Bromwich, D. H., B. Chen, K. M. Hines and R. I. Cullather, 1998: Global atmospheric responses to Antarctic forcing. Annals of Glaciology, 27: 521-527.
To evaluate the greatest impact that sea-ice anomalies around Antarctica could have on the global atmosphere, 15 year seasonal cycle simulations are conducted with the U.S. National Center for Atmospheric Research Community Climate Model version 2.1. Sensitivity simulations are performed with the following conditions: (1) all sea ice in the Southern Hemisphere is replaced by year-round open water, but the permanent ice shelves are retained (NSIS); and (2) all sea ice in the Southern Hemisphere and the major ice shelves are removed and replaced by open water (NISH). The results are compared to a standard run (CNT) with boundary conditions set for the present climate. The comparison shows chat trains of positive and negative anomalies in zonal-mean fields extend into the tropical latitudes of the Northern Hemisphere. Anomalies are largest during April-October. The additional removal of the ice shelves in NISH enhances the response, as zonally averaged anomalies are similar in pattern to those in NSIS but are roughly twice as large poleward of 50 degrees S, and only slightly larger farther north. Anomalies in the eddy fields are found in both hemispheres. In NISH, and to a lesser degree in NSIS, these anomalies appear to be related to a delayed northern advance over China during June of the rain front associated with the summer monsoon. Consequently, precipitation is enhanced in middle and southern China and decreased in northern China. Observational analyses have also found links between Antarctic sea-ice variations and modulations of the East Asian monsoon.
Bromwich, D. H., R. I. Cullather and M. L. VanWoert, 1998: Antarctic precipitation and its contribution to the global sea-level budget. Annals of Glaciology, 27: 220-226.
Antarctic precipitation estimations derived from several new sources ale examined in comparison to results found previously The availability of analyzed atmospheric datasets has been a significant and beneficial tool for atmospheric and climate research for a broad range of research interests. This is particularly true for the polar regions, where the observational arrays are sparsely distributed. In high southern latitudes, a comprehensive assimilation of all available observations, including satellite data, is necessary for an accurate depiction of the atmospheric circulation. Recent studies have found the operational analyses of the European Centre for Medium-range Weather Forecasts to be superior to those of other weather-forecasting centers in depicting the large-scale atmospheric circulation patterns over Antarctica. "Re-analysis" programs at major weather-forecasting centers have produced atmospheric numerical analyses using a "frozen" data-assimilation system. These projects have also derived precipitation and evaporation fields using an ensemble of short-term forecasts. From these new sources, Antarctic P - E (precipitation minus evaporation/sublimation) is compared and evaluated against the long-term glaciological synthesis, as well as results fi om previous studies. The comparisons indicate significant regional disagreements exist between P - E from the re-analysis forecasts and the glaciological data. For the ensemble forecasting method, the continental-average evaporation is the largest area of uncertainty and differs by art order of magnitude between the re-analysis datasets. This finding supports the use of the atmospheric moisture budget fur determining P - E collectively in atmospheric diagnostic studies for Antarctica.
Bromwich, D. H., R. I. Cullather, Q. Chen and B. M. Csatho, 1998: Evaluation of recent precipitation studies for Greenland ice sheet. Journal of Geophysical Research-Atmospheres, 103(D20): 26007-26024.
The retrieval of an accurate spatial and temporal record of contemporary Greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales. A comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long-term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of Greenland. There is general agreement on a long-term Greenland average of about 35 cm yr(-1) and on long-term values for regional scales, although-differences for outlying data sets exceed 50% of the observed glaciological estimate for particular regions. A fundamental problem is the inadequate topographic representation of Greenland in the numerical analyses. Nearly all of the data sets are overly dry for high-elevation areas, as seen from comparisons with glaciological observations from Summit. The east-central region of Greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses. In contrast, there is strong agreement among all methods on the temporal variability for the west-central region over a 15-year period. From the comparison it is concluded that none of the data sets is able to capture all of the regional-scale features. In general, however, the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features. Agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of Greenland's precipitation climatology. In particular, an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses. In the north, anomalously high precipitation is associated with cyclonic development near the Fram Strait. For west-central Greenland the close agreement among methods is related to the dominant contribution of the mean circulation.
Camargo, S. J., M. K. Tippett and I. L. Caldas, 1998: Nonmodal energetics of resistive drift waves. Physical Review E, 58(3): 3693-3704.
The modal and nonmodal linear properties of the Hasegawa-Wakatani system are examined. This linear model for plasma drift waves is nonnormal in the sense of not having a complete set of orthogonal eigenvectors. A consequence of nonnormality is that finite-time nonmodal growth rates can be larger than modal growth rates. In this system, the nonmodal time-dependent behavior depends strongly on the adiabatic parameter and the time scale of interest. For small values of the adiabatic parameter and short time scales, the nonmodal growth rates, wave number, and phase shifts (between the density and potential fluctuations) are time dependent and differ from those obtained by normal mode analysis. On a given time scale, when the adiabatic parameter is less than a critical value, the drift waves are dominated by nonmodal effects while for values of the adiabatic parameter greater than the critical value, the behavior is that given by normal mode analysis. The critical adiabatic parameter decreases with time and modal behavior eventually dominates. The nonmodal linear properties of the Hasegawa-Wakatani system may help to explain features of the full system previously attributed to nonlinearity.
Cane, M. A., V. M. Kamenkovich and A. Krupitsky, 1998: On the utility and disutility of JEBAR. Journal of Physical Oceanography, 28(3): 519-526.
The usefulness of the concept of JEBAR, the joint effect of baroclinicity and relief, in large-scale ocean dynamics is critically analyzed. The authors address two questions. Does the JEBAR term properly characterize the joint impact of stratification and bottom topography on the ocean circulation? Do estimates of the JEBAR term from observational data allow reliable diagnostic calculations?
Carbotte, S., C. Mutter, J. Mutter and G. Ponce-Correa, 1998: Influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer: Insights from the east Pacific rise at lat16 degrees N. Geology, 26(5): 455-458.
Seismic reflection data from the East Pacific Rise at lat 16 degrees N, which is spreading at the high end of intermediate rates, suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate, Rather, magma-chamber depths form two distinct populations, each associated,vith a distinct axial morphology and with an abrupt transition occurring,within the intermediate-spreading-rate range. Our data (1) show that the melt lens at high-intermediate-spreading ridges lies at a shallow level similar to lens depths at faster-spreading ridges, and (2) provide further support for the spreading-rate invariance of ridges with axial highs noted in other ridge properties. The axial morphology of the two ridge segments within the study area differs markedly, and a large contrast in magma supply is inferred. The ridge segment with greater magma supply is associated with a broader and more continuous melt lens, a wider region over which the extrusive crust accumulates, and a thicker extrusive layer off-axis where supply to the ridge segment appears to be centered. However, on-axis, the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed, consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens.
Chen, D. K., M. A. Cane, S. E. Zebiak and A. Kaplan, 1998: The impact of sea level data assimilation on the Lamont model prediction of the 1997/98 El Niño. Geophysical Research Letters, 25(15): 2837-2840.
Assimilating the sea level data from tropical Pacific tide gauges greatly improved the Lament model prediction of the 1997/98 El Niño while retaining its overall performance during the past few decades. The implication is that the Lament forecast system is limited by the observational data used for initialization probably probably as much as by its simplified model physics. The sea level measurements in the vicinity of the equator are extremely effective in correcting the model ocean state and preconditioning it for ENSO prediction.
Cullather, R. I., D. H. Bromwich and M. L. VanWoert, 1998: Spatial and temporal variability of Antarctic precipitation from atmospheric methods. Journal of Climate, 11(3): 334-367.
The spatial and temporal variability of net precipitation (precipitation minus evaporation/sublimation) for Antarctica derived from the European Centre for Medium-Range Weather Forecasts operational analyses via the atmospheric moisture budget is assessed in comparison to a variety of glaciological and meteorological observations and datasets. For the 11-yr period 1985-95, the average continental value is 151 mm yr(-1) water equivalent. Large regional differences with other datasets are identified, and the sources of error are considered. Interannual variability in the Southern Ocean storm tracks is found to be an important mechanism for enhanced precipitation minus evaporation (P - E) in both east and west Antarctica. In relation to the present findings, an evaluation of the rawinsonde method for estimating net precipitation in east Antarctica is conducted. Estimates of P - E using synthetic rawinsondes derived from the analyses are found to compare favorably to glaciological estimates. A significant upward trend of 2.4 mm yr(-1) is found for the Antarctic continent that is consistent with findings from the National Centers for Environmental Prediction, formerly the National Meteorological Center, and the National Center for Atmospheric Research Reanalysis precipitation dataset. Despite large regional discrepancies, the general agreement on the main features of Antarctic precipitation between studies suggests that a threshold has been reached, where the assessment of the smaller terms including evaporation/sublimation and drift snow loss is required to explain the differences.
Curchitser, E. N., M. Iskandarani and D. B. Haidvogel, 1998: A spectral element solution of the shallow-water equations on multiprocessor computers. Journal of Atmospheric and Oceanic Technology, 15(2): 510-521.
A shallow-water spectral element ocean model is implemented on multiple instruction multiple data, distributed memory parallel computers. A communications-minimizing partitioning algorithm for unstructured meshes, based on graph theory, is presented and is shown to improve the efficiency in a limited range of granularities. A domain decomposition implementation with an architecture-independent communications scheme, using message passing, is devised and rested on an nCUBE/2. a Gray T3D, and an IBM SP2. The implementation exhibits high efficiencies over a wide range of granularities. An order of magnitude analysis shows that, to leading order, the efficiency stays constant when KN2 grows proportionally to P, where K is the total number of elements, N is the order of the spectral truncation within an element, and P is the number of processors.
Dery, S. J., P. A. Taylor and J. B. Xiao, 1998: The thermodynamic effects of sublimating, blowing snow in the atmospheric boundary layer. Boundary-Layer Meteorology, 89(2): 251-283.
A seasonal snowcover blankets much of Canada during wintertime. In such an environment, the frequency of blowing snow events is relatively high and can have important meteorological and hydrological impacts. Apart from the transport of snow, the thermodynamic impact of sublimating blowing snow in air near the surface can be investigated. Using a time or fetch-dependent blowing snow model named 'PIEKTUK' that incorporates prognostic equations for a spectrum of sublimating snow particles, plus temperature and humidity distributions, it is found that the sublimation of blowing snow can lead to temperature decreases of the order of 0.5 degrees C and significant water vapour increases in the near-surface air. Typical predicted snow removal rates due to sublimation of blowing snow are several millimetres snow water equivalent per day over open Arctic tundra conditions. The model forecast sublimation rates are most sensitive to humidity, as well as wind speed, temperature and particle distributions, with a maximum value in sublimation typically found approximately 1 km downstream from blowing snow initiation. This suggests that the sublimation process is self-limiting despite ongoing transport of snow by wind, yielding significantly lower values of blowing snow sublimation rates (nearly two-thirds less) compared to situations where the thermodynamic feedbacks are neglected. The PIEKTUK model may provide the necessary thermodynamic inputs or blowing snow parameterizations for mesoscale models, allowing the assessment of the contribution of blowing snow fluxes, in more complex situations, to the moisture budgets of high-latitude regions.
Evans, M. N., A. Kaplan and M. A. Cane, 1998: Optimal sites for coral-based reconstruction of global sea surface temperature. Paleoceanography, 13(5): 502-516.
We determine the structure of a network of sites from which coral-based, proxy measurements of sea surface temperature (SST) variability minimize the error in a reconstruction of the large-scale features of the global SST field. For a wide range of coral-derived SST observational error and several minimization criteria, sites in the equatorial oceans, especially the central and eastern equatorial Pacific, best minimize the error in the reconstruction. If the observational error is sufficiently low, additional optimal sites are in selected subtropical lo,cations. If the observational error is high, the error is minimized by resampling the most important equatorial sites. The marginal return on additional sites diminishes rapidly and is not sensitive to the size of the observational error: two sites reduce the analysis error by half as much as 10 sites, while the first 6-7 sites achieve half the error reduction of all 65 sites in the analysis domain. In the extratropics the reduction of reconstruction error is small and gradual, while error in the tropical Pacific is sharply reduced by 2-3 sites and gradually thereafter. These results suggest that a limited set of redundantly sampled sites with uncorrelated and low observational error (approximate to 0.3 degrees-0.6 degrees C) will provide the best approach to reconstruction of large-scale features of the SST field from coral data for the preinstrumental period.
Gordon, A. L. and R. D. Susanto, 1998: Makassar Strait transport: Initial estimate based on Arlindo results. Marine Technology Society Journal, 32(4): 34-45.
This report presents the initial evaluation of the transport and temperature of ocean water passing through Makassar Strait, which is the primary pathway of the Pacific to Indian Ocean transport called the Indonesian through-flow. The transport was measured as part of the Indonesian USA Arlindo program, by two moorings deployed within the Labani Channel, a deep constriction near 3 degrees S. Both moorings were operative from December 1996 to February 1998; the western most mooring operated until July 1998 The moorings were deployed during a weak La Niña phase. An El Niño condition began in March 1997, becoming extreme during the 1997 Summer and Fall, relaxing in early 1998 The Makassar thermocline depth and transport reflect the phases of ENSO. The thermocline was deeper during La Niña, when the warm pool of the tropical Pacific with its relatively high sea level and deep thermocline, is pressed zip against the Pacific entrance to the Indonesian Seas. A shallower thermocline occurs during El Niño when the warm pool shifts eastward in the Pacific Ocean, reducing its accessibility to the Indonesian seas. ENSO effects are also seen in the velocity data. As El Niño takes hold the deeper instruments recorded reduced speeds, as the throughflow shoals with the thermocline. The 1997 average Makassar Strait throughflow transport is 9.3 Sv (Sv = 10(6) m(3)/sec), assuming the flow in the upper 200 meters equals the flow measured by the current meter moored at 200 m. Other models for the surface flow range from 6.7 Sv (zero surface flow) to 11.3 Sv (thermocline shear is extrapolated to the sea surface). A comparison of the transports for the two December to February periods included in the records also reflects the ENSO effect: the transport during the El Niño months of December 1997 to February 1998 average 5 Sv, while the La Niña months of December 1996 to February 1997 averages 12 Sv, a 2.5 fold difference The Makassar transports determined from the Arlindo data are at the higher side of estimates based in Timer Sea and Indian ocean studies.
Hacker, P., E. Firing, J. Hummon, A. L. Gordon and J. C. Kindle, 1998: Bay of Bengal currents during the northeast monsoon. Geophysical Research Letters, 25(15): 2769-2772.
Velocity and property observations were made during February and March 1995 as part of the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP) expedition in the Indian Ocean. The observed circulation in the upper 300 m of the ocean during the northeast monsoon is compared to the output of a high-resolution, S-layer, nonlinear model forced by European Center for Medium Range Weather Forecasting (ECMWF) winds. The data identify several new features in the Bay of Bengal: a shelf-break coastal current in the northeast corner, an eddy and subsurface jet near the South Preparis Channel, and an eastward countercurrent extending from 80 degrees E to the eastern boundary. The countercurrent transports high-salinity surface water from the northwest Indian Ocean into the bay, and separates the historically observed North Equatorial Current into two currents with separate sources and water properties. The data also detail the spatial structure of the South Equatorial Current and Countercurrent, and show the Equatorial Undercurrent in the eastern half of the Indian Ocean. The model compares well enough with the data to suggest that such realistic models may provide a useful temporal context for the WHP snapshot.
Harnik, N. and R. S. Lindzen, 1998: The effect of basic-state potential vorticity gradients on the growth of baroclinic waves and the height of the tropopause. Journal of the Atmospheric Sciences, 55(3): 344-360.
The stability characteristics of normal mode perturbations on idealized basic states that have meridional potential vorticity (PV) gradients that are zero in the troposphere, very large at the tropopause, and order beta in the stratosphere are checked. The results are compared to the corresponding models that have a lid at the tropopause. The dispersion relations and the vertical structures of the modes are similar in the two models, thus confirming the relevance of the Eady problem to unbounded atmospheres. The effect of replacing the lid with a more realistic tropopause is to complicate the interaction of tropopause and surface waves, such as to inhibit phase locking for a range of wavenumbers. This causes the short-wave cutoff of the Eady model to move to longer waves. Also, there is a slight destabilization of the long waves, which have large amplitudes in the stratosphere
Houghton, R. W. and M. Visbeck, 1998: Upwelling and convergence in the Middle Atlantic Bight shelfbreak front. Geophysical Research Letters, 25(15): 2765-2768.
Convergent and upwelling circulation within the shelfbreak front in the Middle Atlantic Eight are detected using a dye tracer injected into the bottom boundary layer at the foot of the front. From the three day displacement and dispersion of two dye injections within the front we infer Lagrangian isopycnal (diapycnal) velocities and diffusivities of 2 x 10(-2) m/s (4 x 10(-6) m/s) and 9 m(2)/s (6 x 10(-6) m(2)/s). These results substantiate model predictions of Chapman and Lentz  and previous dye tracer observations by Houghton .
Huang, H. P. and W. A. Robinson, 1998: Two-dimensional turbulence and persistent zonal jets in a global barotropic model. Journal of the Atmospheric Sciences, 55(4): 611-632.
The dynamics of two-dimensional turbulence on a rotating sphere are examined. The anisotropic Rhines scale is derived and verified in decaying turbulence simulations. Due to the anisotropic nature of the Rossby waves, the Rhines barrier is displaced toward small total wavenumber n with decreasing zonal wavenumber m. Upscale energy transfer along the zonal axis (m = 0) is not directly arrested by beta. A forced dissipative model with high-wavenumber forcing is used to investigate the dynamics of persistent zonal jets. Persistent jets form in the low energy (strong rotation) cases with the root-mean-square velocity V-rms << a Omega. Under a fixed rotation rate, the jet scale decreases with the energy. The equilibrated jets generally stay at fixed latitudes. The zonal bands are nearly uniformly distributed in latitude, except that bands in the high latitudes tend to be wider and weaker, as clearly affected by a decreasing beta with latitude. The rime-mean zonal winds in the forced simulations appear to be stable, with their absolute vorticity gradient dominated by beta. The increase of the jet scale with energy as required by stability is consistent with the simulated results.
Kaplan, A., M. A. Cane, Y. Kushnir, A. C. Clement, M. B. Blumenthal and B. Rajagopalan, 1998: Analyses of global sea surface temperature 1856-1991. Journal of Geophysical Research-Oceans, 103(C9): 18567-18589.
Global analyses of monthly sea surface temperature (SST) anomalies from 1856 to 1991 are produced using three statistically based methods: optimal smoothing (OS), the Kalman filter (KF) and optimal interpolation (OI). Each of these is accompanied by estimates of the error covariance of the analyzed fields. The spatial covariance function these methods require is estimated from the available data; the time-marching model is a first-order autoregressive model again estimated from data. The data input for the analyses are monthly anomalies from the United Kingdom Meteorological Office historical sea surface temperature data set (MOHSST5) [Parker et al., 1994] of the Global Ocean Surface Temperature Atlas (GOSTA) [Bottomley et al., 1990].
Kessler, W. S., L. M. Rothstein and D. K. Chen, 1998: The annual cycle of SST in the eastern tropical Pacific, diagnosed in an ocean GCM. Journal of Climate, 11(5): 777-799.
The annual onset of the east Pacific cold tongue is diagnosed in an ocean GCM simulation of the tropical Pacific. The model uses a mixed-layer scheme that explicitly simulates the processes of vertical exchange of heat and momentum with the deeper layers of the ocean; comparison with observations of temperature and currents shows that many important aspects of the model fields are realistic. As previous studies have found, the heat balance in the eastern tropical Pacific is notoriously complicated, and virtually every term in the balance plays a significant role at one time or another. However, despite many complications, the three-dimensional ocean advection terms in the cold tongue region tend to cancel each other in the annual cycle and, to first order, the variation of SST can be described as simply following the variation of net solar radiation at the sea surface (sun minus clouds). The cancellation is primarily between cooling due to equatorial upwelling and warming due to tropical instability waves, both of which are strongest in the second half of the year (when the winds are stronger). Even near the equator, where the ocean advection is relatively intense, the terms associated with cloudiness variations are among the largest contributions to the SST balance. The annual cycle of cloudiness transforms the semiannual solar cycle at the top of the atmosphere into a largely 1 cycle yr(-1) variation of insolation at the sea surface. However, the annual cycle of cloudiness appears closely tied to SST in coupled feedbacks (positive for low stratus decks and negative for deep cumulus convection), so the annual cycle of SST cannot be fully diagnosed in an ocean-only modeling context as in the present study. Zonal advection was found to be a relatively small influence on annual equatorial cold tongue variations; in particular, there was little direct (oceanic) connection between the Peru coastal upwelling and equatorial annual cycles. Meridional advection driven by cross-equatorial winds has been conjectured as a key factor leading to the onset of the cold tongue. The results suggest that the SST changes due to this mechanism are modest, and if meridional advection is in fact a major influence, then it must be through interaction with another process (such is a coupled feedback with stratus cloudiness). At present, it is not possible to evaluate this feedback quantitatively.
Krahmann, G. and F. Schott, 1998: Longterm increases in Western Mediterranean salinities and temperatures: anthropogenic and climatic sources. Geophysical Research Letters, 25(22): 4209-4212.
The deep water of the western Mediterranean Sea is known to have become warmer and saltier since about the 1950s. The causes of these changes have, however, not yet been sactisfactorily determined. Previous studies speculated on decreasing precipitation, greenhouse warming and/or anthropogenic reduction of the freshwater flux into the eastern Mediterranean. Here we report on results from a new oceanographic database of the western Mediterranean Sea together with determinations of longterm changes of the fresh water budget. We analyzed temperature and salinity data of the past 40 years to detect deviations from the longterm average. Certain areas and depth ranges are showing increases in temperature or salinity some of which have been found earlier and some which are new. From the regional and vertical distribution we conclude that the observed increases of temperature and:salinity in the western Mediterranean Sea are caused both by changes in atmospheric conditions as described by the NAO-index and by the regulation of Spanish rivers.
Liu, A. Z., M. F. Ting and H. L. Wang, 1998: Maintenance of circulation anomalies during the 1988 drought and 1993 floods over the United States. Journal of the Atmospheric Sciences, 55(17): 2810-2832.
The large-scale circulation anomalies associated with the 1988 drought and the 1993 floods are investigated with the National Centers for Environmental Prediction Reanalysis data and a linear stationary wave model. The transient vorticity and thermal forcings are explicitly calculated and the diabatic heating is derived as a residual in the thermodynamic energy equation. Using the April-June (AMJ) data for 1988, and June-August (JJA) data for 1993, the linear stationary wave model is able to reproduce the main features of the geopotential height anomaly for the two Seasons when all forcings are included. This provides a basis for further investigation of stationary wave response to different forcing mechanisms using the linear model.
Marshall, J., F. Dobson, K. Moore, P. Rhines, M. Visbeck, E. d'Asaro, K. Bumke, S. Chang, R. Davis, K. Fischer, R. Garwood, P. Guest, R. Harcourt, C. Herbaut, T. Holt, J. Lazier, S. Legg, J. McWilliams, R. Pickart, M. Prater, I. Renfrew, F. Schott, U. Send and W. Smethie, 1998: The Labrador Sea deep convection experiment. Bulletin of the American Meteorological Society, 79(10): 2033-2058.
In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.
Martinson, D. G. and R. A. Iannuzzi, 1998: Antarctic Ocean-Ice Interaction: Implications from Ocean Bulk Property Distributions in the Weddell Gyre. In: M. Jeffries (Editor), Antarctic Research Series, Antarctic Sea Ice: Physical Processes, Interactions and Variability. American Geophysical Union, Boston, pp. 243-271.
The sea ice distribution in the Antarctic polar oceans is intimately tied to
McPhee, M. G., T. P. Stanton, J. H. Morison and D. G. Martinson, 1998: Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing? Geophysical Research Letters, 25(10): 1729-1732.
During the Surface Heat Budget of the Arctic (SHEBA) deployment in October, 1997, multiyear ice near the center of the Beaufort Gyre was anomalously thin. The upper ocean was both warmer and less saline than in previous years. The salinity deficit in the upper 100 m, compared with the same region during the Arctic Ice Dynamics Joint Experiment (AIDJEX) in 1975, is equivalent to surface input of about 2.4 m of fresh water. Heat content has increased by 67 MJ m(-2). During AIDJEX the change in salinity over the melt season implied melt equivalent to about 0.8 m of fresh water. As much as 2 m of freshwater input may have occurred during the 1997 summer, possibly resulting from decreased ice concentration from changes in atmospheric circulation early in the summer, in the classic albedo-feedback scenario. Unchecked, the pattern could lead to a significantly different sea-ice regime in the central Arctic.
Phillips, J. G., M. A. Cane and C. Rosenzweig, 1998: ENSO, seasonal rainfall patterns and simulated maize yield variability in Zimbabwe. Agricultural and Forest Meteorology, 90(1-2): 39-50.
A correlation between ENSO (El Niño/Southern Oscillation) and rainfall in southern Africa has been recognized for at least a decade. This recognition has led to the expectation that ENSO-based climate predictions will have significant applications in agricultural management. This study is an analysis of the potential for using ENSO predictions to reduce risk in agricultural production associated with rainfall variability at the site level. Records of sea-surface temperatures in the equatorial Pacific during November, December and January were used to define El Niño, La Niña and neutral years. Climate data from four sites in four of the five agroecological zones (AEZ) in Zimbabwe were analyzed with respect to ENSO phases and used to drive a maize growth simulation model parameterized for soil conditions typical of each area, using two nitrogen fertilizer treatments and three planting dates. The four sites (Karoi, AEZ II; Gweru, AEZ III; Masvingo, AEZ IV; and Beitbridge, AEZ V) all showed a decrease in seasonal precipitation associated with the El Niño phase, compared to both neutral and La Niña years. At sites in zones II and III, within-season rainfall variability increased for both El Niño and La Niña years relative to neutral years. While average simulated maize yields were generally lowest in El Niño years, variability in rainfall pattern and standard deviation of yields at the site level was high within each ENSO phase, indicating that more precise seasonal climate predictions would be necessary for forecasts to be valuable in crop management decisions in Zimbabwe. However, simulation results point towards the relative importance of predicting favorable cropping seasons as opposed to poor ones with respect to better nitrogen management and yield improvement for the more marginal sites. (C) 1998 Elsevier Science B.V.
Rajagopalan, B., Y. Kushnir and Y. M. Tourre, 1998: Observed decadal midlatitude and tropical Atlantic climate variability. Geophysical Research Letters, 25(21): 3967-3970.
Two common indicators of Atlantic climate variability, viz., the North Atlantic oscillation (NAO) and the cross-intertropical convergence zone (ITCZ) sea surface temperature (SST) gradient, are examined for their frequency characteristics and, midlatitude-tropical links. SST anomalies north and south of the ITCZ are found to be uncorrelated on all time scales, while the sea level pressure (SLP) fluctuations associated with the NAO display a coherent seesaw between Iceland and the Azores. This out-of-phase relationship spans a broad range of time scales, but is particularly strong in the 5 - 10 year period band. Strong, broadband coherence between the NAO and the tropical Atlantic cross-ITCZ SST difference is found in the 8-20 year period band, suggesting a significant midlatitude-tropical interaction. Moreover, tropical Atlantic SSTs on both sides of the ITCZ, separately, exhibit significant coherence with the NAO index and SLP variability over Iceland and the Azores. Based on these findings we hypothesize that the tropical Atlantic (TA) ocean-atmosphere interaction is affecting North Atlantic climate variability.
Randall, D., J. Curry, D. Battisti, G. Flato, R. Grumbine, S. Hakkinen, D. Martinson, R. Preller, J. Walsh and J. Weatherly, 1998: Status of and outlook for large-scale modeling of atmosphere-ice-ocean interactions in the Arctic. Bulletin of the American Meteorological Society, 79(2): 197-219.
Arctic air masses have direct impacts on the weather and climatic extremes of midlatitude areas such as central North America. Arctic physical processes pose special and very important problems for global atmospheric models used for climate simulation and numerical weather prediction. At present, the observational database is inadequate to support research aimed at overcoming these problems. Three interdependent Arctic field programs now being planned will help to remedy this situation: SHEBA, which will operate an ice camp in the Arctic for a year; ARM, which will supply instruments for use at the SHEBA ice camp and which will also conduct longer-term measurements near Barrow, Alaska; and FIRE, which will conduct one or more aircraft campaigns, in conjunction with remote-sensing investigations focused on the SHEBA ice camp. This paper provides an introductory overview of the physics of the Arctic from the perspective of large-scale modelers, outlines some of the modeling problems that arise in attempting to simulate these processes, and explains how the data to be provided by the three field programs can be used to test and improve large-scale models.
Rhein, M., L. Stramma and G. Krahmann, 1998: The spreading of Antarctic bottom water in the tropical Atlantic. Deep-Sea Research Part I-Oceanographic Research Papers, 45(4-5): 507-527.
Hydrographic and tracer [chlorofluorocarbon (CFC), component F11] data in the tropical Atlantic off Brazil taken in spring 1994 are used to describe the development of the water mass characteristics of Antarctic Bottom Water (AABW) between 10 degrees S and 11 degrees N. To compute the AABW transports, geostrophic computations and directly measured velocity fields are combined. Velocity profiles were measured with the Pegasus profiling system and an ADCP attached to the CTD.
Rothstein, L. M., R. H. Zhang, A. J. Busalacchi and D. Chen, 1998: A numerical simulation of the mean water pathways in the subtropical and tropical Pacific Ocean. Journal of Physical Oceanography, 28(2): 322-343.
A reduced-gravity, primitive-equation, upper-ocean general circulation model is used to study the mean water pathways in the North Pacific subtropical and tropical ocean. The model features an explicit physical representation of the surface mixed layer, realistic basin geometry, observed wind and heat flux forcing, and a horizontal grid-stretching technique and a vertical sigma coordinate to obtain a realistic simulation of the subtropical/tropical circulation. Velocity fields, and isopycnal and trajectory analyses are used to understand the mean flow of mixed layer and thermocline waters between the subtropics and Tropics.
Stockdale, T. N., A. J. Busalacchi, D. E. Harrison and R. Seager, 1998: Ocean modeling for ENSO. Journal of Geophysical Research-Oceans, 103(C7): 14325-14355.
Tropical ocean modeling has played a major part in the development of our knowledge of El Nino-Southern Oscillation (ENSO) during the Tropical Ocean-Global Atmosphere (TOGA) decade. Although the foundations had already been laid, it was only with the impetus from TOGA that tropical ocean modeling was able to develop so extensively. In this paper we discuss the development of the wide range of tropical ocean models in use today,from the simple to the complex; the ways in which their abilities to reproduce different phenomena have been assessed; and the ways in which they are being used to better understand and predict the behavior of the coupled ocean-atmosphere system in the tropics. Ocean model development is far from finished, however. Outstanding issues such as forcing fields, model improvements, testing strategies, and applications are also discussed. There is at least as much work to be done in the next decade as was achieved in the last.
Susanto, R. D., Q. N. Zheng and X. H. Yan, 1998: Complex singular value decomposition analysis of equatorial waves in the Pacific observed by TOPEX/Poseidon altimeter. Journal of Atmospheric and Oceanic Technology, 15(3): 764-774.
The mean of the sea level deviation data derived from the TOPEX/Poseidon altimeter in the equatorial Pacific, between 10 degrees S and 10 degrees N, and between 120 degrees E and 78 degrees W, from cycles 2 to 136 (3 October 1992-2 June 1996), are extracted using a maximum-minimum average method. Then, two-dimensional (2D) sea level deviation time series are developed to visualize the dynamics of equatorial waves. The complex singular value decomposition (CSVD) method is applied to decompose these 2D time series into empirical orthogonal modes. Using this method, zonal and meridional structures, propagation directions, periods,and propagation speeds of these empirical modes are obtained.
Ting, M. F. and L. H. Yu, 1998: Steady response to tropical heating in wavy linear and nonlinear baroclinic models. Journal of the Atmospheric Sciences, 55(24): 3565-3582.
The linear, steady-state, baroclinic model response to a tropical heating superimposed on a three-dimensional basic state is examined in this study. The emphasis is on the relevance of the linear model solution as compared to a fully nonlinear baroclinic model. The direct response to heating in the fully nonlinear, time-dependent model is obtained as the day-30 model response, following the Jin and Hoskins approach. When a 15-day linear damping is included in addition to Rayleigh friction, Newtonian cooling, and a scale-selective biharmonic diffusion, the comparison of the linear and the nonlinear model responses to a 2 degrees C/day tropical heating reveals a striking similarity in both the spatial distribution and amplitude. Thus nonlinearity appears to be a secondary effect and may be crudely represented by the 15-day linear damping, and the linear steady-state model can be a useful tool in diagnostic studies.
Tremblay, L. B. and L. A. Mysak, 1998: On the origin and evolution of sea-ice anomalies in the Beaufort-Chukchi sea. Climate Dynamics, 14(6): 451-460.
The origin and space-time evolution of Beaufort-Chukchi Sea ice anomalies are studied using data and a recently developed dynamic-thermodynamic sea-ice model. First, the relative importance of anomalies of river runoff, atmospheric temperature and wind in creating anomalous sea-ice conditions in the Beaufort-Chukchi Sea is investigated. The results indicate that wind anomalies are the dominant factor responsible for creating interannual variability in the Beaufort-Chukchi Sea ice cover. Temperature anomalies appear to play a major role for longer time scale fluctuations, whereas the effects of runoff anomalies are small. The sea-ice model is then used to track the position of a positive sea-ice anomaly as it is transported by the Beaufort Gyre toward the Transpolar Drift Stream and then exported out of the Arctic Basin into the Greenland Sea via Fram Strait. The model integration shows that sea-ice anomalies originating in the western Beaufort Sea can survive a few seasonal cycles as they propogate through the Arctic Basin and can account for a notable amount of anomalous ice export into the Greenland Sea. These anomalies, however, represent a small contribution to the fresh water budget in this area when compared with sea-ice fluctuations generated by interannually varying local winds.
Tziperman, E., M. A. Cane, S. E. Zebiak, Y. Xue and B. Blumenthal, 1998: Locking of El Niño's peak time to the end of the calendar year in the delayed oscillator picture of ENSO. Journal of Climate, 11(9): 2191-2199.
El Nino events owe their name to their tendency to be locked to the seasonal cycle. A simple explanation is proposed here for the lacking of the peak of ENSO's basin-scale warming to the end of the calendar year. The explanation is based on incorporating a seasonally varying coupled ocean-atmosphere instability strength into the delayed oscillator mechanism for the ENSO cycle. It is shown that the seasonally varying amplification of the Rossby and Kelvin ocean waves by the coupled instability forces the events to peak when this amplification is at its minimum strength, at the end of the calendar year. The mechanism is demonstrated using a simple delayed oscillator model and is further analyzed using the Cane-Zebiak model. Being based on the oversimplified delayed oscillator paradigm of ENSO, the proposed mechanism cannot be expected to fully explain the locking of observed events to the end of the year. However, the wave dynamics perspective it offers to approaching the ENSO phase-lacking problem may serve as a first step toward a fuller explanation based on more realistic models and additional data analysis.
Uvo, C. B., C. A. Repelli, S. E. Zebiak and Y. Kushnir, 1998: The relationships between tropical Pacific and Atlantic SST and northeast Brazil monthly precipitation. Journal of Climate, 11(4): 551-562.
The monthly patterns of northeast Brazil (NEB) precipitation are analyzed in relation to sea surface temperature (SST) in the tropical Pacific and Atlantic Oceans, using singular value decomposition. It is found that the relationships between precipitation and SST in both basins vary considerably throughout the rainy season (February-May). In January, equatorial Pacific SST is weakly correlated with precipitation in small areas of southern NEB, but Atlantic SST shows no significant correlation with regional precipitation. In February, Pacific SST is not well related to precipitation, but south equatorial Atlantic SST is positively correlated with precipitation over the northern Nordeste, the latter most likely reflecting an anomalously early (or late) southward migration of the ITCZ precipitation zone. During March, equatorial Pacific SST is negatively correlated with Nordeste precipitation, but no consistent relationship between precipitation and Atlantic SST is found. Atlantic SST-precipitation correlations for April and May are the strongest found among all months or either ocean. Precipitation in the Nordeste is positively correlated with SST in the south tropical Atlantic and negatively correlated with SST in the north tropical Atlantic. These relationships are strong enough to determine the structure of the seasonal mean SST-precipitation correlations, even though the corresponding patterns for the earlier months of the season are quite different. Pacific SST-precipitation correlations for April and May are similar to those for March. Extreme wet (dry) years for the Nordeste occur when both Pacific and Atlantic SST patterns for April and May occur simultaneously. A separate analysis reinforces previous findings in showing that SST in the tropical Pacific and the northern tropical Atlantic are positively correlated and that tropical Pacific-south Atlantic correlations are negligible.
Visbeck, M., H. Cullen, G. Krahmann and N. H. Naik(Henderson), 1998: An ocean model's response to North Atlantic Oscillation-like wind forcing. Geophysical Research Letters, 25(24): 4521-4524.
The response of the Atlantic Ocean to North Atlantic Oscillation (NAO)-like wind forcing was investigated using an ocean-only general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the interannual to multi-decadal frequency response of the ocean to a winter wind anomaly pattern. Overall, the strength of the SST response increased slightly with longer forcing periods. In the subpolar gyre, however, the model showed a broad response maximum in the decadal band (12-16 years).
Zappa, C. J., A. T. Jessup and H. Yeh, 1998: Skin layer recovery of free-surface wakes: Relationship to surface renewal and dependence on heat flux and background turbulence. Journal of Geophysical Research-Oceans, 103(C10): 21711-21722.
The thermal signatures of free-surface wakes observed in the open ocean show that the recovery of the cool skin layer is related to the degree of surface mixing and to ambient environmental conditions. Wakes produced by two surface-piercing cables of O(10(-2) m) in diameter are analyzed using infrared imagery. Under low-wind-speed conditions when the swell and surface current were aligned, the wakes exhibited distinctive patchlike features of O(1 m) in diameter that were generated by the passage of individual waves. The time t* required by the skin layer to recover from these disturbances is compared to the surface-renewal timescale tau used in heat and gas flux models. At low wind speeds, t* is comparable to tau, but at moderate wind speeds the agreement is poor. The spatial and temporal variations in the skin temperature of these wakes are related to a wave Reynolds number used to characterize the strength of the disturbance due to the waves. The recovery process is characterized in terms of the restoring internal energy flux J(r) which is proportional to both the initial thickness and the thermal recovery rate of the skin layer and was found to be directly related to the strength of the surface disruption. Comparison of the wake results with laboratory and other field measurements of breaking waves implies that J(r) is also a strong function of the net heat flux and background turbulence, which relate directly to the existing environmental conditions such as wind stress and sea state. Our results demonstrate that J(r) may vary by several orders of magnitude, depending on the environmental conditions.
Zheng, Q., R. D. Susanto, X. H. Yan, W. T. Liu and C. R. Ho, 1998: Observation of equatorial Kelvin solitary waves in a slowly varying thermocline. Nonlinear Processes in Geophysics, 5(3): 153-165.
TOPEX/POSEIDON (T/P) sea level deviation (SLD) time series from October 3, 1992 to May 15, 1997 combined with upper ocean thermal structures are used to observe the characteristics and analyze the dynamics of equatorial waves in the Pacific Ocean. The evolution of the Kelvin wave propagating along an eastward shoaling thermocline in the equatorial Pacific is investigated. The behavior of this wave as it propagates eastward can be approximately described with the solutions of the perturbed Korteweg-de Vries (PKdV) equation and modified Green's Law. Assuming that the nonlinear term and dispersive term of this equation are balanced, the amplitude increases as the thermocline decreases to the power -3/8. Approaching the eastern Pacific, the nonlinearity increases and the relation changes to the power -9/8. The dispersion relation, and mass and energy conservations are investigated. The results indicate that under a varying thermocline, the nonlinear Kelvin solitary waves indeed exist in the real ocean.
van-de-Kreeke, J. and R. A. Iannuzzi, 1998: Second-order solution for damped cooscillating tide in narrow canal. Journal of Hydraulic Engineering-Asce, 124(12): 1253-1260.
The M-0 and M-4 tidal currents are important for tide-induced net transport of sediment. To develop a physical understanding of the processes responsible for generating M-0 and M-4, a second-order analytical solution for a damped cooscillating tide in a closed-end canal with a horizontal bottom is presented. In deriving the analytical solution, it is assumed that the governing shallow water equations are weakly nonlinear, allowing the use of a perturbation technique. For this, the friction term in the momentum equation is linearized by requiring the tidally and spatially averaged energy dissipation by M-2 to be the same for the nonlinear and linearized friction. To determine the accuracy, for selected canals, the analytical solution is compared with numerical solutions. For M-0 and M-4, numerical and analytical values are within 20%. The analytical solution is used to demonstrate the internal generation of M-0 and M-4 through the nonlinear terms in the equations.
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