North Atlantic Oscillation (NAG) and a SWH dipole in spatial quadrature relation to it. Using the CCA results, an extended statistical hindcast of monthly wave fields is generated from sea level pressure data and used to quantitatively estimate the systematic increase in wave heights since the 1960s. It is shown that an increasing trend in SWH at several northeast Atlantic locations since 1960 or so is related to the systematic deepening of the Icelandic low and intensification of the Azores high over the last three decades. The analysis suggests that wave height south of 40 degrees N has decreased during the same period.
Rajagopalan, B., U. Lall and M. A. Cane, 1997: Anomalous ENSO occurrences: An alternate view. Journal of Climate, 10(9): 2351-2357.
There has been an apparent increase in the frequency and duration of El Niño-Southern Oscillation events in the last two decades relative to the prior period of record. Furthermore, 1990-95 was the longest period of sustained high Darwin sea level pressure in the instrumental record. Variations in the frequency and duration of such events are of considerable interest because of their implications for understanding global climatic variability and also the possibility that the climate system may be changing due to external factors such as the increased concentration of greenhouse gases in the atmosphere. Nonparametric statistical methods for time series analysis are applied to a 1882 to 1995 seasonal Darwin sea level pressure (DSLP) anomaly time series to explore the variations in El Niño-like anomaly occurrence and persistence over the period of record. Return periods for the duration of the 1990-95 event are estimated to be considerably smaller than those recently obtained by Trenberth and Hear using a linear ARMA model with the same time series. The likelihood of a positive anomaly of the DSLP, as well as its persistence, is found to exhibit decadal- to centennial-scale variability and was nearly as high at the end of the last century as it has been recently. The 1990-95 event has a much lower return period if the analysis is based on the 1882-1921 DSLP data. The authors suggest that conclusions that the 1990-95 event may be an effect of greenhouse gas-induced warming be tempered by a recognition of the natural variability in the system.
Reverdin, G., D. Cayan and Y. Kushnir, 1997: Decadal variability of hydrography in the upper northern North Atlantic in 1948-1990. Journal of Geophysical Research-Oceans, 102(C4): 8505-8531.
We investigate the variability of the North Atlantic subarctic gyre in recent decades from time series of station temperature and salinity. Decadal variability stronger at the surface is identified, which exhibits vertical coherence over a layer deeper than the late winter mixed layer. In the northwestern Atlantic, it corresponds to the layer with a component of water from the Arctic Ocean or from the Canadian Arctic. The spatial coherence of the signal is investigated. An empirical orthogonal function decomposition of lagged time series indicates that a single pattern explains 70% of the variance in upper ocean salt content, corresponding to a propagating signal from the west to the northeast in the subarctic gyre. The most likely interpretation is that the salinity signal originates in the slope currents of the Labrador Sea and is diffused/advected eastward of the Grand Banks over the near western Atlantic. In the northwestern Atlantic, temperature fluctuations are strongly correlated to salinity fluctuations and are aligned along the average T-S characteristics. This signal suggests large variations in the outflow of fresh, cold water in the slope current, and is strongly correlated with ice cover. A basin scale atmospheric circulation of weakened westerlies at 55 degrees N, weaker northwesterlies west of Greenland and weaker southerlies over the central and eastern North Atlantic is associated with the high salinity and warm water phase of the first principal component. This circulation pattern leads fluctuations in the northeast Atlantic and lags those in the northwestern part of the basin. The wind indices also suggest that the fluctuations of the fresh water outflow occur during intervals of anomalously northerly winds, either east of Greenland (1965, 1968-1969) or off the Canadian Archipelago (1983-1984).
Rind, D., C. Rosenzweig and M. Stieglitz, 1997: The role of moisture transport between ground and atmosphere in global change. Annual Review of Energy and the Environment, 22: 47-74.
Projections of the effect of climate change on future water availability are examined by reviewing the formulations used to calculate moisture transport between the ground and the atmosphere. General circulation models and climate change impact models have substantially different formulations for evapotranspiration, so their projections of future water availability often disagree, even though they use the same temperature and precipitation forecasts. General circulation models forecast little change in tropical and subtropical water availability, while impact models show severe water and agricultural shortages. A comparison of observations and modeling techniques shows that the parameterizations in general circulation models likely lead to an underestimate of the impacts of global warming on soil moisture and vegetation. Such errors would crucially affect the temperature and precipitation forecasts used in impact models. Some impact model evaporation formulations are probably more appropriate than those in general circulation models, but important questions remain. More observations are needed, especially in the vicinity of forests, to determine appropriate parameterizations.
Rind, D., R. Healy, C. Parkinson and D. Martinson, 1997: The Role of Sea-Ice in 2x Co2 Climate Model Sensitivity .2. Hemispheric dependencies. Geophysical Research Letters, 24(12): 1491-1494.
How sensitive are doubled CO2 simulations to GCM control-run sea ice thickness and extent? This issue is examined in a series of 10 control-run simulations with different sea ice and corresponding doubled CO2 simulations. Results show that with increased control-run sea ice coverage in the Southern Hemisphere, temperature sensitivity with climate change is enhanced, while there is little effect on temperature sensitivity of (reasonable) variations in control-run sea ice thickness. In the Northern Hemisphere the situation is reversed: sea ice thickness is the key parameter, while (reasonable) variations in control-run sea ice coverage are of less importance. In both cases, the quantity of sea ice that can be removed in the warmer climate is the determining factor. Overall, the Southern Hemisphere sea ice coverage change had a larger impact on global temperature, because Northern Hemisphere sea ice was sufficiently thick to limit its response to doubled CO2, and sea ice changes generally occurred at higher latitudes, reducing the sea ice-albedo feedback. In both these experiments and earlier ones in which sea ice was not allowed to change, the model displayed a sensitivity of similar to 0.02 degrees C global warming per percent change in Southern Hemisphere sea ice coverage.
Rodgers, K. B., M. A. Cane and D. P. Schrag, 1997: Seasonal variability of sea surface Delta C-14 in the equatorial Pacific in an ocean circulation model. Journal of Geophysical Research-Oceans, 102(C8): 18627-18639.
The object of this modeling study is to identify the physical mechanisms responsible for seasonal variability in sea surface Delta(14)C for the equatorial Pacific Ocean. Analyses of Delta(14)C in corals from Guam, Galapagos, Fanning, and Canton reveal seasonal variability between 30 and 50 per mil during the 1970s and early 1980s. Given that this variability occurs on seasonal timescales, whereas air-sea isotopic equilibration occurs on a timescale of 5 to 10 years, the variability must be due to seasonal variability in the physical circulation of the ocean. We use the primitive equation ocean circulation model of Gent and Cane , along with the hybrid mixed layer model of Chen et al. [1994a], to study the dynamical mechanisms responsible. Upwelling in the eastern equatorial Pacific brings up Delta(14)C-depleted waters, and air-sea exchange creates high Delta(14)C in the western equatorial Pacific, establishing horizontal gradients in sea surface Delta(14)C. Seasonally varying lateral advection, acting on these gradients, is the dominant mechanism for Delta(14)C variability in the equatorial Pacific. In addition to the runs which were forced with seasonally varying winds, a run which used interannual winds between 1971 and 1985 was performed. The substantial interannual Delta(14)C variability present in this run is associated with advective anomalies in the equatorial waveguide.
Seager, R. and R. Murtugudde, 1997: Ocean dynamics, thermocline adjustment, and regulation of tropical SST. Journal of Climate, 10(3): 521-534.
The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of st previously identified ''ocean dynamical thermostat.''
Send, U., G. Krahmann, D. Mauuary, Y. Desaubies, F. Gaillard, T. Terre, J. Papadakis, M. Taroudakis, E. Skarsoulis and C. Millot, 1997: Acoustic observations of heat content across the Mediterranean Sea. Nature, 385(6617): 615-617.
The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean can be routinely measured on large scales by satellite remote sensing (providing altimetric and infrared data) and with expendable probes dropped from commercial vessels, the deep interior of the ocean is more difficult to monitor. Ocean acoustic tomography(1) is a promising technique for such applications, as it has the potential to provide systematic, instantaneous and repeated measurements of the ocean interior over large parts of an ocean basin. Here we demonstrate the capability of this technique for measuring the heat content across an entire (albeit small) ocean basin-the western Mediterranean Sea.
Stieglitz, M., D. Rind, J. Famiglietti and C. Rosenzweig, 1997: An efficient approach to modeling the topographic control of surface hydrology for regional and global climate modeling. Journal of Climate, 10(1): 118-137.
The current generation of land-surface models used in GCMs view the soil column as the fundamental hydrologic unit. While this may be effective in simulating such processes as the evolution of ground temperatures and the growth/ablation of a snowpack at the soil plot scale, it effectively ignores the role topography plays in the development of soil moisture heterogeneity and the subsequent impacts of this soil moisture heterogeneity on watershed evapotranspiration and the partitioning of surface fluxes. This view also ignores the role topography plays in the timing of discharge and the partitioning of discharge into surface runoff and baseflow. In this paper an approach to land-surface modeling is presented that allows us to view the watershed as the fundamental hydrologic unit. The analytic form of TOPMODEL equations are incorporated into the soil column framework and the resulting model is used to predict the saturated fraction of the watershed and baseflow in a consistent fashion. Soil moisture heterogeneity represented by saturated lowlands subsequently impacts the partitioning of surface fluxes, including evapotranspiration and runoff. The approach is computationally efficient, allows for a greatly improved simulation of the hydrologic cycle, and is easily coupled into the existing framework of the current generation of single column land-surface models. Because this approach uses the statistics of the topography rather than the details of the topography, it is compatible with the large spatial scales of today's regional and global climate models. Five years of meteorological and hydrological data from the Sleepers River watershed located in the northeastern United States where winter snow cover is significant were used to drive the new model. Site validation data were sufficient to evaluate model performance with regard to various aspects of the watershed water balance, including snowpack growth/ablation, the spring snowmelt hydrograph, storm hydrographs, and the seasonal development of watershed evapotranspiration and soil moisture.
Ting, M. F. and H. Wang, 1997: Summertime US precipitation variability and its relation to Pacific sea surface temperature. Journal of Climate, 10(8): 1853-1873.
The year-to-year fluctuations in summertime precipitation over the U.S. Great Plains are examined in this study using data from 1950 to 1990. There are large interannual variabilities in precipitation amounts over the Great Plains during the period considered. A long-term trend in Great Plains precipitation from relatively wet conditions in the 1950s to relatively dry conditions in the 1980s is also identified. The spatial scale of the anomalous precipitation covers a large portion of the United States on seasonal mean timescales.
Tremblay, L. B. and L. A. Mysak, 1997: Modeling sea ice as a granular material, including the dilatancy effect. Journal of Physical Oceanography, 27(11): 2342-2360.
A dynamic sea ice model based on granular material rheology is presented. The sea lee model is coupled to both a mixed layer ocean model and a one-layer thermodynamic atmospheric model, which allows for an ice albedo feedback. Land is represented by a 6-m thick layer with a constant base temperature. A 10-year integration including both thermodynamic and dynamic effects and incorporating prescribed climatological wind stress and ocean current data was performed in order for the model to reach ii stable periodic seasonal cycle. The commonly observed lead complexes, along which sliding and opening of adjacent ice floes occur in the Arctic sea ice cover, are well reproduced in this simulation. In particular, shear lines extending from the western Canadian Archipelago toward the central Arctic, often observed in winter satellite images, are present. The ice edge is well positioned both in winter and summer using this thermodynamically coupled ocean-ice-atmosphere model. The results also yield a sea ice circulation and thickness distribution over the Arctic, which are in good agreement with observations. The model also produces an increase in ice formation associated with the dilatation of the ice medium along sliding lines. In this model, incident energy absorbed by the ocean melts ice laterally and warms the mixed layer, causing a smaller ice retreat in the summer. This cures a problem common to many existing thermodynamic-dynamic sea ice models.
Tremblay, L. B., L. A. Mysak and A. S. Dyke, 1997: Evidence from driftwood records for century-to-millennial scale variations of the high latitude atmospheric circulation during the Holocene. Geophysical Research Letters, 24(16): 2027-2030.
Different Holocene sea-ice drift patterns in the Arctic Ocean have been hypothesized by Dyke et al. from radiometric analyses of driftwood collected in the Canadian Arctic Archipelago. A dynamic-thermodynamic sea-ice model is used to simulate the modes of Arctic Ocean ice circulation for different atmospheric forcings, and hence determine the atmospheric circulations which may have accounted for the inferred ice drift patterns. The model is forced with the monthly mean wind stresses from 1968 (a year with very large ice export) and 1984 (very low ice export), two years with drastically different winter sea level pressure patterns and with different phases of the NAO index. The simula tions show that for the 1968 wind stresses, a weak Beaufort Gyre with a broad Transpolar Drift Stream (TDS) shifted to the east are produced, leading to a large ice export from the Arctic. Similarly, the 1984 wind stresses lead to an expanded Beaufort Gyre with a weak TDS shifted to the west and a low ice export. These results correspond to the patterns inferred by Dyke et al. Based on the simulations, the driftwood record suggests that for centuries to millennia during the Holocene, the high latitude average atmospheric circulation may have resembled that of 1968, 1984 and today's climatology, with abrupt changes from one state to the other.
Tziperman, E., H. Scher, S. E. Zebiak and M. A. Cane, 1997: Controlling spatiotemporal chaos in a realistic El Niño prediction model. Physical Review Letters, 79(6): 1034-1037.
A method for controlling low-order chaotic behavior of continuous spatiotemporal systems is developed and demonstrated in a complex, realistic 3D partial differential equation model that is used successfully for predicting El Nino events in the equatorial Pacific. An unstable periodic orbit that involves a full-domain oscillation is stabilized using a feedback control applied to a single degree of freedom at a carefully chosen single ''choke point'' in space. A general criterion is presented for determining the optimal points In reconstructed delay-coordinate phase space at which to apply the feedback control.
Tziperman, E., S. E. Zebiak and M. A. Cane, 1997: Mechanisms of seasonal - ENSO interaction. Journal of the Atmospheric Sciences, 54(1): 61-71.
The mechanisms by which the seasonal cycle in the equatorial Pacific affects ENSO are investigated using the Zebiak and Cane ENSO prediction model. The most dominant seasonal effect is found to be due to the wind divergence field, as determined by the seasonal motion of the ITCZ, through its effect on the atmospheric heating. The next-order seasonal effects are due to the seasonality of the background SST and ocean upwelling velocity, and the corresponding mechanisms are analyzed. It is suggested that the seasonal forcing has a first-order effect on ENSO's dynamics and that important aspects of the seasonal forcing may be included in idealized delayed oscillator ENSO models by making the model background shift seasonally from stable to unstable states.
Visbeck, M., J. Marshall, T. Haine and M. Spall, 1997: Specification of eddy transfer coefficients in coarse-resolution ocean circulation models. Journal of Physical Oceanography, 27(3): 381-402.
Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging from horizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams (GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of a transformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-dimensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective ''chimney'' where the baroclinic zone is created by differential surface cooling; 2) spindown of a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a form for the horizontal transfer coefficient given by
Xue, Y., M. A. Cane and S. E. Zebiak, 1997: Predictability of a coupled model of ENSO using singular vector analysis .1. Optimal growth in seasonal background and ENSO cycles. Monthly Weather Review, 125(9): 2043-2056.
The fastest initial error growth (optimal growth) in the Zebiak and Cane (ZC) forecast model for the El Niño-Southern Oscillation (ENSO) is analyzed by singular value decomposition of a forward tangent model along a trajectory in a reduced EOF space. In this paper (Part I of II), optimal growth about the seasonally varying background and ENSO cycles from a long model run are discussed.
Xue, Y., M. A. Cane and S. E. Zebiak, 1997: Predictability of a coupled model of ENSO using singular vector analysis .2. Optimal growth and forecast skill. Monthly Weather Review, 125(9): 2057-2073.
The fastest perturbation growth (optimal growth) in forecasts of El Niño-Southern Oscillation (ENSO) with the Zebiak and Cane model is analyzed by singular value decomposition of forward tangent models along forecast trajectories in a reduced EOF space. The authors study optimal growth in forecast runs using two different initialization procedures and discuss the relationship between optimal growth and forecast skill.
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