Belkin, I. M. and A. L. Gordon, 1996: Southern Ocean fronts from the Greenwich meridian to Tasmania. Journal of Geophysical Research-Oceans, 101(C2): 3675-3696.
All available meridional sections have been analyzed to investigate the evolution of main fronts between 0 degrees and 150 degrees E. The central South Atlantic is featured by the Subtropical Frontal Zone (STFZ), bordered by the North and South Subtropical Fronts (NSTF and SSTF, respectively), and by the Polar Frontal Zone (PFZ), bordered by the Subantarctic and Polar Fronts (SAF and PF, respectively). This structure becomes more complex in the African sector as the Agulhas Retroflection and the bottom topography force a more convoluted pattern. The Retroflection and associated Agulhas Front (AF) press the SSTF from 38 degrees to 42 degrees-43 degrees S. Strong interactions of the AF, SSTF, and SAF with topography shift the fronts but do not obliterate them. The AF can be traced reliably up to 52 degrees E, sometimes up to 75 degrees E. The SAF is deflected from 45 degrees to 43 degrees S by the Mid-Ocean Ridge and converges with the SSTF north of the Prince Edward Islands to form a combined SSTF/SAF, This front intensifies east of 50 degrees-52 degrees E as a result of the confluence with the AF, and between 52 degrees and 65 degrees E a triple AF/SSTF/SAF (''the Crozet Front'') is observed. The PF continues along 49 degrees and 50 degrees S between the Crozet Plateau and the Ob-Lena (Conrad) Rise, passing north of Kerguelen,; nearly joining the triple Crozet Front. Downstream of the Kerguelen-Amsterdam Passage the canonical structure is being restored (SSTF, SAF, PF); however, the front parameters in the Australian sector are different from the African sector, largely because of strong air-sea interaction and cross-frontal exchanges in the Crozet-Kerguelen region. The SSTF, squeezed between the AF and SAF, loses characteristics to both. The SSTF/SAF interaction results in the Australian SAF being warmer and saltier downstream, while the SSTF becomes shallower and weaker. The Australian STF derives its characteristics mostly from the AF, thus bringing the modified Agulhas waters' to the Pacific Ocean. The newly defined North Subtropical Front (NSTF) was distinguished in the Indian Ocean between 31 degrees and 38 degrees S. The front marks the southern boundary of the subtropical salty, warm water pool of the central South Indian Ocean. The NSTF location is coincident with the position of the wind convergence between westerlies and easterlies, suggesting the possible wind-driven frontogenesis.
Camargo, S. J., B. D. Scott and D. Biskamp, 1996: The influence of magnetic fluctuations on collisional drift-wave turbulence. Physics of Plasmas, 3(11): 3912-3931.
A two-dimensional isothermal collisional drift-wave turbulence model including magnetic fluctuations is studied numerically. The model has as limits the electrostatic collisional drift-wave and two-dimensional magnetohydrodynamic systems. The electromagnetic and electrostatic regimes for thermal gradient-driven (drift-wave) turbulence are decided by the parameter <(beta)over cap> = (4 pi nT/B-2)(L(s)(2)/L(n)(2)), where L(s) and L(n) are the parallel and background profile scale lengths, respectively. Significant electromagnetic effects were found only for <(beta)over cap>similar to 10 for most parameters, and were most pronounced in the strongly adiabatic regime for drift waves. The principal effect of the magnetic fluctuations is magnetic induction in the parallel force balance for electrons, which is linear. This diminishes the adiabaticity of the system by reducing the immediacy of the dissipative coupling between the density and electrostatic potential fluctuations. The transport was still found to be dominantly electrostatic even for <(beta)over cap>=10, although its level decreased with <(beta)over cap> due to reduced coherency in the coupling between EXB velocity and density fluctuations. (C) 1996 American Institute of Physics.
Cane, M.A., A. Kaplan, R.N. Miller, B.Y. Tang, E.C. Hackert and A.J. Busalacchi, 1996: Mapping tropical Pacific sea level: Data assimilation via a reduced state space Kalman filter. J. Geophysical Research-Oceans, 101(C10): 22599-22617.
The well-known fact that tropical sea level can be usefully simulated by linear wind driven models recommends it as a realistic test problem for data assimilation schemes. Here we report on an assimilation of monthly data for the period 1975-1992 from 34 tropical Pacific tide gauges into such a model using a Kalman filter. We present an approach to the Kalman filter that uses a reduced state space representation for the required error covariance matrices. This reduction makes the calculation highly feasible. We argue that a more complete representation will be of no value in typical oceanographic practice, that in principle it is unlikely to be helpful, and that it may even be harmful if the data coverage is sparse, the usual case in oceanography. This is in part a consequence of ignorance of the correct error statistics for the data and model, but only in part. The reduced state space is obtained from a truncated set of multivariate empirical orthogonal functions (EOFs) derived from a long model run without assimilation. The reduced state space filter is compared with a full grid point Kalman filter using the same dynamical model for the period 1979-1985, assimilating eight tide guage stations and using an additional seven for verification [Miller et al., 1995]. Results are not inferior to the full grid point filter, even when the reduced filter retains only nine EOFs. Five sets of reduced space filter assimilations are run with all tide gauge data for the period 1975-1992. In each set a different number of EOFs is retained: 5, 9, 17, 32, and 93, accounting for 60, 70, 80, 90, and 99% of the model variance, respectively. Each set consists of 34 runs, in each of which one station is withheld for verification. Comparing each set to the nonassimilation run, the average rms error at the withheld stations decreases by more than 1 cm. The improvement is generally larger for the stations at lowest latitudes. Increasing the number of EOFs increases agreement with data at locations where data are assimilated; the added structures allow better fits locally. In contrast, results at withheld stations are almost insensitive to the number of EOFs retained. We also compare the Kalman filter theoretical error estimates with the actual errors of the assimilations. Features agree on average, but not in detail, a reminder of the fact that the quality of theoretical estimates is limited by the quality of error models they assume. We briefly discuss the implications of our work for future studies, including the application of the method to full ocean general circulation models and coupled models.
Clement, A. C., R. Seager, M. A. Cane and S. E. Zebiak, 1996: An ocean dynamical thermostat. Journal of Climate, 9(9): 2190-2196.
The role of ocean dynamics in the regulation of tropical sea surface temperatures (SSTs) is investigated using the Zebiak-Cane coupled ocean-atmosphere model. The model is forced with a uniform heating, or cooling, varying between +/-40 W m(-2) into the ocean surface. a new climatological SST pattern is established for which the area-averaged temperature change is smaller in magnitude than the imposed forcing. The forcing is balanced almost equally by a change in the heat flux out of the ocean and by vertical advection of heat in the ocean through anomalous equatorial ocean upwelling. The generation of anomalous upwelling is identified here as a possible mechanism capable of regulating tropical SSTs. This ocean dynamical thermostat mechanism has a seasonally varying efficiency that causes amplification (weakening) of the seasonal cycle for the heating (cooling). The interannual variability also changes under the imposed forcing. These results suggest that the role of ocean dynamics should be included in any discussion of the regulation of the tropical climate.
Comiso, J. C. and A. L. Gordon, 1996: Cosmonaut polynya in the Southern Ocean: Structure and variability. Journal of Geophysical Research-Oceans, 101(C8): 18297-18313.
Along the far eastern margin of the Weddell Gyre is a persistent feature in the middle of the ice pack which we previously reported and called the Cosmonaut polynya. A study of polynya occurrences from 1973 to 1993 reveals that since 1986 the polynya has become more active with an average size of about 7.2 x 10(4) km(2) and an average location at 52 degrees E and 65 degrees S. Satellite observations indicate that the polynya has recurred several times during winter in recent years with intervals ranging from a few days to a few weeks. The centroid of the polynya varies only slightly with each formation during the year and from one year to another, suggesting a controlling influence of the ocean and bottom topography that may be initially induced by wind. The daily time series indicates two primary modes of formation: one that is initiated in the early winter during a storm at a site usually preceded by an embayment of the ice edge and another that occurs during midwinter often preceded by a coastal polynya event adjacent to Cape Ann. The Cosmonaut polynya region is characterized in this study by compression of the westward flowing coastal current and the eastward flowing southern edge of the Antarctic Circumpolar Current. Following the principle of conservation of potential vorticity, vertical stretching of the water column would ensue, enhancing upwelling. Such a process accelerates the injection of relatively warm salty deep water into the surface layer, inhibiting sea ice growth and causing the polynya formation. This theory appears to explain the general behavior of the polynya in terms of frequency, duration, size, and location.
Cullather, R. I., D. H. Bromwich and M. L. VanWoert, 1996: Interannual variations in Antarctic precipitation related to El Niño southern oscillation. Journal of Geophysical Research-Atmospheres, 101(D14): 19109-19118.
The accurate estimation of Antarctic precipitation variability is an essential component in understanding global sea level fluctuations; direct measurement techniques, however, are replete with practical difficulties. In this study, net precipitation (precipitation minus sublimation) for the Antarctic continent is computed for 1980-1994 using operational numerical analyses obtained from the ECMWF (European Centre for Medium-Range Weather Forecasts). The resulting estimations reveal a strong interannual variability for the Antarctic continent, implying a +/-1.2 - 1.5 mm yr(-1) maximum range in the Antarctic eustatic change contribution. In particular, variability for the South Pacific sector (120 degrees W-180 degrees W) is shown to be correlated with the El Niño-Southern Oscillation (ENSO) phenomenon for 1980-1990. The relation becomes anticorrelated after 1990, associated with a strong East Antarctic ridging pattern that coincides with the start of the prolonged series of warm events of the early 1990s. This result is relevant to other studies relating ENSO variability to high southern latitudes, and a more elaborate picture of this teleconnection pattern is presented. Comparisons of sea level pressure values using available ship observations show good agreement and offer a confirmation of the analyses in this data-sparse region. Additionally, a comparison of results with values obtained from the precipitation fields of the NCEP/NCAR (NCEP: National Centers for Environmental Prediction; NCAR: National Center for Atmospheric Research) reanalysis project are discussed.
Dery, S. J. and P. A. Taylor, 1996: Some aspects of the interaction of blowing snow with the atmospheric boundary layer. Hydrological Processes, 10(10): 1345-1358.
Several possible effects of blowing snow on the atmospheric boundary layer are investigated, mostly within the general framework of the Prairie Blowing Snow Model (PBSM). The processes of snow saltation and suspension are first described. Variations to the drift density profile are tested and the effects of stratification and density variation calculations are evaluated. Despite high density gradients of blowing snow, stratification effects on turbulence and the velocity profiles can generally be neglected. However, with saltating or suspended snow in a constant shear stress layer, part of the shear stress is carried by the particles. A highly simplified, single-phase approach, based on the density variation of the air-snow mixture coupled to a simple turbulent stress-strain relationship, is used to illustrate this. Sublimation rates in a column of blowing snow are calculated using the PBSM and results are compared with those obtained with a modified formulation which incorporates a spectrum of sublimating particles of varying sizes at each height in a steady-state surface boundary layer and different specifications of the ventilation velocity.
Ffield, A. and A. L. Gordon, 1996: Tidal mixing signatures in the Indonesian seas. Journal of Physical Oceanography, 26(9): 1924-1937.
Expressions of low-frequency tidal periods are found throughout the Indonesian Seas' temperature field, supporting the hypothesis that vertical mixing is enhanced within the Indonesian Seas by the tides. The thermal signatures of tidal mixing vary mostly at the fortnightly and monthly tidal periods due to nonlinear dynamics redistributing tidal energy into these periods. Away from the coasts, the largest tidal mixing signatures are observed in sea surface temperature within the Seram and Banda Seas. Most of the Indonesian Throughflow passes through the Banda Sea where strong vertical mixing modifies the thermocline by transferring surface heat and freshwater to deeper layers before the upper water column is exported to the Indian Ocean. Modulation of vertical eddy fluxes within the Indonesian Seas by fortnightly and monthly tides may act to regulate ocean-atmosphere fluxes.
Garzoli, S. L. and A. L. Gordon, 1996: Origins and variability of the Benguela Current. Journal of Geophysical Research-Oceans, 101(C1): 897-906.
A subset of the Benguela Sources and Transports (BEST) 1992-1993 data is analyzed to study the magnitude and variability of the large scale transports in the area. The data consist of inverted echo sounder series and conductivity-temperature-depth stations. The mean 16 month transport values for the upper 1000 m indicate that of the 13-Sv northward transport within the Benguela Current, 50% is derived from the central Atlantic (which from geometry may be chiefly South Atlantic water), 25% comes from the Indian Ocean (which may be chiefly Agulhas water), and the remaining 25% may be a blend of Agulhas and tropical Atlantic water. A simple schematic of the transport pattern with a somewhat restricted corridor for Agulhas eddies translation is envisioned. To the west of the eddy corridor flows the South Atlantic source for the Benguela Current; to the east is the Agulhas (Indian Ocean) source. The corridor is breached by South Atlantic and Indian water as the transient eddy field stirs these water masses.
Garzoli, S. L., A. L. Gordon, V. M. Kamenkovich, D. Pillsbury and C. DuncombeRae, 1996: Variability and sources of the southeastern Atlantic circulation. Journal of Marine Research, 54(6): 1039-1071.
The 1992-1993 Benguela Sources and Transport (BEST) time series provide a quantitative view of the Benguela Current transport and the eddy held crossing 30S, as well as an estimate of the relation between its barotropic and baroclinic components. This is done by a simultaneous analysis of the BEST data derived from inverted echo sounders, pressure sensors, current meter moorings, CTD, and ADCP stations. The analysis of the time Series indicates that the annual mean baroclinic transport of the BengueIa Current is 13 Sv with a total transport of 16 Sv. Through the combination of instruments the total baroclinic plus barotropic transport of the upper 2600 m was obtained without making any assumption about the level of no motion. Results from this calculation corroborated the assumption that 1000 m as a level of no motion could be used as a fairly good approximation. The stationary flow of the Benguela Current is mostly confined near the African Continent while a transient flow, composed by large eddies shed from the Agulhas retroflection, dominates the western portion of the Benguela Current. In the stationary part of the Benguela Current, both barotropic and baroclinic components are equally important while in the transient part, the barotropic is more substantial. Several rings were observed during the experiment that migrated toward the west. An initial speed of 12 km/day diminished to 6 to 7 km/day at the Walvis Ridge. The water mass source of the Benguela Current includes Indian and South Atlantic subtropical thermocline water; relatively saline, low oxygen tropical Atlantic water; and the cooler, fresher subantarctic water. Changes in thermocline salinity correlate with transport: in general when the northward transport is increasing the thermocline salinity also increases, without a decrease in oxygen. This indicates that the Benguela Current increases in strength by bringing in more subtropical thermocline water. As the Agulhas input is most effective in boosting the salinity of the upper thermocline (the South Atlantic Current water being deficit in salinity relative to the Indian Ocean source) we suggest that the spatial variations in transport are tied to Agulhas water influx, presumably associated with the eddy field.
Gordon, A. L. and R. A. Fine, 1996: Pathways of water between the Pacific and Indian oceans in the Indonesian seas. Nature, 379(6561): 146-149.
THE physical structure of the Pacific and Indian oceans is substantially affected by the inter-ocean transport of excess fresh water from the North Pacific Ocean through the Indonesian seas(1,2). The efficiency of this transport is an important regulator of the meridional overturning of these oceans(1,2), and hence perhaps of the global thermohaline circulation(3); in addition the seepage of warm water out of the Pacific affects the volume of the western Pacific warm pool, and thus may influence El Niño events(24). But the sources, pathways and physical properties of the Indonesian throughflow are not well enough characterized to allow its influence on ocean circulation and the climate system to be quantified. Here we report salinity, temperature and chemical-tracer data from the Indonesian seas which show that the throughflow is dominated by two components: one of low-salinity, well ventilated North Pacific water through the upper thermocline of the Makassar Strait, and the other of more saline South Pacific water through the lower thermocline of the eastern Indonesian seas. Seasonal (monosonal) variations in the ratio of these components, perhaps modulated by El Niño conditions, imply the existence of potentially important variable feedbacks to the ocean circulation and climate system.
Houghton, R. W., 1996: Subsurface quasi-decadal fluctuations in the North Atlantic. Journal of Climate, 9(6): 1363-1373.
Analysis of hydrographic time series data at ocean station S (Panulirus) and along the Labrador continental margin is used to investigate the subsurface structure associated with the North Atlantic quasi-decadal SST fluctuation. Quasi-decadal fluctuations of both temperature and salinity are confined to the upper water column that is ventilated by the seasonal cycle. In the subtropical gyre, they are independent of the variations of the gyre baroclinic transport. On the Labrador continental margin, the quasi-decadal fluctuations are confined to the upper portion of the Labrador Current.
Ilahude, A. G. and A. L. Gordon, 1996: Thermocline stratification within the Indonesian Seas. Journal of Geophysical Research-Oceans, 101(C5): 12401-12409.
An extensive suite of conductivity-temperature-depth stations was obtained from the Baruna Jaya I during the southeast monsoon of 1993 and northwest monsoon of 1994, as part of the Indonesian/U.S. Arlindo project. The main objective of these cruises was to determine sources, pathways, and mixing histories of the throughflow water masses for the monsoon extremes. Water mass analysis indicates that the most penetrating route followed by Pacific water occurs within the Makassar Strait. This supports the notion that this strait carries the bulk of the Pacific to Indian throughflow, consisting of North Pacific Subtropical Water (upper thermocline S-max) and North Pacific Intermediate Water (lower thermocline S-min). The more attenuated S-max core during the northwest monsoon relative to the southeast monsoon suggests that the throughflow may slacken in that season. There is only minor contribution within the possible throughflow pathway east of Sulawesi. However, relative salty water of South Pacific origin is observed in the lower thermocline within the Seram and southern Maluku Seas, particularly in the northwest monsoon. Density-driven, sill depth overflow into the deep Banda Sea basin via the Lifamatola Passage also contributes to the total throughflow, though this contribution is likely to be minor. While some of the throughflow has been shown to pass through the Lombok Strait, water mass analysis clearly shows the Makassar throughflow turning into the Flores Sea and Banda Sea before curling southward into the Timer Sea and Indian Ocean.
Kamenkovich, V. M., Y. P. Leonov, D. A. Nechaev, D. A. Byrne and A. L. Gordon, 1996: On the influence of bottom topography on the Agulhas eddy. Journal of Physical Oceanography, 26(6): 892-912.
A series of numerical experiments with a two-layer primitive equation model is presented to study the dynamics Of Agulhas eddies. The main goal of the paper is to examine the influence of an underwater meridional ridge (modeled after the Walvis Ridge) on an Agulhas eddy hitting it. First, the propagation of an eddy of the specified vertical structure over a flat bottom is considered, varying the initial eddy horizontal scale from 40 to 120 km. Unlike small nonlinear eddies, large nonlinear eddies (on the scale of Agulhas eddies) do not rapidly evolve into a compensated state (no motion in the lower layer). Second, the influence of a ridge on eddies of differing vertical structures having a specified intensity in the upper layer and a prescribed horizontal scale is analyzed. Significantly baroclinic eddies can cross the Walvis Ridge, but barotropic or near-barotropic ones cannot.
Krupitsky, A., V. M. Kamenkovich, N. H. Naik(Henderson) and M. A. Cane, 1996: A linear equivalent barotropic model of the Antarctic circumpolar current with realistic coastlines and bottom topography. Journal of Physical Oceanography, 26(9): 1803-1824.
A linear equivalent barotropic (EB) model is applied to study the effects of the bottom topography H and baroclinicity on the total transport and the position of the Antarctic Circumpolar Current (ACC). The model is based on the observation of Killworth that the time mean velocity held of the FRAM Model is self-similar in the vertical.
Kushnir, Y. and I. M. Held, 1996: Equilibrium atmospheric response to North Atlantic SST anomalies. Journal of Climate, 9(6): 1208-1220.
The equilibrium general circulation model (GCM) response to sea surface temperature (SST) anomalies in tile western North Atlantic region is studied. A coarse resolution GCM, with realistic lower boundary conditions including topography and climatological SST distribution, is integrated in perpetual January and perpetual October modes, distinguished from one another by the strength of the midlatitude westerlies. An SST anomaly with a maximum of 4 degrees C is added to the climatological SST distribution of the model with both positive and negative polarity, These anomaly runs are compared to one another, and to a control integration, to determine the atmospheric response. In al cases warming (cooling) of the midlatitude ocean surface yields a warming (cooling) of the atmosphere over and to the east of the SST anomaly center. The atmospheric temperature change is largest near tile surface and decreases upward. Consistent with this simple thermal response, the geopotential height field displays a baroclinic response with a shallow anomalous low somewhat downstream from the warm SST anomaly. The equivalent barotropic, downstream response is weak and not robust: To help interpret the results, the realistic GCM integrations are compared with parallel idealized model runs. The idealized model has full physics and a similar horizontal and vertical resolution, bur an all-ocean surface with a single, permanent zonal asymmetry. The idealized and realistic versions of the GCM display compatible response patterns that are qualitatively consistent with stationary. linear, quasigeostrophic theory. However, the idealized model response is stronger and more coherent. The differences between the two model response patterns can be reconciled based on the size of the anomaly, the model treatment of cloud-radiation interaction,and the static stability of the model atmosphere in the vicinity of the ST anomaly. Modal results are contrasted with other GCM studies and observations.
Legg, S. Y., H. Jones and M. Visbeck, 1996: A heton perspective of baroclinic eddy transfer in localized open ocean convection. Journal of Physical Oceanography, 26(10): 2251-2266.
A simple point-vortex ''heton'' model is used to study localized ocean convection. In particular, the statistically steady state that is established when lateral buoyancy transfer, effected by baroclinic instability, offsets the localized surface buoyancy loss is investigated. Properties of the steady state, such as the statistically steady density anomaly of the convection region, are predicted using the hypothesis of a balance between baroclinic eddy transfer and the localized surface buoyancy loss. These predictions compare favorably with the values obtained through numerical integration of the heton model.
Marchese, P. J. and A. L. Gordon, 1996: The eastern boundary of the Gulf Stream recirculation. Journal of Marine Research, 54(3): 521-540.
A meridionally aligned thermocline front near 60W in the subtropical North Atlantic is revealed by the 1992 Trident data set. The front separates saltier thermocline water to the east from less salty water to the west. The eastern water is subjected to excess evaporation of the subtropics, while the western water is fed by lower salinity Gulf Stream water, which derives water from the wet tropical Atlantic. It is suggested that the front marks the eastern edge of the Gulf Stream recirculation cell, hence refer to it as the recirculation front. ?The surface layer displays a fan-like TIS scatter above the 18 degrees C Subtropical Mode Water, with the fresher surface water located west of the recirculation front, and a subsurface salinity maximum to the east. In the lower thermocline (8 to 12 degrees C) there is a step-like salinity increase of about 0.04 toward the east as measured along isotherms, producing two modes in the TIS scatter. At the intermediate water level (approximately in the 4 to 8 degrees C range) the extent of the low salinity Antarctic Intermediate Water and salty Mediterranean outflow water are also reflected in the position of the recirculation front. That the front marks the easternmost extent of the Gulf Stream recirculation is supported by the potential vorticity, which reveals a region of high homogeneous values within the recirculation cell. East of the front, the potential vorticity field is sloped along isopycnals indicating the meridional flow of the Sverdrup interior. Mapping of the recirculation front using archived data reveals that it extends deep into the subtropical convergence zone (STCZ), a region whose fronts have all been previously attributed to Ekman convergence.
McPhee, M. G., S. F. Ackley, P. Guest, B. A. Huber, D. G. Martinson, J. H. Morison, R. D. Muench, L. Padman and T. P. Stanton, 1996: The Antarctic Zone Flux Experiment. Bulletin of the American Meteorological Society, 77(6): 1221-1232.
In winter the eastern Weddell Sea in the Atlantic sector of the Southern Ocean hosts some of the most dynamic air-ice-sea interactions found on earth. Sea ice in the region is kept relatively thin by heat flux from below, maintained by upper-ocean stirring associated with the passage of intense, fast-moving cyclones. Ocean stratification is so weak that the possibility of deep convection exists, and indeed, satellite imagery from the Weddell Sea in the 1970s shows a large expanse of open water (the Weddell Polynya) that persisted through several seasons and may have significantly altered global deep-water production. Understanding what environmental conditions could again trigger widespread oceanic overturn may thus be an important key in determining the role of high latitudes in deep-ocean ventilation and global atmospheric warming. During the Antarctic Zone Flux Experiment in July and August 1994, response of the upper ocean and its ice cover to a series of storms was measured at two drifting stations supported by the National Science Foundation research icebreaker Nathaniel B. Palmer. This article describes the experiment, in which fluxes of heat, mass, and momentum were measured in the upper ocean, sea ice, and lower-atmospheric boundary layer. Initial results illustrate the importance of oceanic heat flux at the ice undersurface for determining the character of the sea ice cover. They also show how the heat flux depends both on high levels of turbulent mixing during intermittent storm events and on large variability in the stratified upper ocean below the mixed layer.
Murtugudde, R., R. Seager and A. J. Busalacchi, 1996: Simulation of the tropical oceans with an ocean GCM coupled to an atmospheric mixed-layer model. Journal of Climate, 9(8): 1795-1815.
A reduced gravity, primitive equation, ocean general circulation model (GCM) is coupled to an advective atmospheric mixed-layer (AML) model to demonstrate the importance of a nonlocal atmospheric mixed-layer parameterization for a proper simulation of surface heat fluxes and sea surface temperatures (SST). Seasonal variability of the model SSTs and the circulation are generally in good agreement with the observations in each of the tropical oceans. These results are compared to other simulations that use a local equilibrium mixed-layer model. Inclusion of the advective AML model is demonstrated to lead to a significant improvement in the SST simulation in all three oceans. Advection and diffusion of the air humidity play significant roles in determining SSTs even in the tropical Pacific where the local equilibrium assumption was previously deemed quite accurate. The main, and serious, model flaw is an inadequate representation of the seasonal cycle in the upwelling regions of the eastern Atlantic and Pacific Oceans. The results indicate that the feedback between mixed-layer depths and SSTs can amplify SST errors, implying that increased realism in the modeling of the ocean mixed layer increases the demand for realism in the representation of the surface heat fluxes. The performance of the GCM with a local-equilibrium mixed-layer model in the Atlantic is as poor as previous simple ocean model simulations of the Atlantic. The conclusion of earlier studies that the simple ocean model was at fault may, in fact, not be correct. Instead the local-equilibrium heat flux parameterization appears to have been the major source of error. Accurate SST predictions may, hence, be feasible by coupling the AML model to computationally efficient simple ocean models.
Rae, C. M. D., S. L. Garzoli and A. L. Gordon, 1996: The eddy field of the southeast Atlantic Ocean: A statistical census from the Benguela Sources and Transports project. Journal of Geophysical Research-Oceans, 101(C5): 11949-11964.
Data collected during the Benguela Sources and Transports project were examined to determine some statistical properties of the eddy field observed in the Cape Basin. Seven anticyclonic eddies were encountered during the hydrographic surveys. Two of these were shown to be of Brazil Current origin (this paper and Smythe-Wright Et al. ). Inverted echosounder (IES) records of acoustic travel time were scaled to the depth of the thermocline, represented in the Cape Basin by the 10 degrees C isotherm. These records indicate that a minimum of four to six eddies, assumed to be of Agulhas origin, entered the Cape Basin per year during the sampling period. They were associated with depressions in the 10 degrees C isotherm records ranging from 100 to 400 m and of a duration between 30 and 100 days. The thermocline appears to shallow appreciably after the passage of an eddy before relaxing to the local mean. Estimates of the heat and salt contents of the hydrographically surveyed eddies indicated that the mean available heat and salt anomalies of the eddies were 0.55 x 10(20) J and 3.5 x 10(12) kg, respectively. Extrapolating the hydrographic data to the eddies detected in the IES record shows the eddy field responsible for the transfer of 2.2 to 3.3 x 10(20) J yr(-1) (0.007 PW), 14 to 21 x 10(12) kg salt yr(-1), and 2.6 to 3.8 x 10(6) m(3) s(-1).
Reverdin, G., A. Kaplan and M. A. Cane, 1996: Sea level from temperature profiles in the tropical Pacific Ocean, 1975-1982. Journal of Geophysical Research-Oceans, 101(C8): 18105-18119.
We investigate how well the 1975-1992 sea level interannual variability in the tropical Pacific is captured by dynamic height from temperature profiles. For each temperature profile, a surface dynamic height relative to 300 m is estimated, assuming a constant temperature-salinity relationship. After multiplication by a latitudinally varying factor and the removal of a seasonal cycle, the dynamic height deviations fit the tide gauge sea level variability to within the sampling errors, except at a few sites near the equator west of the date line, where surface salinity variability is large. The dynamic height data are assimilated into a wind-forced linear numerical model of the sea level in the tropical Pacific, applying a Kalman filter in a space of reduced dimension. A limited number of empirical orthogonal functions of the unfiltered run (1975-1992) define the reduced space, into which the Kalman Filter covariance evolution calculation is done [Cane et at, 1996]. Experiments indicate that results are better with 32 functions than with a smaller number but are not improved by retaining more functions. The resulting analyzed fields of sea level are compared to withheld dynamic height estimates from moorings, sea level data from tide gauges, and sea level analyses made with the same Kalman filter formalism applied to tide gauge measurements. The comparisons to observations suggest that the temperature profiles were usually sufficient to constrain the monthly analyzed fields to be close to the observed sea level with errors typically less than 3 cm neat the equator. The comparison to tide gauge sea level reveals that this analysis is often more accurate than the analysis of tide gauge sea level data with which it shares many characteristics. Near the equator west of the date line, salinity variations ate large and their neglect in estimating dynamic height has a negative impact on the analysis. The analyzed signal is underestimated in the southwest Pacific and at more than 20 degrees off the equator. The reanalysis of the temperature data done with a primitive equation model at the National Meteorological Center (NMC) [Ji et at, 1995; Enfield and Harris, 1995] does not share this problem. On the other hand, NMC reanalysis (RA4) departs more from the observations elsewhere, although more data were included than in our analysis.
Schott, F., M. Visbeck, U. Send, J. Fischer, L. Stramma and Y. Desaubies, 1996: Observations of deep convection in the Gulf of Lions, northern Mediterranean, during the winter of 1991/92. Journal of Physical Oceanography, 26(4): 505-524.
During December 1991 to April 1992 measurements with moored acoustic Doppler current profiler (ADCP) stations and shipboard surveys were carried out in the convection regime of the Gulf of Lions, northwestern Mediterranean. First significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late December. Mixed layer deepening to about 400 m, eroding the salinity maximum layer of saltier and warmer Levantine Intermediate Water and causing temporary surface-layer warming, followed during a second cooling period of late January.
SmytheWright, D., A. L. Gordon, P. Chapman and M. S. Jones, 1996: CFC-113 shows Brazil eddy crossing the South Atlantic to the Agulhas Retroflection region. Journal of Geophysical Research-Oceans, 101(C1): 885-895.
Using the new ocean tracer CFC-113 and other hydrographic and chemical data, we have identified and ''aged'' a Brazil eddy transported across the South Atlantic to the Cape Basin. This is the first observation of such an eddy so far east. It was observed during two RRS Discovery cruises in January and May 1993, and using TOPEX-POSEIDON altimeter data, we have been able to track it between the two locations. TS and nutrient characteristics show that the eddy could not have an eastern Atlantic origin. Comparison with eddies found in the Brazil/Falklands Confluence suggests that this is the most likely source. From the CFC-113:CFC-11 ratio it appears that the eddy is in excess of 4 years old, and from the two sightings and the altimeter data we have estimated a transport rate for the eddy and given credence to the CFC-113:CFC-11 and CFC-113:CFC-12 ageing technique in surface waters.
Ting, M. F., 1996: Steady linear response to tropical heating in barotropic and baroclinic models. Journal of the Atmospheric Sciences, 53(12): 1698-1709.
The atmospheric response to tropical heating is examined using both the linear, multilevel baroclinic model with;io imposed tropical heat source, and the one-level barotropic model with a tropical divergence forcing. The divergent component of the response in the baroclinic model is characterized by a tropical divergence confined to the heated region, plus convergence and divergence centers away from the tropical heated region at the outflow level. The rotational component of the response is depicted by a local baroclinic response in the Tropics and a remote equivalent barotropic wave train in the extratropics.
Ting, M. F., M. P. Hoerling, T. Y. Xu and A. Kumar, 1996: Northern hemisphere teleconnection patterns during extreme phases of the zonal-mean circulation. Journal of Climate, 9(10): 2614-2633.
Regional climate anomalies associated with year-to-year changes in the tropospheric zonal-mean zonal wind ((u) over bar) are examined. This study focuses on the wintertime Northern Hemisphere extratropics and compares seasonal mean anomalies associated with (u) over bar to those associated with the El Nino-Southern Oscillation during the 1947-94 period.
Visbeck, M., J. Marshall and H. Jones, 1996: Dynamics of isolated convective regions in the ocean. Journal of Physical Oceanography, 26(9): 1721-1734.
An initially resting ocean of stratification N is considered, subject to buoyancy loss at its surface of magnitude B-0 over a circular region of radius r, at a latitude where the Coriolis parameter is f. Initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer. However, as deepening proceeds, horizontal density gradients at the edge of the forcing region support a geostrophic rim current, which develops growing meanders through baroclinic instability. Eventually finite-amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below, setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface. In this final state quasi-horizontal baroclinic eddy transfer dominates upright ''plume'' convection.
Weppernig, R., P. Schlosser, S. Khatiwala and R. G. Fairbanks, 1996: Isotope data from Ice Station Weddell: Implications for deep water formation in the Weddell Sea. Journal of Geophysical Research-Oceans, 101(C11): 25723-25739.
Helium isotope data (He-3/He-4 ratios and He-4 concentrations) and (H2O)-O-18/(H2O)-O-16 ratios obtained from stations occupied during the drift of Ice Station Weddell (February to June 1992) are used, together with hydrographic data, to study formation of deep and bottom water in the western Weddell Sea. The data indicate deep and bottom water formation along the entire track of the ice station (71.4 to 65.8 degrees S, approximate to 53 degrees W). Ice Shelf Water (ISW) seems to contribute significantly to the formation of Weddell Sea Deep Water (WSDW) and Weddell Sea Bottom Water (WSBW) in the southern part of the drift track, Toward the north, the fraction of ISW contained in WSDW/WSBW decreases, This trend is overlaid by high ISW fractions in the deep and bottom waters found in the vicinity of the Larsen Ice Shelf, The fraction of Western Shelf Water (WSW) in WSBW shows the opposite trend, increasing from south to north, The combined fraction of ISW and WSW in waters with potential temperatures below 0 degrees C is about 20%, corresponding to a roughly 200 m thick layer, Overall, WSW seems to contribute approximately 2 to 3 times more water than ISW to the water column below the 0 degrees C isotherm, Using the estimated flow of ISW over the sill north of the Filchner Depression of 1 Sv [Foldvik et al,, 1985a] together with the ratio of WSW plus Winter Water (WW) to ISW, we calculate a value of about 5 Sv for the formation rate of WSBW with a potential temperature of -0.7 degrees C, About one third of this flux represents near-surface waters (WSW/WW) which have recently been equilibrated with the atmosphere, whereas pure ISW contributes about 10%.
Yuan, X. J. and L. D. Talley, 1996: The subarctic frontal zone in the North Pacific: Characteristics of frontal structure from climatological data and synoptic surveys. Journal of Geophysical Research-Oceans, 101(C7): 16491-16508.
The subarctic front is a thermohaline structure across the North Pacific, separating colder, fresher water to the north from warmer, saltier water to the south. Levitus's  data and 72 conductivity-temperature-depth/salinity-temperature-depth sections are used to show the spatial and seasonal variations of the climatological frontal zone and the characteristics of the frontal structure in synoptic surveys. The temperature gradient in the mean frontal zone is stronger in the western Pacific and decreases eastward, while the salinity gradient has less variation across the Pacific. The temperature gradient also has larger seasonal variation, with a maximum in spring, than the salinity gradient. The synoptic surveys show that the frontal zone is narrower and individual fronts tend to be stronger in the western Pacific than in the eastern Pacific. Density gradients tend to be more compensated at the strongest salinity fronts than at the strongest temperature fronts. A horizontal minimum of vertical stability is found south of the subarctic halocline outcrop. The northern boundary of the North Pacific Intermediate Water merges with the frontal zone west of 175 degrees W and is north of the northern boundary of the subarctic frontal zone in the eastern Pacific. The shallow salinity minima start within the subarctic frontal zone in the eastern Pacific.
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