Bromwich, D. H., F. M. Robasky, R. I. Cullather and M. L. Vanwoert, 1995: Atmospheric Hydrologic-Cycle over the Southern-Ocean and Antarctica from Operational Numerical-Analyses. Monthly Weather Review, 123(12): 3518-3538.
Moisture budget calculations for Antarctica and the Southern Ocean (40 degrees-72 degrees S) are performed using operational numerical analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Meteorological Center (NMC), and the Australian Bureau of Meteorology (ABM). The analyses are intercompared for an 8-yr period from 1985 to 1992 and are evaluated against representative rawinsonde sites, which are considered accurate depictions of moisture transport at these sites.
Byrne, D. A., A. L. Gordon and W. F. Haxby, 1995: Agulhas Eddies - a Synoptic View Using Geosat Erm Data. Journal of Physical Oceanography, 25(5): 902-917.
Warm core rings formed in the Agulhas Retroflection transfer water from the Indian Ocean to the South Atlantic. In an attempt to measure the strength of this exchange, a combination of satellite altimeter and hydrographic data are used to examine Agulhas eddy paths and decay rates in the South Atlantic, Because the surface dynamic height of a warm core eddy is higher than surrounding waters the rings are visible in satellite altimeter measurements. Over 20 Agulhas eddies have been tracked from maps of anomalous sea surface height (SSH) derived from the Geosat Exact Repeat Mission (ERM) dataset. The correlation (r(2)) of dynamic height referenced to 2000 dbar and anomaly SSH for one coincidentally sampled area is 97% within an Agulhas eddy, dropping to a fraction of that outside of it, indicating that the SSH anomaly signal is a reliable measure for strong features like Agulhas eddies.
Bürger, G., S. E. Zebiak and M. A. Cane, 1995a: Quasi-Fixed Points and Periodic-Orbits in the Zebiak-Cane ENSO Model with Applications in Kalman Filtering .1. Monthly Quasi-Fixed Points. Monthly Weather Review, 123(9): 2802-2813.
In an effort to apply the interactive Kalman filter to higher-dimensional systems, the concept of a quasi-fixed point is introduced. This is defined to be a system state where the tendency, in a suitable reduced space, is at a minimum. It allows one to use conventional search algorithms for the detection of quasi-fixed points. In Part I quasi-fixed points of the ENSO model of Zebiak and Cane are found when run in a permanent monthly mode, the reduced space being defined via a multiple EOF projection. The stability characteristics of the quasi-fixed points are analyzed, and it is shown that they are significantly different from the (in)stabilities of the average monthly models. With these quasi-fixed points, assimilation experiments are carried out with the interactive Kalman filter for the Zebiak-Cane model in the reduced space. It is demonstrated that the results are superior to both a seasonal Kalman filter and the extended Kalman filter.
Bürger, G., S. E. Zebiak and M. A. Cane, 1995b: Quasi-Fixed Points and Periodic-Orbits in the Zebiak-Cane ENSO Model with Applications in Kalman Filtering .2. Periodic-Orbits. Monthly Weather Review, 123(9): 2814-2824.
In part II of this study on the application of the interactive Kalman filter to higher-dimensional systems, a modification suited to periodically forced systems is introduced. As in Part I, the object of study here is the ENSO model of Zebiak and Cane, but here the technique of quasi-fixed points is applied to certain Poincare maps of that system that are related to the forcing period of 1 year. As a result, it is possible to search the model systematically for possible periodic orbits, no matter whether they are stable or unstable. An unstable 4-year cycle is found in the model, and it is argued that this cycle can be traced back to a 4-year limit cycle, which is known to exist under weak atmosphere-ocean coupling. All other quasi-fixed points are related to orbits that do not appear to be periodic. The findings are applied to the modified version of the interactive Kalman filter, which deals with cycles as regimes. Comparing these results with the findings in Part I, it is found that the filter performances improve using, in the following order, the extended filter, the interactive filter with cycles, a seasonal average filter, and the original interactive Kalman filter from Part I.
Chen, D., S. E. Zebiak, A. J. Busalacchi and M. A. Cane, 1995: An Improved Procedure for El Niño Forecasting - Implications for Predictability. Science, 269(5231): 1699-1702.
A coupled ocean-atmosphere data assimilation procedure yields improved forecasts of El Niño for the 1980s compared with previous forecasting procedures. As in earlier forecasts with the same model, no oceanic data were used, and only wind information was assimilated. The improvement is attributed to the explicit consideration of air-sea interaction in the initialization. These results suggest that El Niño is more predictable than previously estimated, but that predictability may vary on decadal or longer time scales. This procedure also eliminates the well-known spring barrier to El Niño prediction, which implies that it may not be intrinsic to the real climate system.
Clement, A. C. and A. L. Gordon, 1995: The Absolute Velocity-Field of Agulhas Eddies and the Benguela Current. Journal of Geophysical Research-Oceans, 100(C11): 22591-22601.
Acoustic Doppler current profiler (ADCP) data referenced to Global Positioning System navigation were obtained in May 1993 from the Royal Research ShipDiscovery within the Benguela Current as part of the Benguela Source and Transport (BEST) project. These data are used in combination with hydrographic data collected during the cruise to investigate the absolute geostrophic velocities in the Benguela Current and the transient eddies. Four anticyclonic eddies were encountered during the cruise, of which three were determined to be Agulhas Retroflection eddies of various ages and one was determined to be an eddy derived from the Brazil Current. ADCP velocities averaged between conductivity-temperature-depth stations have a high linear correlation with geostrophic velocities derived from the hydrographic data (correlation coefficient of 0.93) along the entire cruise track. The magnitudes of the two velocity estimates, however, were notably different within the eddies. It was determined that these discrepancies are probably due to a significant barotropic component of the flow near the eddy center. As much as 50% of the total flow in the eddy is barotropic. The horizontal length scale (radius of maximum velocity) of this eddy determined from both the ADCP data and the thermal field was found to be approximately 60 km, considerably smaller than previous estimates, which are about 120 km. The barotropic component in Agulhas eddies leads to an equal partitioning of total mechanical energy between available potential and kinetic energy. It is also expected to have a significant effect on the climatically important exchange of mass between the Indian and South Atlantic Oceans. Total geostrophic velocities were computed for the Benguela Current using the averaged ADCP at 250 m as a reference. The ADCP referenced geostrophic transport across 30 degrees S of water warmer than 9 degrees C in the Benguela Current was found to be 17 Sv (1 Sv= 10(6) m(3) s(-1)) to the north and that of the upper kilometer was 25 Sv to the north. These values are largely consistent with previous estimates, suggesting that the upper layer flow across this section is dominated by the baroclinic field.
Gordon, A. L. and B. A. Huber, 1995: Warm Weddell Deep-Water West of Maud Rise. Journal of Geophysical Research-Oceans, 100(C7): 13747-13753.
A pool of relatively warm (>1.0 degrees C) Weddell Deep Water (WDW) immediately west of Maud Rise appears to be a quasi-stationary feature. The ''warm pool'' is derived from the flow of warm WDW around the flanks of Maud Rise. An austral spring 1989 expedition of the RN Akademik Fedorov obtained detailed measurements of the warm pool. It displays the general regional relationship of temperature maximum (t-max) warmth to shallowness of the pycnocline, as well as mixed layer oxygen concentration well below full saturation. Depression of oxygen values is a product of injection of WDW into the mixed layer during the ice-covered winter period. Associated with this transfer are high vertical fluxes of heat and salt which limit the thickness of the sea ice cover. In the winter the atmosphere is sufficiently cold to remove the WDW heat without massive sea ice melting, though the regional ice thickness is restricted. In the spring the atmospheric conditions cannot remove the ocean hear, and ice melting ensues before the atmospheric heat budget alone can account for the ice melt. This is clearly seen in the warm pool as very low mixed layer oxygen, which is a reflection of high WDW entrainment and vertical heat flux and is normally associated with high mixed layer salinity. Here it is coupled instead with reduced salinity, a result of approximately 0.5 m of ice melt. The Fedorov data confirm the role of oceanic heat flux in early removal of Southern Ocean sea ice at the end of winter, in that the region of highest WDW entertainment is associated with greater amounts of melt water.
Gordon, A. L., 1995: When Is Appearance Reality - a Comment on Why Does the Indonesian Throughflow Appear to Originate from the North Pacific. Journal of Physical Oceanography, 25(6): 1560-1567.
The transfer of water from the Pacific to the Indian Oceans within the Indonesian Seas is comprised primarily of North Pacific water masses. To state that this water is in reality South Pacific water and that it only ''appears'' to be North Pacific water is misleading and does not properly reflect the large-scale climate role of the North Pacific Ocean.
Gordon, A. L., K. T. Bosley and F. Aikman, 1995: Tropical Atlantic Water within the Benguela Upwelling System at 27-Degrees-S. Deep-Sea Research Part I-Oceanographic Research Papers, 42(1): 1-12.
A CTD-O-2 and ADCP section across the African Atlantic continental margin near 27 degrees S, obtained during R.R.S. Discovery cruise 165B in May 1987, reveals the water mass structure and associated velocity field of the shelf and upper slope of the Benguela upwelling system. Continental shelf water upwelling within the Benguela Current is drawn from the 12 degrees C (about 200 m) level. The upwelling water is drawn from oxygen depleted, tropical South Atlantic thermocline water that is advected along the shelf floor by a southward flowing subsurface current. Lower thermocline and intermediate water from the tropical South Atlantic are also observed flowing southward over the continental slope. Tropical Atlantic water generally resides north of the Angola-Benguela Front at 16 degrees S. A narrow band of upwelled water is observed well seaward of the shelf, along the western edge of a large Agulhas eddy, indicating that Agulhas eddies play a role in stirring eastern boundary upwelled water into the ocean interior. These eddies also draw into the interior tropical Atlantic water found over the upper continental slope. The net transport between the 120 and 350 isobaths as measured by the ship-mounted ADCP, referenced to the sea floor, is 0.9 x 10(6) m(3) s(-1) to the south, with 1.6 x 10(6) m(3) s(-1) of southward Rowing tropical Atlantic water and 0.7 x 10(6) m(3) s(-1) of northward Bowing upwelled surface water. The tropical thermocline water mass advected to the south is not observed offshore within the northward flowing Benguela Current, in an unaltered state, thus the 0.9 x 10(6) m(3) s(-1) must feed shelf upwelling south of 27 degrees S, implying a net offshore flux of upwelled water between Luderitz (26 degrees) and Cape Columbine (33 degrees S).
Hoerling, M. P., M. F. Ting and A. Kumar, 1995: Zonal Flow-Stationary Wave Relationship During El Niño - Implications for Seasonal Forecasting. Journal of Climate, 8(7): 1838-1852.
An analysis of the Northern Hemispheric zonal mean flow anomalies during El Nino is performed, and the dynamical effect of such atmospheric flows on the wintertime climatological stationary waves over the Pacific/ North American (PNA) region is assessed. Only in the subtropical latitudes can one identify a statistically significant zonal flow anomaly during the El Ninos of the historical record, Strong zonal flow anomalies in the midlatitudes are observed during individual El Nino events, although these appear to be manifestations of chaotic atmospheric behavior. The observational results are confirmed by GCM climate simulations using prescribed SSTs for the 1982-93 period. The principal SST-forced zonal flow signal in these experiments is located on the equatorward flank of the subtropical jet.
Houghton, R. W., 1995: The Bottom Boundary-Layer Structure in the Vicinity of the Middle Atlantic Bight Shelfbreak Front. Continental Shelf Research, 15(10): 1173-1194.
The velocity structure in the bottom boundary layer (BBL) in the Middle Atlantic Eight (MAB) is investigated using data derived from the Shelf Edge Exchange Processes (SEEP-II) experiment to test theoretical predictions of the effects of cross-shelf buoyancy flux on the BBL which are thought to contribute to frontogenesis at the shelfbreak. The cross-shelf how is bottom intensified with a direction veering consistent with that of a frictional boundary layer with rotation. The BBL thickness based on veering angle ranges from 8 to 40 m with a velocity dependence complicated by stratification that is dependent on frontal position. There is evidence of the asymmetry in the BEL thickness expected for the cross-shelf buoyancy flux during up- and downwelling events. However, the velocity structure does not define a sharp upper boundary to the BBL as given by simple one-dimensional model calculations and suggests that there must be other sources of turbulence in addition to bottom friction.
Huang, H. P. and W. A. Robinson, 1995: Barotropic Model Simulations of the North Pacific Retrograde Disturbances. Journal of the Atmospheric Sciences, 52(10): 1630-1641.
Branstator-Kushnir-type large-scale westward propagating waves are investigated using linear and nonlinear global barotropic models with an idealized zonally asymmetric basic state. Retrograde waves are found in the most unstable normal mode of the zonally asymmetric basic state with a jet in the Northern Hemisphere. Westward propagating waves also exist in nonlinear equilibrium states under a wide range of supercriticality and in both periodic and chaotic regimes. The frequency of the most unstable mode remains as a peak in the frequency spectrum through the nonlinear equilibration process. That frequency matches the frequency of the westward propagating waves in the nonlinear equilibrium states. Local energetics analyses of the linear and nonlinear cases show that the barotropic energy conversion concentrated in the jet exit supplies the perturbation energy of the disturbances ah over the globe. Under a traditional spherical-harmonic decomposition, the westward propagating waves consist of several spherical-harmonic components. In the weakly chaotic nonlinear equilibrium states, these components show higher regularity in time than the others and may possess higher predictability.
Muench, R. D. and A. L. Gordon, 1995: Circulation and Transport of Water Along the Western Weddell Sea Margin. Journal of Geophysical Research-Oceans, 100(C9): 18503-18515.
Ocean current, temperature, and salinity data obtained from the western Weddell Sea during the austral winter 1992 U.S.-Russian drifting ice station experiment Ice Station Weddell 1 (ISW-1) are used to describe water circulation and transport. Surface-to-bottom baroclinic currents were computed by applying the geostrophic approximation to derived density data. These were corrected using current measurements obtained from drifting current meter arrays, and the resulting total currents were vertically integrated to obtain volume transports. Transport was found to be northward in the region, which encompassed the western boundary current of the cyclonic Weddell Sea gyre. This northward transport increased from south to north by more than a factor of 2, from about 12 x 10(6) m(3) s(-)1 in the southwestern Weddell to about 28 x 10(6) m(3) s(-)1 farther north. The increase in northward transport was compensated for by westward flow from the interior of the gyre into the western boundary region. About 5-6 x 10(6) m(3) s(-1) of the northward transport was contained in a 300-500 m thick bottom layer of cold water. This layer, whose transport increased by about 1 x 10(6) m(3) s(-1) from south to north, was identifiable by its water mass characteristics as Weddell Sea Bottom Water originating on the southwestern and western shelf regions. Its north flowing volume was consistent with past estimates of a 1.5-2 x 10(6) m(3) s(-1) production rate coupled with a 300-400% transport increase due to entrainment during downslope flow from the shelves to the deep basin. The maximum (northernmost) northward transport, less the bottom water transport, is consistent with previous estimates for wind-driven transport in the Weddell Gyre provided that bottom friction and the sea ice influence on wind forcing are taken into consideration.
Murtugudde, Ragu , Mark Cane and Vishwanath Prasad, 1995: A Reduced-Gravity, Primitive Equation, Isopycnal Ocean GCM: Formulation and Simulations. Monthly Weather Review, 123(9): 2864-2887.
A reduced gravity, primitive equation, ocean GCM with an isopycnal vertical coordinate is developed. A "buffer" layer is introduced to allow the mixed layer to detrain mass at arbitrary densities without the coordinate drift or the heat loss suffered by other isopycnal models. The diapycnal velocity is derived from the thermodynamic equation. Negative layers are removed by a heat- and mass-conserving convective adjustment scheme. The model formulation on a ? plane employs an A grid and allows irregular coastlines and local grid stretching.
Naik(Henderson), N. H., M. A. Cane, S. Basin and M. Israeli, 1995: A Solver for the Barotropic Mode in the Presence of Variable Topography and Islands. Monthly Weather Review, 123(3): 817-832.
A scheme is presented for solving the equation for barotropic ocean circulation, taking into account the special character of the problem: nearly inviscid motion following f/H contours in the ocean interior, with viscous effects closing the flow near western boundaries. Using a special compact finite-difference discretization, the scheme generates boundary layers without spurious oscillations and without demanding very high resolution. Sharp changes in topography and closed f/H contours (e.g., in the vicinity of high sea mounts) are also handled by the scheme in a way that localizes errors due to underresolved topographic features. Strategies are formulated for simplifying the connectedness of the domain by ''sinking'' the islands.
Rind, D., R. Healy, C. Parkinson and D. Martinson, 1995: The Role of Sea-Ice in 2x Co2 Climate Model Sensitivity .1. The Total Influence of Sea-Ice Thickness and Extent. Journal of Climate, 8(3): 449-463.
As a first step in investigating the effects of sea ice changes on the climate sensitivity to doubled atmospheric CO2, the authors use a standard simple sea ice model while varying the sea ice distributions and thicknesses in the control run. Thinner ice amplifies the atmospheric temperature sensitivity in these experiments by about 15% (to a warming of 4.8 degrees C), because it is easier for the thinner ice to be removed as the climate warms. Thus, its impact on sensitivity is similar to that of greater sea ice extent in the control run, which provides more opportunity for sea ice reduction. An experiment with sea ice not allowed to change between the control and doubled CO2 simulations illustrates that the total effect of sea ice on surface air temperature changes, including cloud cover and water vapor feedbacks that arise in response to sea ice variations, amounts to 37% of the temperature sensitivity to the CO2 doubling, accounting for 1.56 degrees C of the 4.17 degrees C global warming. This is about four times larger than the sea ice impact when no feedbacks are allowed. The different experiments produce a range of results for southern high latitudes with the hydrologic budget over Antarctica implying sea level increases of varying magnitude or no change. These results highlight the importance of properly constraining the sea ice response to climate perturbations, necessitating the use of more realistic sea ice and ocean models.
Robertson, R., L. Padman and M. D. Levine, 1995: Fine-Structure, Microstructure, and Vertical Mixing Processes in the Upper Ocean in the Western Weddell Sea. Journal of Geophysical Research-Oceans, 100(C9): 18517-18535.
The upward flux of heat from the subsurface core of Warm Deep Water (WDW) to the perennially ice-covered sea surface over the continental slope in the western Weddell Sea is estimated using data obtained during February-June 1992 from a drifting ice station. Through the permanent pycnocline the diapycnal heat flux is estimated to be about 3 W m(-2), predominantly because of double-diffusive convection. There is no evidence that shear-driven mixing is important in the pycnocline. The estimated mean rate of heat transfer from the mixed layer to the ice is 1.7 W m(-2), although peak heat fluxes of up to 15 W m(-2) are found during storms. It is hypothesized that isopycnal mixing along sloping intrusions also contributes to the loss of heat from the WDW in this region; however, we are unable to quantify the fluxes associated with this process. Intrusions occur intermittently throughout this experiment but are most commonly found near the boundary of the warm-core current and the shelf-modified water to the east. These heat fluxes are significantly lower than the basin-averaged value of 19 W m(-2) (Fahrbach et al., 1994) that is required to balance the heat budget of the Weddell Gyre. Other studies suggest that shelf processes to the west of the ice station drift track and more energetic double-diffusive convection in the midgyre to the east could account for the difference between our flux estimates for this region and those based on the basin-scale heat budget.
Seager, R. and S. E. Zebiak, 1995: Simulation of Tropical Climate with a Linear Primitive Equation Model. Journal of Climate, 8(10): 2497-2520.
The tropical climate simulated with a new global atmosphere model is presented. The model is purposely designed for climate studies and is still under development. It is designed to bridge the gap between very efficient but simple models of the tropical atmosphere and sophisticated but inefficient general circulation models (GCMs). In this paper the authors examine the sensitivity of the model's climate to specific formulations of convection, boundary-layer physics, and radiation.
Seager, R., M. B. Blumenthal and Y. Kushnir, 1995: An Advective Atmospheric Mixed-Layer Model for Ocean Modeling Purposes - Global Simulation of Surface Heat Fluxes. Journal of Climate, 8(8): 1951-1964.
A simple model of the lowest layer of the atmosphere is developed for coupling to ocean models used to simulate sea surface temperature (SST). The model calculates the turbulent fluxes of sensible and latent heat in terms of variables that an ocean model either calculates (SST) oris forced by (winds). It is designed to avoid the need to specify observed atmospheric data (other than surface winds), or the SST, in the surface flux calculations of ocean models and, hence, to allow a realistic representation of the feedbacks between SST and the fluxes. The modeled layer is considered to be either a dry convective layer or the subcloud layer that underlies marine clouds. The turbulent fluxes are determined through a balance of horizontal advection and diffusion, the surface flux and the flux at the mixed layer top, and, for temperature, radiative cooling. Reasonable simulations of the global distribution of latent and sensible heat flux are obtained. This includes the large fluxes that occur east of the Northern Hemisphere continents in winter that were found to be related to both diffusion (taken to be a parameterization of baroclinic eddies) and advection of cold, dry air from the continent. However, east of North America during winter the sensible heat flux is underestimated and, generally, the region of enhanced fluxes does not extend far enough east compared to observations. Reasons for these discrepancies are discussed and remedies suggested.
Seager, R., Y. Kushnir and M. A. Cane, 1995: On heat flux boundary conditions for ocean models. Journal of Physical Oceanography, 25(12): 3219-3230.
Recent modeling studies of thermohaline variability have imposed rapid damping of modeled sea surface temperature (SST) anomalies equivalent to assuming the atmosphere has an infinite heat capacity. Such surface heat Bur parameterizations effectively exclude the possibility of SST playing an active role in the thermohaline circulation. The authors present results of simple thermodynamic modeling of the lower atmosphere that suggest the sensitivity of the surface heat fluxes to variations in SST is much smaller than often assumed. It is found that the flux response is strongly dependent on the scale of the SST anomaly. For the very largest scales the fluxes increase by only a few watts per square meter per kelvin change of SST. For the scales typical of observed anomalies the nonlocality of the response enhances the sensitivity, which may reach up to similar to 15 W m(-2) K-1. This extreme is still less than half of the values typically assumed in ocean models. The small sensitivity arises from the adjustment of the lower atmosphere to the underlying ocean in accord with its relatively much smaller ability to store heat and moisture. The increase in fluxes with SST is dominated by the latent heat Aux but offset significantly by reduced net longwave radiative cooling of the surface.
Ting, M. F. and S. L. Peng, 1995: Dynamics of the Early and Middle Winter Atmospheric Responses to the Northwest Atlantic SST Anomalies. Journal of Climate, 8(9): 2239-2254.
The differences between early and middle winter atmospheric responses to the sea surface temperature anomalies (SSTA) in the northwest Atlantic are examined using a linear baroclinic model. Using a global spectral model, Peng et al. found a positive height anomaly in the perpetual November and a negative height anomaly in the Perpetual January experiments in response to a warm SSTA over the northwest Atlantic. These height anomalies are found to be associated with the reduced Atlantic jet stream in November and enhanced jet in January. Linear model diagnostics suggest that the difference in jet stream response may induce anomalous storm track eddy vorticity fluxes, which in turn maintain the different atmospheric responses under the early and middle winter conditions.
Tziperman, E., M. A. Cane and S. E. Zebiak, 1995: Irregularity and Locking to the Seasonal Cycle in an ENSO Prediction Model as Explained by the Quasi-Periodicity Route to Chaos. Journal of the Atmospheric Sciences, 52(3): 293-306.
The behavior of the Cane-Zebiak ENSO prediction model is analyzed as a function of model parameters measuring the strength of coupling between the model ocean and atmosphere and the amplitude of the background seasonal cycle specified in the model. As either of these two parameters is increased, the model undergoes a transition from periodic to chaotic behavior according to the universal quasi-periodicity route to chaos. Thus, the irregularity of model ENSO events and their partial locking to the seasonal cycle can both be explained as low-order chaotic behavior driven by the seasonal cycle. The chaos is due to irregular jumping of the Pacific natural ocean-atmosphere oscillator between different nonlinear resonances with the seasonal forcing.
Ukita, J. and R. E. Moritz, 1995: Yield Curves and Flow Rules of Pack Ice. Journal of Geophysical Research-Oceans, 100(C3): 4545-4557.
A theoretical framework is developed, which relates small-scale pack ice energy transformations dominated by ridging and sliding processes to large-scale dynamics described by internal ice stress and strain rate. The framework consists of an energy equation, a kinematic model, and the minimization of maximum shear stress. From the kinematic model and energy equation we derive an expression for the maximum shear stress and compressive stress in terms of a deformational pattern and geometry of cracks. For each choice of a kinematic model the minimization principle applied to this expression gives an explicit constitutive relationship as follows: a yield curve, a flow rule, and the directional relationship between stress and strain rate. The theory provides an energetic explanation for different yield curves and flow rules. The cavitating fluid is realized with the absence of energetic transformations on shear and divergent motions. Hibler's (1979) constitutive relationship corresponds to the energetic function whose form depends on the strain rate magnitude, reflecting the viscous-plastic coupling. It is found that for a class of energy functions associated with smooth and strictly convex yield curves the flow rule is normal. The Mohr-Coulomb yield criterion is identified as a special case of the normal flow rule. Preferred orientation of cracks is also explained by the same minimization of the maximum shear stress. The theory predicts that the preferred orientation depends on ice geometry, as well as the energetic contribution from sliding relative to ridging. For uniform square- and diamond-shaped floes the maximum shear stress associated with ridging obtains its minimum value when the axes of symmetry of the floes coincide with the principal axes of the strain rate. A field of isotropically oriented square-shaped floes is simulated, resulting in a sine lens-shaped yield curve.
Visbeck, M. and J. Fischer, 1995: Sea-Surface Conditions Remotely-Sensed by Upward-Looking Adcps. Journal of Atmospheric and Oceanic Technology, 12(1): 141-149.
Surface data obtained from 153-kHz acoustic Doppler current profilers deployed in the Greenland Sea at about 350-m depth during the winter of 1988/89 were investigated under several aspects. First a method is described to improve the instrument depth measurements using the binned backscattered energy profile near the surface. The accurcy of the depth estimates is found to be significantly better than 0.5 m.
Visbeck, M., J. Fischer and F. Schott, 1995: Preconditioning the Greenland Sea for Deep Convection - Ice Formation and Ice Drift. Journal of Geophysical Research-Oceans, 100(C9): 18489-18502.
The role of sea ice in preconditioning the mixed layers of the central Greenland Sea for deep convection is investigated, with particular emphasis on the formation of the ''Nordbukta.'' The opening of the ice free bay in late January 1989 indicated that the upper layer was well preconditioned for deep convection which reached down to 1500 m depth in March 1989. We propose that the ice free bay occurred due to diminishing new ice formation without extensive ice melt. A key process is wind-driven ice drift to the southwest, as observed by upward looking acoustic Doppler current profilers, which will alter the upper ocean freshwater budget when an ice volume gradient along the ice-drift direction exists. We investigated the importance and effects of such an ice-drift-induced freshwater loss on upper ocean properties using an ice-ocean mixed-layer model. Observed temperature and salt profiles from December 1988 served as initial conditions, and the model was integrated over the winter season. Given the one-dimensional physics and climatological surface fluxes, the model was not able to produce a reasonable ice and mixed-layer evolution. However, allowing ice drift to reduce the local ice thickness improved the ice-ocean model performance dramatically. An average ice export of 5-8 mm d(-1) was needed to be consistent with the observed evolution of mixed-layer properties and ice cover. Using the same fluxes and ice export, but initial conditions from the ''Is Odden'' region, yielded ice cover throughout the winter over a shallow mixed layer, both of which are consistent with the observations from the Odden region.
Wang, Z. , J. Bao and C. Dayong, 1995: A Coupled Model of Ocean-Atmosphere and Its Experiment. Acta Oceanologica Sinica, Vol.17(No.5): 50-58.
A simple coupled model of atmosphere-ocean within limit area is designed in this paper. The model is used to simulate the ENSO circulation in the tropical Pacific Ocean. It is made up of a layer of atmospheric mode and a layer of oceanic mode. The former includes the equations of movement, heating, and state; the latter includes the equations of movement, quality, and SSTa. The tmospheric mode and the oceanic mode are coupled by the diabetic heating and the wind stress. The atmospheric mode affects the oceanic mode mainly by the wind stress, and the oceanic mode affects the atmospheric mode mainly by the diabetic heating.
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