Showing papers on "Ocean current published in 1993"

On the North Atlantic Circulation

Abstract: A summary for North Atlantic circulation is proposed to replace the circulation scheme hypothesized by Worthington in 1976. Divergences from the previous model are in thermohaline circulation, cross-equatorical transport and Florida Current sources, flow in the eastern Atlantic, circulation in the Newfoundland Basin, slope water currents, and flow pattern near the Bahamas. The circulation patterns presented here are consistent with the majority of of published accounts of flow components. 77 refs., 14 figs., 3 tabs.

Abrupt climate change and transient climates during the Paleogene: a marine perspective.

Abstract: Detailed investigations of high latitude sequences recently collected by the Ocean Drilling Program (ODP) indicate that periods of rapid climate change often culminated in brief transient climates, with more extreme conditions than subsequent long term climates. Two examples of such events have been identified in the Paleogene; the first in latest Paleocene time in the middle of a warming trend that began several million years earlier: the second in earliest Oligocene time near the end of a Middle Eocene to Late Oligocene global cooling trend. Superimposed on the earlier event was a sudden and extreme warming of both high latitude sea surface and deep ocean waters. Imbedded in the latter transition was an abrupt decline in high latitude temperatures and the brief appearance of a full size continental ice-sheet on Antarctica. In both cases the climate extremes were not stable, lasting for less than a few hundred thousand years, indicating a temporary or transient climate state. Geochemical and sedimentological evidence suggest that both Paleogene climate events were accompanied by reorganizations in ocean circulation, and major perturbations in marine productivity and the global carbon cycle. The Paleocene-Eocene thermal maximum was marked by reduced oceanic turnover and decreases in global delta 13C and in marine productivity, while the Early Oligocene glacial maximum was accompanied by intensification of deep ocean circulation and elevated delta 13C and productivity. It has been suggested that sudden changes in climate and/or ocean circulation might occur as a result of gradual forcing as certain physical thresholds are exceeded. We investigate the possibility that sudden reorganizations in ocean and/or atmosphere circulation during these abrupt transitions generated short-term positive feedbacks that briefly sustained these transient climatic states.

Mean Circulation of the Hamburg LSG OGCM and Its Sensitivity to the Thermohaline Surface Forcing

Abstract: The sensitivity of the global ocean circulation to changes in surface heat flux forcing is studied using the Hamburg Large Scale Geostrophic (LSG) ocean circulation model. The simulated mean ocean circulation for appropriately chosen surface forcing fields reproduces the principal water mass properties, residence times, and large-scale transport properties of the observed ocean circulation quite realistically within the constraints of the model resolution. However, rather minor changes in the formulation of the high-latitude air–sea heat flux can produce dramatic changes in the structure of the ocean circulation. These strongly affect the deep-ocean overturning rates and residence times, the oceanic heat transport, and the rate of oceanic uptake of CO2. The sensitivity is largely controlled by the mechanism of deep-water formation in high latitudes. The experiments support similar findings by other authors on the sensitivity of the ocean circulation to changes in the fresh-water flux and are cons...

Geochemical cycles in an ocean general circulation model. Preindustrial tracer distributions

Abstract: A state-of-the-art report is given of the Hamburg model of the oceanic carbon cycle. The model advects geochemical tracers important to the carbon cycle by the currents of a general circulation model. The geochemical cycling is driven by a Michaelis-Menten type production kinetics. The model is an extension of the Bacastow and Maier-Reimer (1990) model. It is based on a more realistic current field and includes a mechanism of lysocline-sediment interaction. Principal variables are ∑CO2, alkalinity, phosphate, oxygen, and silicate. The carbon variables are defined for 12C, 13C, and 14C separately. In addition to these carbon isotopes, 39A and δ18O of dissolved oxygen are predicted. The model predicts realistic global patterns of tracer distribution. In the equatorial eastern Pacific, however, the structures are exaggerated due to a strong upwelling which is a common feature of coarse resolution models of the general circulation of the ocean.

The Role of Indonesian Throughflow in a Global Ocean GCM

Abstract: The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absenc...

A hydrographic section across the subtropical South Indian Ocean

Abstract: Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 106 m3 s−1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 106 m3 s−1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 106 m3 s−1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 106mm3 s−1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 106 m3 s−1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10−5 cm s−1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.

Stability and Variability of the Thermohaline Circulation

Abstract: The stability and internal variability of the ocean's thermohaline circulation is investigated using a coarse-resolution general circulation model of an idealized ocean basin, in one hemisphere. The model circulation is driven, in addition to wind forcing, by restoring the surface temperature to prescribed values, and by specifying freshwater fluxes in the surface salinity budget (mixed boundary conditions). All forcing functions are constant in time. The surface freshwater forcing is the dominant factor in determining the model's stability and internal variability. Increasing the relative importance of freshwater flux versus thermal forcing, in turn, one stable steady state of the model, two stable ones, one stable, and one unstable equilibrium, or no stable steady states at all are found. If the freshwater forcing is sufficiently strong, self-sustained oscillations exist in the deep-water formation rate, which last thousands of years. One type of oscillation occurs on the time scale of decades ...

The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Abstract: Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well-defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January-May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.

Subtidal circulation over the northern California shelf

Abstract: The circulation over the shelf and upper slope off northern California, between 38°N and 42°N, was observed using moored current and temperature recorders deployed as part of the Northern California Coastal Circulation Study (NCCCS), from March 1987 through October 1989. The results of this study provide a larger-scale view of the wind-driven circulation than that described through the 1981–1982 Coastal Ocean Dynamics Experiment (CODE), particularly with regard to alongshore and temporal variations. From an improved description of the frequency structure of wind and current, a very low frequency (VLF) signal is identified in the observed currents at frequencies below those typical of wind forcing. The majority of this VLF variance appears to be accounted for by persistent flow events associated with the presence of mesoscale circulation in the adjacent ocean. The longer duration of the NCCCS also allows an improved description of the seasonality of flow regimes off northern California. Three oceanic seasons are identified: an upwelling season (April-July), a relaxation season (August-November), and a storm season (December-March). Alongshore variations in the strength of upwelling, in the strength of the alongshore flow, both near-surface and undercurrent, and in water temperature not only are a function of latitude, as is the wind, but they also correspond to location relative to promontories, notably Cape Mendocino. Immediately south of Cape Mendocino, the near-surface flow exhibits an equatorward minimum and a temperature minimum, whereas the undercurrent exhibits a poleward maximum. Conversely, at the moorings immediately north of the cape, temperatures are a maximum and the undercurrent exhibits a minimum. The maximum in near-surface temperature relates to a minimum in upwelling; no significant correlation was found between local wind and current immediately north of Cape Mendocino. This upwelling minimum and the upwelling maximum south of the cape were also observed as persistent sea surface temperature patterns in satellite imagery.

Effect of drake and panamanian gateways on the circulation of an Ocean model

Abstract: Geologic studies indicate that prior to ∼40 Ma the Drake Passage was closed and the Central American Isthmus was open. The effect of these changes has been examined in an ocean general circulation model. Several sensitivity experiments were conducted, all with atmospheric forcing and other boundary conditions from the present climate, but with different combinations of closed and open gateways. In the first experiment, the only change involved closure of the Drake Passage. In agreement with earlier studies the barrier modified the geostrophic balance that now maintains the circumpolar flow in the southern ocean, with the net effect being decreased transport of the Antarctic Current and an approximate fourfold increase in outflow of Antarctic deep-bottom waters. The very large increase in Antarctic outflow suppresses North Atlantic Deep Water (NADW) formation. In addition to corroboration of results from earlier studies, our simulations provide several new insights into the role of a closed Drake Passage. A more geologically realistic closed Drake/open central American isthmus experiment produces essentially the same pattern of deepwater circulation from the first experiment, except that Antarctic outflow is about 20% less than the first experiment. The resultant unipolar deepwater circulation pattern for the second experiment is consistent with paleoceanographic observations from the early Cenozoic. A third experiment involved an open Drake and open central American isthmus. In this experiment, Antarctic outflow is diminished to slightly above present levels but NADW production is still low due to free exchange of low-salinity surface water between the North Pacific and North Atlantic. The low level of thermohaline overturn should have reduced oceanic productivity in the Oligocene (∼30 Ma), a result in agreement with geologic observations. Finally, simulations with an energy balance model demonstrate that the changes in surface heat flux south of 60°S due to breaching of the Drake barrier do not result in temperature changes large enough to have triggered Antarctic glaciation. This last result suggests that some other factor (CO2?) may be required for Antarctic ice sheet expansion in the Oligocene (∼30–34 Ma). Our results lend further support to the concept that even in the absence of changing boundary conditions due to ice sheet growth, variations in the geometry of the ocean basins can significantly influence ocean circulation patterns and the sediment record. The results also suggest that the primary polarities of the Cenozoic deepwater circulation may have been controlled by opening and closing of these two gateways.

Large-Scale Dynamics and Global Warming

Abstract: Predictions of future climate change raise a variety of issues in large-scale atmospheric and oceanic dynamics. Several of these are reviewed in this essay, including the sensitivity of the circulation of the Atlantic Ocean to increasing freshwater input at high latitudes; the possibility of greenhouse cooling in the southern oceans; the sensitivity of monsoonal circulations to differential warming of the two hemispheres; the response of midlatitude storms to changing temperature gradients and increasing water vapor in the atmosphere; and the possible importance of positive feedback between the mean winds and eddy-induced heating in the polar stratosphere.

Observations of vertical currents and convection in the central greenland sea during the winter of 1988-1989

Abstract: 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−l. 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.

Circulation of Antarctic intermediate water in the South Indian Ocean

Abstract: Chlorofluorocarbon (CFC) and hydrographic data collected on the R.R.S. Charles Darwin Cruise 29 along 32°S during November-December 1987, are used to examine the circulation in the South Indian Ocean. The emphasis is on Antarctic Intermediate Water (AAIW); bottom waters and mode waters are also examined. Bottom waters entering in the western boundary of the Crozet Basin (about 60°E) and in the Mozambique Basin (about 40°E) have low concentrations of anthropogenic CFCs. The rest of the bottom and deep waters up to about 2000 m have concentrations that are below blank levels. Above the intermediate waters there are injections of mode waters, which are progressively denser in the eastward direction. They form a broad subsurface CFC maximum between 200 and 400 m. The injections of recently ventilated (with respect to CFCs and oxygen) Subantarctic Mode Waters (SAMWs) at different densities indicate that there is considerable exchange between the subtropical and subantarctic regions. The tracer data presented show that the circulation of AAIW in the South Indian Ocean is different from that in the South Atlantic and South Pacific oceans in several ways. (1) The most recently ventilated AAIW is observed in a compact anticyclonic gyre west of 72°E. The shallow topography (e.g. that extending northeastward from the Kerguelen Plateau) may deflect and limit the eastward extent of the most recently ventilated AAIW. As a consequence, there is a zonal offset in the South Indian Ocean of the location of the most recently ventilated SAMW and AAIW, which does not occur in the other two oceans. The strongest component of SAMW is in the east, while the AAIW is strongest in the western-central South Indian Ocean. The offset results in a higher vertical gradient in CFCs in the east. (2) The Agulhas Current may impede input of AAIW along the western boundary. (3) Tracers are consistent with an inter-ocean flow from the South Pacific into the Eastern Indian Ocean, similar to the South Atlantic to Indian linkage. (4) It appears that the high wind stress curl forces an equatorward component of the circulation that is strongest around 60°E. As a result the highest concentrations of CFCs and oxygens in bottom waters, AAIW, and the lightest component of SAMW are co-located along the 32°S track at about 60°E. Thus, the most recently ventilated circumpolar waters, participating in both the wind driven and the thermohaline circulations, follow similar paths equatorward into the subtropical Indian Ocean.

On the Transport and Angular Momentum Balance of Channel Models of the Antarctic Circumpolar Current

Abstract: Angular momentum balances are discussed, both in general as well as in the context of simple channel models of the Antarctic Circumpolar Current (ACC). Particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation. It is found that topographic form drag is established very early in the integration, whereas interfacial form drag can take much longer to develop. Restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady-state transport. The estimate assumes there to be little or no circumpolar flow at great depth, an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the Southern Ocean does not extend to the ocean floor. The predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite...

Tropical Pacific Interannual Variability and CO2 Climate Change

Abstract: In this paper, an attempt is made to estimate possible sensitivities of El Nino-Southern Oscillation (ENSO)-related effects in a climate with increased carbon dioxide (CO2). To illustrate this sensitivity, results are shown from two different interactive ocean-atmosphere model configurations and an atmospheric model with prescribed heating anomalies. In the first, an atmospheric general circulation model (GCM) is coupled to a global coarse-grid dynamical ocean GCM (coupled model). In the second, the same atmospheric model is coupled to a simple nondynamic slab-ocean mixed-layer model (mixed-layer model). In the third, an atmospheric model is run in perpetual January mode with observed sea surface temperatures (SSTs) and prescribed tropical tropospheric heating anomalies (prescribed-heating model). Results from the coupled model show that interannual SST variability (with warm and cold events relative to the mean SST) continues to occur in the tropics with a doubling of CO2. This variability is su...

A synthesis of the Levantine Basin circulation and hydrography, 1985–1990

Abstract: The Levantine Basin circulation derived from recent data consists of a series of sub-basin-scale to mesoscale eddies interconnected by jets. The basin-scale circulation is masked by eddy variability that modulates and modifies it on seasonal and interannual time scales. Long-term qualitative changes in the circulation are reflected in the bifurcation pattterns of ther mid-basin jets, relative strengths of eddies and the hydrographic properties at the core of these eddies. Confinement within the Basin geometry strongly influences the co-evolution of the circulation features. Surface measurements, satellite images and the mass field indicate an entire range of scales of dynamical features in the region. The complexity of the circulation is consistent with the basin-wide and mesoscale heterogeneity of the hydrographic properties. The interannual variability of LIW (Levantine Intermediate Water) formation in the region appears correlated with the changes in the circulation. Wintertime convective overturning of water masses reach intermediate depths and constitute a dominant mechanism of LIW formation, especially in anticyclonic eddies and along the coasts of the northern Levantine Basin.

Temporal and spatial variability of volume transport of the Kuroshio in the East China Sea

Abstract: The temporal and spatial variability of absolute geostrophic volume transport of the Kuroshio is examined using hydrographic and surface current data collected by the Japan Meteorological Agency on 39 transport sections made at six transects in the Fast China Sea and one transect just outside the East China Sea during the three-year period 1986–1988. The Kuroshio volume transport ( KVT ) is estimated by integration over each transect of the downstream (positive) part of the geostrophic velocity referenced to the observed surface current. Our analysis indicates that, on average, only about 25% of the KVT inside the East China Sea is due to the baroclinic or vertically varying component. While KVT exhibits a small-scale temporal and spatial variability with downstream phase speeds of order 8–19 km day −1 and wavelengths of order 150–375 km, mean KVT is spatially uniform within the East China Sea. KVT has significant annual and interannual variations: KVT is large in summer, is positively correlated with local downstream wind stress, and is large during large meander events of the Kuroshio south of Japan. Our best estimate of the mean KVT in the East China Sea during 1986–1988 is 23.7 Sv ± 2.0 Sv. This represents about one-half of the mean interior Sverdrup transport at this latitude. The Kuroshio quickly absorbs most of the remaining interior Sverdrup transport after it exits the East China Sea through the Tokara Strait.

Mean circulation of the upper layers of the western equatorial Pacific Ocean

Abstract: E and on O.soN at l42°E. No variation in its transport (15.0 x 10 6 m 3 s·l) is found between those longitudes. We fmd indication of the Equatorial Undercurrent at 137°E-0.75°N in the geostrophic field. The northern and southern Subsurface Countercurrents are clearly identified by extrema of eastward velocity at l65°E around 3°N and 3°S (250 dbar). No evidence of a southern Subsurface Countercurrent is found at l42°E. At 137°E the northern Subsurface Countercurrent is not characterized by a local extrema of eastward velocity: the North Equatorial Countercurrent seems to extend from the surface to 400 dbar with a south ward shi ft of its core. Analysis of the seasonal variability at l6s o E indicates that the E~atorial Undercurrent transport increases by a factor 2 between January (10.7 X 10 6 m 3 s-l) and JuIy (21.5 x 10 m 3 s·l) and the Equatorial Interrnediate Current transport is halved (6.3 x 10 6 m 3 s·l in January, 3.5 x 10 6 m 3 s·l in JuIy). In contrast, the transport of the Subsurface Countercurrents does not vary substantially between those two months. The meridional distributions of salinity and potential vorticity show that the axes of the main eastward currents are associated with strong meridional property gradients, not with property extrema. The eastward currents thus represent a barrier to the riorthward extension of the high salinity Tropical Water. Relatively weak meridional gradients of salinity and potential vorticity are observed in the westward directed South Equatorial Current and Equatorial Interrnediate Current.

Flow of deep and bottom waters in the Pacific at 10°N

Abstract: Flow of Lower Circumpolar Water and North Pacific Deep Water is described in the Pacific at 10°N primarily using a recent trans-Pacific CTD/O 2 section. Water-mass properties are used to define boundaries at potential isotherms for these two water masses. In turn, zero-velocity surfaces are set at different potential isotherms in each ocean basin along 10°N, their selection guided by the water-mass properties and the thermal wind field. These surfaces are then applied to estimate transport for each of the water masses. Net transport of Lower Circumpolar Water is estimated at 5.8 × 10 6 m 3 s −1 (positive northward) in the East Mariana Basin and 8.1 × 10 6 m 3 s −1 in the Central Pacific Basin, while a small flow of 0.4 × 10 6 m 3 s −1 through the Yap Trench into the Philippines Basin is inferred.

Climate Variability in a Coupled GCM. Part I: The Tropical Pacific

Abstract: A 26-year integration has been performed with a coupled ocean-atmosphere general circulation model (CGCM). The oceanic part resolves all three oceans in the latitude band 70°N–70°S but is dynamically active only between 30°N and 30°S. The atmosphere is represented by a global low-order spectral model. The coupled model was forced by seasonally varying insolation. Although the simulated time-averaged mean conditions in both atmosphere and ocean show significant deviations from the observed climatology, the CGCM realistically simulates the interannual variability in the tropical Pacific. In particular, the CGCM simulates an irregular ENSO with a preferred time scale of about 3 years. The mechanism for the simulated interannual variability in the tropical Pacific is related to both the “delayed action oscillator” and the “slow SST mode.” It therefore appears likely that either both modes can coexist or they degenerate to one mode within certain locations of the parameter space. This hypothesis is also supported by calculations performed with simplified coupled models, in which the atmospheric GCM was replaced by linear steady-state atmosphere models. Further, evidence is found for an eastward migration of zonal wind anomalies over the western Pacific prior to the extremes of the simulated ENSO, indicating a link to circulation systems over Asia. Because an earlier version of the CGCM did not simulate interannual variability in the tropical Pacific, additional experiments with a simplified coupled model have been conducted to study the sensitivity of coupled systems to varying mean oceanic background conditions. It is shown that even modest changes in the background conditions can push the coupled system from one flow regime into another.

Equatorial variability and resonance in a wind-driven Indian Ocean model

Abstract: A numerical isopycnal ocean model has been designed and applied to model the Indian Ocean north of 25°S. Vertical normal modes are used in the open boundary conditions and for selections of initial layer depths. A 21-year integration with a reduced Hellerman-Rosenstein monthly averaged wind stress has been made with 3.5-layer and 1.5-layer versions of the model. Both solutions reproduce the main features of the observed wind-driven seasonal circulation in the Indian Ocean above the main thermocline. The transient semiannual equatorial surface jets are more intense, more coherent, and in better phase agreement with observations when three layers are active. The associated undercurrents below the main thermocline are also included in the 3.5-layer model solution. Second baroclinic-mode, reflecting, equatorial Kelvin and Rossby waves combine to give a semiannual, resonant basin mode. Experiments with an equatorial band of semiannual zonal winds suggest a very strong response of the Indian Ocean to wind forcing with this period. Further, the amplitudes of the 28–30 day oscillations in the western equatorial model region are found to be strongly damped with depth; they have upward phase propagation and downward energy propagation.

The Mechanism for Antarctic Intermediate Water Renewal in a World Ocean Model

Abstract: Realistic representation of the low-salinity tongue of Antarctic Intermediate Water (AAIW) has been achieved in a coarse-resolution ocean general circulation model. The authors find that this water mass is not generated by direct subduction of surface water near the polar front. Instead, the renewal process is concentrated in the southeast Pacific Ocean off southern Chile. The outflow of the East Australian Current progressively cools (by heat loss to the atmosphere) and freshens (by assimilation of polar water, carried north by the surface Ekman drift) during its slow movement across the South Pacific toward the AAIW formation zone. Further, deep, warm advection near Chile enables more convective overturn, resulting in very deep mixed layers from which AAIW is fed into the South Pacific and also into the Malvinas Current. Along with this isolated region of AAIW renewal, the model relies on alongisopycnal mixing of fresh surface water from the polar front to capture a realistic circumpolar tongue...

Seasonal variability in the southwestern Atlantic

Abstract: The circulation of the southwestern Atlantic Ocean is dominated by the Subtropical Gyre and the confluence of the Brazil and Malvinas currents. Observations indicate that the latitude of this confluence changes seasonally, lying farther north during the austral winter than during the summer. This phenomenon has important consequences for the local climate and marine population, as the latitude of the confluence also marks the boundary between the warm waters of the subtropical gyre and the cold waters of the Antarctic Circumpolar Current. We present evidence that these seasonal migrations may be related to changes in the transport of both the Brazil and Malvinas currents. A numerical model forced by climatological wind stress indicates that the transport of the Brazil Current decreases during winter months and increases during summer months. Geosat altimeter data corroborate the model results and also indicate that the transport of the Malvinas Current undergoes a seasonal cycle with phase opposite to that of the Brazil Current. Our hypothesis is that during the austral summer, a southward displacement of the latitude of the confluence is coincident with an acceleration of the flow in the subtropical gyre and a weakening of the transport of the Malvinas Current. This situation reverses during the winter when the Malvinas Current grows stronger, the Brazil Current transport decreases, and the latitude of the confluence of these two currents moves northward.

Weddell Gyre structure of the eastern boundary

Abstract: In January–February 1988 a survey in the eastern part of the Weddell Gyre by three U.S.S.R. research vessels showed a complicated hydrographic structure that results from the intensive interaction between the waters of the Weddell Gyre and Antarctic Circumpolar Current. Three types of mesoscale features were found: warm-core eddies of circumpolar origin, with maximum temperature values in the range 1.7–2.1°C; warm-core eddies formed at the Weddell Front with maximum temperatures of 0.8–1.4°C; and cold-core features from the Cold Regime of the Weddell Gyre with a maximum deep water temperature of 0.7°C. The inflow of the Circumpolar Deep Water into the gyre is facilitated by the sharp southward excursion of the Antarctic Circumpolar Currentt core at about 26°E due to the topographical constraint of the mid-ocean ridge. Intense warm-core eddies forming at the Polar Front and drifting southward are visible in GEOSAT altimeter data as relatively high sea-level variability. The pure Cold Regime deep water with a maximum temperature

Past climate and the role of ocean heat transport: Model simulations for the Cretaceous

Abstract: A series of general circulation model experiments using Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) were executed to evaluate the sensitivity of simulated mid-Cretaceous climate to small perturbations in ocean heat transport. Three experiments were performed: (1) mixed layer ocean with no ocean heat transport, ZEROQ, (2) ocean heat transport specified as required for GENESIS to best match modern observations, ONEQ, and (3) doubled ocean heat transport, TWOQ. The ONEQ experiment represents an ocean heat transport which is actually about 15% of the values given by Carissimo et al. (1985) from modern observations. As a sensitivity experiment these model simulations represent a doubling of the role of the ocean. However, relative to the observations, they represent small perturbations to the total poleward heat transport in the model. With the exception of the tropics, no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport. However, relatively small increases in ocean heat transport resulted in a number of significant differences between simulations, including tropical cooling, polar warming, weakened equator-to-pole surface temperature gradients, weakened midlatitude jets, decreased land-sea pressure contrast, and decreased midlatitude storminess. The experiments indicate that changes in ocean heat transport which are well within the realm of possibility for Earth history can have significant climatic impact. Although ocean heat transport may be a significant factor in explaining Cretaceous polar warmth, the changes specified in these experiments alone are not sufficient to explain the polar warmth of the Cretaceous.