Deveaux, R. D., A. L. Gordon, J. C. Comiso and N. E. Bacherer, 1993: Modeling of Topographic Effects on Antarctic Sea-Ice Using Multivariate Adaptive Regression Splines. Journal of Geophysical Research-Oceans, 98(C11): 20307-20319.
The role of seafloor topography in the spatial variations of the southern ocean sea ice cover as observed (every other clay) by the Nimbus 7 scanning multichannel microwave radiometer satellite in the yews 1980, 1983, and 1984 is studied. Bottom bathymetry can affect sea ice surface characteristics because of the basically barotropic circulation of the ocean south of the Antarctic Circumpolar current. The main statistical tool used to quantify this effect is a local nonparametric regression model of sea ice concentration as a function of the depth and its first two derivatives in both meridional and zonal directions. First, we model the relationship of bathymetry to sea ice concentration in two study areas, one over the Maud Rise and the other over the Ross Sea shelf region. The multiple correlation coefficient is found to average 44% in the Maud Rise study area and 62% in the Ross Sea study area over the years 1980, 1983, and 1984. Second, a strategy of dividing the entire Antarctic region into an overlapping mosaic of small areas, or windows, is considered. Keeping the windows small reduces the correlation of bathymetry with other factors such as wind, sea temperature, and distance to the continent. We find that although the form of the model varies from window to window due to the changing role of other relevant environmental variables, we are left with a spatially consistent ordering of the relative importance of the topographic predictors. For a set of three representative days in the Austral winter of 1980, the analysis shows that an average of 54% of the spatial variation in sea ice concentration over the entire ice cover can be attributed to topographic variables. The results thus support the hypothesis that there is a sea ice to bottom bathymetry link. However, this should not undermine the considerable influence of wind, current, and temperature which affect the ice distribution directly and are partly responsible for the observed bathymetric effects.
Fischer, J. and M. Visbeck, 1993: Deep Velocity Profiling with Self-Contained Adcps. Journal of Atmospheric and Oceanic Technology, 10(5): 764-773.
Ocean deep velocity profiles were obtained by lowering a self-contained 153.6-kHz acoustic Doppler current profiler (ADCP) attached to a CTD-rosette sampler. The data were sampled during two Meteor cruises in the western tropical Atlantic.
Fischer, J. and M. Visbeck, 1993: Seasonal-Variation of the Daily Zooplankton Migration in the Greenland Sea. Deep-Sea Research Part I-Oceanographic Research Papers, 40(8): 1547-1557.
Moored Acoustic, Doppler Current Profilers (ADCPs) were used to analyse the daily vertical zooplankton migration and its seasonality. One-year records of vertical velocity and acoustic backscatter were obtained at four stations in the Greenland Sea. Both parameters exhibited a diurnal cycle typical for vertically migrating zooplankton. Upward and downward migration occurred in short periods approximately 5 h long, and peak migration velocities were around 1.5 cm s-1.
Gordon, A. L., B. A. Huber, H. H. Hellmer and A. Ffield, 1993: Deep and Bottom Water of the Weddell Seas Western Rim. Science, 262(5130): 95-97.
Oceanographic observations from the Ice Station Weddell 1 show that the western rim of the Weddell Gyre contributes to Weddell Sea Bottom Water. A thin (<300 meters), highly oxygenated benthic layer is composed of a low-salinity type of bottom water overlying a high-salinity component. This complex layering disappears near 66-degrees-S because of vertical mixing and further inflow from the continental margin. The bottom water flowing out of the western rim is a blend of the two types. Additionally, the data show that a narrow band of warmer Weddell Deep Water hugged the continental margin as it flowed into the western rim, providing the continental margin with the salt required for bottom-water production.
Imbrie, J., A. Berger, E. A. Boyle, S. C. Clemens, A. Duffy, W. R. Howard, G. Kukla, J. Kutzbach, D. G. Martinson, A. Mcintyre, A. C. Mix, B. Molfino, J. J. Morley, L. C. Peterson, N. G. Pisias, W. L. Prell, M. E. Raymo, N. J. Shackleton and J. R. Toggweiler, 1993: On the Structure and Origin of Major Glaciation Cycles .2. The 100,000-Year Cycle. Paleoceanography, 8(6): 699-735.
Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (similar to 15,000 years) much larger than the other cycles (similar to 5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations.
Imbrie, J., A. C. Mix and D. G. Martinson, 1993: Milankovitch Theory Viewed from Devils Hole. Nature, 363(6429): 531-533.
VARIATIONS in the oxygen isotope content (deltaO-18) of late Quaternary deep-sea sediments mainly reflect changes in continental ice mass1, and hence provide important information about the timing of past ice ages. Because these sediments cannot yet be dated directly beyond the range of radiocarbon dating (40-50 kyr), ages for the deltaO-18 record have been generated2,3 by matching the phase of the changes in deltaO-18 to that of variations in the Earth's precession and obliquity. Adopting this timescale yields a close correspondence between the time-varying amplitudes of these orbital variations and those of a wide range of climate proxies4, lending support to the Milankovitch theory that the Earth's glacial-interglacial cycles are driven by orbital variations. Recently Winograd et al.5 reported a record of deltaO-18 variations in a fresh-water carbonate sequence from Devils Hole, Nevada, dated by U-Th disequilibrium6. They concluded that the timing of several of the features in the record, which reflects changes in the temperature of precipitation over Nevada as well as changes in the isotopic composition of the moisture source5,7, showed significant deviations from that predicted by Milankovitch theory. Here we demonstrate that applying the Devils Hole chronology to ocean cores requires physically implausible changes in sedimentation rate. Moreover, spectral analysis of the Devils Hole record shows clear evidence of orbital influence. We therefore conclude that transfer of the Devils Hole chronology to the marine record is inappropriate, and that the evidence in favour of Milankovitch theory remains strong.
Naik(Henderson), N. H. and J. Vanrosendale, 1993: The Improved Robustness of Multigrid Elliptic Solvers Based on Multiple Semicoarsened Grids. Siam Journal on Numerical Analysis, 30(1): 215-229.
Multigrid convergence rates degenerate on problems with stretched grids or anisotropic operators, unless one uses line or plane relaxation. For three-dimensional problems, only plane relaxation suffices, in general. While line and plane relaxation algorithms are efficient on sequential machines, they are quite awkward and inefficient on parallel machines. This paper presents a new multigrid algorithm, based on the use of multiple coarse grids, that eliminates the need for line or plane relaxation in anisotropic problems. This algorithm is developed, and the standard multigrid theory is extended to establish rapid convergence for this class of algorithms. The new algorithm uses only point relaxation, allowing easy and efficient parallel implementation, yet achieves robustness and convergence rates comparable to line and plane relaxation multigrid algorithms.
Schott, F., M. Visbeck and J. Fischer, 1993: Observations of Vertical Currents and Convection in the Central Greenland Sea During the Winter of 1988-1989. Journal of Geophysical Research-Oceans, 98(C8): 14401-14421.
During the winter of 1988-1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60-260 m, two were looking downward from 200 m, and one was looking upward from 1400 m. First maxima of vertical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline. Beginning in late November the marginal ice zone expanded eastward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the ''is odden,'' curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m-2. Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratified layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to >350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of >50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6-16, associated with downward velocities at the 1400-m level of about 3 cm s-1. One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses. However, different from the surface pool that was governed by mixed-layer physics, the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermediate depths in winter 1988-1989.
Simpson, H. J., M. A. Cane, A. L. Herczeg, S. E. Zebiak and J. H. Simpson, 1993: Annual River Discharge in Southeastern Australia Related to El Niño Southern Oscillation Forecasts of Sea-Surface Temperatures. Water Resources Research, 29(11): 3671-3680.
Annual natural discharge (Q) of the River Murray and its most extensive tributary, the Darling River system, is often inversely related to sea surface temperature (SST) anomalies in the eastern tropical Pacific Ocean. These SST variations are components of a planetary-scale phenomenon referred to as El Nino-Southern Oscillation (ENSO). Darling and Murray river historical values of Q indicate that annual surface runoff from regions dominated by subtropical summer monsoon precipitation and annual surface runoff primarily responding to temperate winter storms are both strongly influenced by ENSO cycles. Forecasting, approximately 1 year in advance, of ENSO-related SST from geophysical model calculations thus provides a mechanism for estimating probabilities of annual river discharge amount. Contingency tables relating annual Q to SST, based on combining observed data for 95 years and forecast SST over a period of 15 years, provide probabilities of expected annual Q as a function of forecast SST. The SST of the eastern tropical Pacific was successfully forecast to be appreciably warmer than long-term mean conditions for much of the year beginning in mid 1991. Precipitation data through 1991 indicated that annual natural Q for the Darling River was probably substantially below the mean. However, winter precipitation in higher-runoff portions of the Murray Basin was above average during this El Nino episode, contrary to the trend for most such events over the past century.
Simpson, H. J., M. A. Cane, S. K. Lin, S. E. Zebiak and A. L. Herczeg, 1993: Forecasting Annual Discharge of River Murray, Australia, from a Geophysical Model of ENSO. Journal of Climate, 6(2): 386-391.
Annual discharge (Q) in the largest river system in Australia, the River Murray (including the extensive tributary network of the Darling River), is often inversely related to sea surface temperature (SST) anomalies in the eastern equatorial Pacific Ocean. Conditional probability tables were constructed, with annual natural Q of the Murray for the period 1891-1985 divided into three amount categories; SST values were also divided into three groups. These tables permit probabilities of Q falling in each of three discharge categories to be estimated from either observed or forecast SST values. Using forecasts from a geophysical model, which indicated higher-than-average SST for most of calendar year 1991, natural Q of the River Murray from June 1991 to May 1992 is forecast to be in the lower half of annual discharges since 1891 (64% probability). Using similar assumptions, the probability of annual natural Q for the year beginning June 1991 falling in the highest one-third discharge category is only 21%.
Ting, M. F. and M. P. Hoerling, 1993: Dynamics of Stationary Wave Anomalies During the 1986/87 El Niño. Climate Dynamics, 9(3): 147-164.
The dynamics of the wintertime atmospheric response to the 1986/87 El Nino SST anomalies is studied. A GCM used for this purpose simulates a wave train over the Pacific/North American (PNA) region that agrees closely in amplitude with that observed, but phase shifted 30-degrees to the east. Linear baroclinic model experiments are performed in order to determine the origin of the GCM and observed stationary wave anomalies, with particular focus on the cause for GCM failure. Diagnostics with the linear model reveal that the GCM and observed wave train anomalies are maintained by very different processes. In the GCM, the forcing due to tropical diabatic heating and transient vorticity fluxes are equally important over the PNA region. In the observations, the transient vorticity fluxes assume the primary role. The cause for these discrepancies is traced to the different dynamic influences of suppressed rainfall near Indonesia. The associated diabatic cooling is found to excite a large amplitude wave train over the PNA region in the GCM, while no significant extratropical response to cooling is found in the observations. The combined effects of the diabatic cooling and the reorganization of the storm track transients by the remotely forced wave train acts to shift the GCM's wave train well to the east of that observed. Due to uncertainties in the observed diabatic forcing, however, it is not clear to what extent the GCM's failure is due to errors in the simulated anomalous forcing and/or to the GCM's mean climate error.
Ting, M. F. and N. C. Lau, 1993: A Diagnostic and Modeling Study of the Monthly Mean Wintertime Anomalies Appearing in a 100-Year GCM Experiment. Journal of the Atmospheric Sciences, 50(17): 2845-2867.
The nature of simulated atmospheric variability on monthly time scales has been investigated by analyzing the output from a 100-year integration of a spectral GCM with rhomboidal wavenumber 15 truncation. In this experiment, the seasonally varying, climatological sea surface temperature was prescribed throughout the world oceans. The principal modes of variability in the model experiment were identified by applying a rotated empirical orthogonal function (EOF) analysis to the Northern Hemisphere monthly averaged 515-mb geopotential height for the winter season (November through March). The individual leading spatial modes are similar to the observed north-south dipoles over the North Atlantic and North Pacific, as well as wavelike patterns in the Pacific/North American and Northern Asian sectors.
Ting, M. F. and P. D. Sardeshmukh, 1993: Factors Determining the Extratropical Response to Equatorial Diabatic Heating Anomalies. Journal of the Atmospheric Sciences, 50(6): 907-918.
The steady linear response of a spherical baroclinic atmosphere to an equatorial diabatic heat source having a simple horizontal and vertical structure is examined. This source is imposed upon representative zonally symmetric as well as zonally varying flows during the boreal winter. Two climatologies are considered. One is a 6-year average of global observations analyzed at the European Centre for Medium-Range Weather Forecasts (ECMWF). The other is a 30-year average, taken from a general circulation model (GCM) run at the Geophysical Fluid Dynamics Laboratory in Princeton.
Wamser, C. and D. G. Martinson, 1993: Drag Coefficients for Winter Antarctic Pack Ice. Journal of Geophysical Research-Oceans, 98(C7): 12431-12437.
This paper presents air-ice and ice-water drag coefficients referenced to 10-m-height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP-86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS-89). The optimal values of the air-ice drag coefficients, made from turbulent flux measurements, are C-10 = (1.79 +/- 0.06) x 10(-3) for WWSP-86 and (1.45 +/- 0.09) x 10(-3) for WWGS-89. Neutral drag coefficient values are C(N10) = 1.68 x 10(-3) for WWSP-86 and 1.44 x 10(-3) for WWGS-89. The slightly lower values for WWGS-89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median z0BAR = 0.47 mm for WWSP-86 and 0.27 mm for WWGS-89). These values are consistent with Arctic measurements for 80-100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice-water drag coefficient for both WWSP-86 and WWGS-89, estimated from periods of ice drift thought to represent free-drift conditions (air-ice stress balanced by ice-water drag and Coriolis force), is (1.13 +/- 0.26) x 10(-3), and the ice-water turning angle betaBAR = 18 +/- 18-degrees. This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first-year Arctic ice. Consistent with previous findings for WWSP-86, the free-drift form of the momentum balance can be used to describe the observed WWGS-89 ice drift observations by using an ''effective'' drag coefficient and turning angle that subsume the influence of ice-ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by approximately 33%.
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