Showing papers on "Ocean current published in 1996"

On the freshwater forcing and transport of the Atlantic thermohaline circulation

Abstract: The 'conveyor belt' circulation of the Atlantic Ocean transports large amounts of heat northward, acting as a heating system for the northern North Atlantic region. It is widely thought that this circulation is driven by atmospheric freshwater export from the Atlantic catchment region, and that it transports freshwater northward to balance the loss to the atmosphere. Using results from a simple conceptual model and a global circulation model, it is argued here that the freshwater loss to the atmosphere arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven subtropical gyre, while the thermohaline circulation transports freshwater southward. It is further argued that the direction of freshwater transport is closely linked to the dynamical regime and stability of the 'conveyor belt': if its freshwater transport is indeed southward, then its flow is purely thermally driven and inhibited by the freshwater forcing. In this case the circulation is not far from Stommel's saddle-node bifurcation, and a circulation state without NADW formation would also be stable.

The Hawaii Ocean Time-series (HOT) program: Background, rationale and field implementation

Abstract: Long-term ocean observations are needed to gain a comprehensive understanding of natural habitat variability as well as global environmental change that might arise from human activities. In 1988, a multidisciplinary deep-water oceanographic station was established at a site north of Oahu, Hawaii, with the intent of establishing a long-term ( > 20 years) data base on oceanic variability. The primary objective of the Hawaii Ocean Time-series (HOT) program is to obtain high-quality time-series measurements of selected oceanographic properties, including: water mass structure, dynamic height, currents, dissolved and particulate chemical constituents, biological processes and particulate matter fluxes. These data will be used, in part, to help achieve the goals of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) research programs. More importantly, these data sets will be used to improve our description and understanding of ocean circulation and ocean climatology, to elucidate further the processes that govern the fluxes of carbon into and from the oceans, and to generate novel hypotheses. These are necessary prerequisites for developing a predictive capability for global environmental change.

Pathways of water between the Pacific and Indian oceans in the Indonesian seas

Abstract: 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 seas1,2. The efficiency of this transport is an important regulator of the meridional overturning of these oceans1,2, and hence perhaps of the global thermohaline circulation3; in addition, the seepage of warm water out of the Pacific affects the volume of the western Pacific warm pool, and thus may influence EI Nino events24. 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 EI Nino conditions, imply the existence of potentially important variable feedbacks to the ocean circulation and climate system.

Ocean Circulation Theory

Abstract: 1 Sverdrup Theory.- 2 Homogeneous Models of the Ocean Circulation.- 3 Vertical Structure: Baroclinic Quasi-Geostrophic Models.- 4 Theory of the Ventilated Thermocline.- 5 Buoyancy Forced Circulation and Cross-Gyre Flow.- 6 Equatorial Dynamics of the Thermocline: The Equatorial Undercurrent.- 7 Abyssal Circulation.

An improved method for detecting anthropogenic CO2 in the oceans

Abstract: An improved method has been developed for the separation of the anthropogenic CO 2 from the large natural background variability of dissolved inorganic carbon (C) in the ocean. This technique employs a new quasi-conservative carbon tracer ΔC * , which reflects the uptake of anthropogenic CO 2 and the air-sea disequilibrium when a water parcel loses contact with the atmosphere. The air-sea disequilibrium component can be discriminated from the anthropogenic signal using either information about the water age or the distribution of ΔC * in regions not affected by the anthropogenic transient. This technique has been applied to data from the North Atlantic sampled during the Transient Tracers in the Ocean North Atlantic (TTO NAS) and Tropical Atlantic study (TTO TAS) cruises in 1981-1983. The highest anthropogenic CO 2 concentrations and specific inventories (inventory per square meter) are found in the subtropical convergence zone. In the North Atlantic, anthropogenic CO 2 has already invaded deeply into the interior of the ocean, north of 50°N it has even reached the bottom. Only waters below 3000 m and south of 30°N are not yet affected. We estimate an anthropogenic CO 2 inventory of 20 ± 4 Gt C in the North Atlantic between 10°N and 80°N. The 2.5-dimensional ocean circulation model of Stocker et al. [1994] and the three-dimensional ocean general circulation biogeochemistry model of Sarmiento et al. [1995] predict anthropogenic CO 2 inventories of 18.7 Gt C and 18.4 Gt C, respectively, in good agreement with the observed inventory. Important differences exist on a more regional scale, associated with known deficiencies of the models.

Introduction to physical oceanography

Abstract: Preface Acknowledgments 1. The Oceans 2. Basic Equations 3. Boundary Conditions at the Air-Sea Interface 4. Geostrophic Flow 5. Planetary Boundary Layers 6. Barotropic Ocean Circulation 7. Baroclinic Ocean Flows 8. General Circulation of a Baroclinic Ocean with Bottom Topography 9. Surface Gravity Waves 10. Inertial Motions 11. Astronomical Tides 12. Vorticity Appendices References Recommended Books Index

Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data

Abstract: Today, the ocean thermohaline circulation transports half of the 231Pa produced by radioactive decay in the Atlantic Ocean water column to the Southern Ocean. This export respectively imparts low and high 231Pa/230Th ratios to the surface sediments of these oceans. Ocean sediments from the Last Glacial Maximum bear a similar isotopic fingerprint, implying that advection of North Atlantic Deep/Intermediate Water into the Circumpolar Deep Water of the Southern Ocean occurred at a similar—or slightly higher—rate during the last glacial period.

An estimate of global ocean circulation and heat fluxes

Abstract: ALTHOUGH ocean circulation and the consequent exchange of heat and gases with the atmosphere exert a strong influence on climate, discussions of global circulation have previously been highly schematic1–3 (invoking laminar flow patterns that ignore the turbulent nature of the real flow), non-quantitative and/or based upon mutually inconsistent regional studies1–8. Here we present a dynamically and kinematically consistent estimate of the magnitude and structure of global ocean circulation and its associated heat fluxes, derived by integrating hydrographic velocity data over the rapid spatial variations that they show. We find no single overturning cell, but instead a complex and probably time-varying circulation pattern. The simplest interpretation suggests that there are two nearly independent cells: one connecting overturning in the Atlantic Ocean to other basins through the Southern Ocean, and the other connecting the Indian and Pacific basins through the Indonesian archipelago.

The effect of the Indonesian throughflow on ocean circulation and heat exchange with the atmosphere: A review

Abstract: A new version of the island rule, which relates transport around an island to wind and pressure forcing, provides a basis for comparing various published theoretical estimates of the long-term mean Indonesian throughflow magnitude. It is found, among other things, that nonlinear effects near Halmahera and pressure gradients across New Zealand may modify the long-term throughflow magnitude. Recent theoretical estimates of interannual throughflow variations are in approximate agreement with observation; Indian Ocean Kelvin waves play an important role. On still shorter timescales and for understanding flow details in different channels, consideration of the full details of the Indonesian region via numerical modeling becomes essential. Most of the throughflow enters from the Mindanao Current. The process by which the South Pacific waters eventually reach the Mindanao Current appears to involve some nonlinear retroflection process (particularly in northern summer) and subsequent freshening along long pathways in the North Pacific. Observed water mass transformations in Indonesian waters demand a vertical eddy diffusivity of about 10−4 m2 s−1, large enough to generate turbulent heat fluxes of order 40 W m−2 at the base of the mixed layer. According to numerical models, changes in ocean circulation associated with the throughflow are likely to affect patterns of heat exchange with the atmosphere in widely separated regions of the world ocean. In particular, an increased throughflow will result in more heat loss to the atmosphere in the subtropical Indian Ocean and less in the Pacific Ocean. Simple, physically reasonable mechanisms have been offered for these model results. Observational evidence to support or refute these mechanisms is rather fragmentary but is reviewed here. Our present understanding of the throughflow permits some informed speculations as to the possible role of the throughflow in coupled ocean-atmosphere phenomena, such as the El Nino-Southern Oscillation (ENSO). Earlier estimates of western Pacific reflectivity seem largely confirmed by recent observational results. However, it is suggested that tidal mixing in the Indonesian seas may generate ENSO-related sea surface temperature anomalies there, possibly affecting the development of westerly wind bursts.

Hydrography and circulation in the western Bay of Bengal during the northeast monsoon

Abstract: The Bay of Bengal, a semienclosed tropical basin that comes under the influence of monsoonal wind and freshwater influx, is distinguished by a strongly stratified surface layer and a seasonally reversing circulation. We discuss characteristics of these features in the western Bay during the northeast monsoon, when the East India Coastal Current (EICC) flows southward, using hydrographic data collected during December 1991. Vertical profiles show uniform temperature and salinity in a homogeneous surface layer, on average, 25 m deep but shallower northward and coastward. The halocline, immediately below, is approximately 50 m thick; salinity changes by approximately 3 parts per thousand. About two thirds of the profiles show temperature inversions in this layer. Salinity below the halocline hardly changes, and stratification is predominantly due to temperature variation, The halocline is noticeably better developed and the surface homogeneous layer is thinner in a low-salinity plume that hugs the coastline along the entire east coast of India, The plume is, on average, 50 km wide, with isohalines sloping down toward the coast. Most prominent in the geostrophic velocity field is the equatorward EICC. Its transport north of about 13 degrees N, computed with 1000 dbar as the level of reference, varies between 2.6 and 7.1 x 10(6) m(3) s(-1); just south of this latitude, a northwestward flow from offshore recurves and merges with the coastal current. At the southern end of the region surveyed, the transport is 7.7 x 10(6) m(3) s(-1). Recent model studies lead us to conclude that the EICC during the northeast monsoon is driven by winds along the east coast of India and Ekman pumping in the interior bay. In the south, Ekman pumping over the southwestern bay is responsible for the northwestward flow that merges with the EICC.

Coupled dynamics of the South China Sea, the Sulu Sea, and the Pacific Ocean

Abstract: The complex geometry, the seasonally reversing monsoon winds, and the connectivity with the Pacific Ocean all contribute to the coupled dynamics of the circulation in the South China Sea (SCS), the Sulu Sea, and the region around the Philippine Islands The 1/2°, 15-layer global reduced gravity thermodynamic Navy layered ocean model (NLOM) is used to separate these components and to investigate the role of each one When forced by the Hellerman and Rosenstein [1983] (HR) monthly wind stress climatology, the basic features of the model solution compare well with observations, and with higher-resolution NLOM versions The dynamics of the flow from the Pacific Ocean into the SCS via the Luzon Strait are emphasized The effects of Ekman suction/pumping due to wind curl are examined by forming monthly spatial averages of the winds over the SCS/Sulu Sea basins This maintains a monthly varying stress but with a region of zero curl Forcing the model with these modified winds leaves the mean Luzon Strait transport unchanged, and the variability actually increases slightly These results suggest that it is the pressure head created by the pileup of water from the monsoonal wind stress that controls the variability of the Luzon Strait transport The forcing for wind stress pileup effects could be either internal or external to the SCS/Sulu Sea basin The effects of internal forcing are studied by applying monthly winds within this basin but annual HR winds outside the region With this forcing the mean Luzon Strait transport is essentially unchanged, but the variability is only 44% of the standard case value The external forcing is defined as zero stress in the SCS/Sulu Sea basins and HR monthly winds outside Again, the mean Luzon Strait transport is unchanged, and here the variability is 60% of the standard case The mean Luzon Strait transport is largely a function of the model geometry When the Sulu archipelago is opened, a net cyclonic flow develops around the Philippines, which is essentially an extension of the northern tropical gyre The bifurcation latitude of the North Equatorial Current (NEC) at the Philippine coast is also affected by the amount of transport through the Sulu archipelago Opening this archipelago causes the NEC split point to move southward and increases the transport of the Kuroshio east of Luzon while decreasing the Mindanao Current Opening or closing the Sunda Shelf/Java Sea or the Sulu archipelago does not affect the transport of the Pacific to Indian Ocean throughflow

Effects of plankton dynamics on seasonal carbon fluxes in an ocean general circulation model

Abstract: We discuss the effect of embedding a simple plankton model in the Hamburg model of the oceanic carbon cycle (HAMOCC3) [Maier-Reimer, 1993]. The plankton model consists of five components: phytoplankton, zooplankton, detritus, dissolved organic carbon, and nutrients. Interactions between compartments are described by one global set of parameters. Despite its simplicity the plankton model reproduces regional differences in seasonal oceanic pCO2 and improves the biogeochemical tracer distributions at the depth of the oxygen minimum in the Pacific Ocean. The predicted seasonal turnover of organic material is consistent with recent atmospheric O2 measurements in the remote areas of the Southern Ocean.

Remotely sensed surface currents in Monterey Bay from shore-based HF radar (Coastal Ocean Dynamics Application Radar)

Abstract: Near-surface currents in Monterey Bay derived from a network of shore-based HF radars are presented for August–December 1994 and compared with those from April to September 1992. Focus is placed on the low-frequency (2- to 30-day period) motions in the remotely sensed data and on comparison of radar-derived currents with moored current and wind observations, ship-based acoustic Doppler current profiler observations, satellite-based surface temperature imagery, and surface drifter velocities. The radar-derived picture of the late summer mean flow is very similar in the two realizations and is consistent with historical data. Flow is equatorward in the outer part of the bay, poleward in a narrow band nearshore, and very sluggish in the middle of the bay. Low-pass-filtered time series of radar-derived currents are highly correlated with moored current observations and with winds in the outer part of the bay. The vector time series are also coherent across a broad frequency band with currents typically in phase between 1- and 9-m depths and with 1-m currents typically 40°–60° to the right of the wind. Overall, these results confirm the utility of Coastal Ocean Dynamics Applications Radar (CODAR)-type HF radars for the study of coastal surface currents out to ranges ∼50 km from shore, particularly for highly averaged fields. Data variability and comparison with in situ observations for high-frequency (1- to 48-hour period) motions point to the need to better characterize and minimize sources of error in the radar observations.

Coastal upwelling indices, west coast of North America, 1946-1995

Abstract: Long time series of ocean surface currents are not available, however reasonable estimates of surface transport and coastal upwelling may be made using planetary boundary layer theory and the geostrophic wind approximation. PFEG generates daily and monthly indices of coastal upwelling at 15 standard geographic points along the west coast of North America. The first set, beginning in 1967, is comprised of daily means of six-hourly upwelling indices, estimated from six-hourly synoptic pressure fields. The second set of indices are derived from monthly-mean pressure fields, and extend back over 50 years to 1946. The annual cycle is estimated at each point by a least-squares regression of the 1967-91 daily data to an annual and semiannual harmonic signal. The means and standard deviations of monthly values are calculated for the same 25-year period to compare their annual climatologies to those from the daily indices. Upwelling north of 30N has a strong annual signal. Greatest upwelling rates occur at 33N, with a linear decrease in the maximum upwelling indices north to about 54N. The greatest annual range occurs at 39N. The date of maximum upwelling increases linearly from late April at 21N to mid-July at 48N. Minima occur within 15 days of 1 January at all latitudes, with negative (downwelling-favorable) indices occurring north of 36N during at least part of the year. Downwelling is a year-around feature along the British Columbia coast. The annual range off Baja California is relatively small. This region is also characterized by secondary minima and maxima in August and October, respectively. Highest interannual variance at a location occurs during months of greatest absolute index values. Variance south (north) of 45N is greatest in summer (winter). Indices derived from monthly pressure fields are consistently greater than monthly averages of the daily indices. This discrepancy is greatest during winter downwelling at northern points, and summer upwelling at southern points. Daily standard errors are consistently greater than the associated monthly deviations, due to the high variability on a given Julian day associated with synoptic atmospheric motions. Monthly standard deviations off central and southern California peak in spring and remain relatively high through the summer, while daily standard errors decline rapidly between late winter and summer. One of the most striking highlights is the relatively short duration of significant positive and negative anomalies (< 1 year), despite the large latitudinal extent of most anomalies. The indices also feature sudden shifts in their temporal patterns. These may be attributed to changes in the source of the monthly pressure fields, or in the methodology used to interpolate the gridded pressure fields used to calculate these indices, rather than true environmental changes.

How well does a 1/4° global circulation model simulate large-scale oceanic observations?

Abstract: Numerical high-resolution ocean general circulation models have experienced a revolutionary development during the last decade Today they are run globally in realistic configuration with realistic surface boundary forcing To fully use the results of those models in understanding various aspects of the ocean general circulation and to combine ocean observations with models (state estimation) in a manner consistent with the data and model dynamics, stringent model-data comparisons are a necessary first step In this paper a quantitative model-data comparison is carried out for the global Parallel Ocean Climate Model (POCM), known also as the Semtner and Chervin model, with nominal lateral resolution of 1/4° The focus is on various aspects of the simulated large-scale circulation and their relation to the TOPEX/POSEIDON sea surface height (SSH) observations and World Ocean Circulation Experiment (WOCE) hydrography Comparisons are made for (1) the global mean sea surface circulation and absolute slopes, (2) rms SSH variability and eddy kinetic energy, (3) the simulation of the observed seasonal cycle in SSH, (4) two-dimensional frequency-wavenumber spectra of the large-scale fluctuations, as well as (5) the hydrography for WOCE sections Recent improvements in external surface forcing fields including daily wind-stress fields and sea surface heat fluxes lead to a significant improvement in the overall agreement of the simulated and observed large-scale mean circulation and its variability However, simulated amplitudes of variability remain low by about a factor of 2 to 4 over a broad spectral range, including the long wavelengths and periods Both the causes and consequences of this low variability remain obscure

Deep water ventilation in the South China Sea

Abstract: As the western Pacific intermediate water funnels into the deep basin of the South China Sea through the Luzon Strait, how does it eventually upwell? This question is examined by releasing a passive tracer, which is uniformly distributed in the deep basin initially, in a three-dimensional, climatology-driven circulation model. Subsequent advection and diffusion of the tracer are studied in relation to the circulation in the deep basin. Two primary regions of deep upwelling responsible for deep water renewal are identified: southwest of Taiwan and off Vietnam. The former is associated with inflow of water at the sill depth of the Luzon Strait, while the latter is induced by orographic lifting of currents over the continental margin. Upwelling off Vietnam begins in August near a topographic bump at 16°N, 114°E, expands southwestward until December, and is weakened by downward motion during the rest of the year. Simultaneous occurrence of deep downwelling off Luzon and Palawan with deep upwelling off Vietnam suggests that the upward tracer dispersion is modulated by basin-wide circulation at mid-depths. At shallow depths, upwelling is present off Vietnam in summer and off northwest Luzon in winter, and shelf break upwelling appears on the edge of Sunda Shelf from October to December, when the southward coastal jet impinges on the shelf. However, these shallow upwelling processes have little effect on the deep water renewal. These findings are supported by climatological data.

Dynamics of the Kuroshio/Oyashio current system using eddy-resolving models of the North Pacific Ocean

Abstract: A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5-layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model-data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actually show mixed barotropic-baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six-layer simulations show a 3-Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy-driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.

Dynamic Ocean-Atmosphere Coupling: A Thermostat for the Tropics

Abstract: The ocean currents connecting the western tropical Pacific Ocean with the eastern tropical Pacific Ocean are driven by surface winds. The surface winds are in turn driven by the sea-surface temperature (SST) differences between these two regions. This dynamic coupling between the atmosphere and ocean may limit the SST in the tropical Pacific Ocean to below 305 kelvin even in the absence of cloud feedbacks.

An atmospheric energy-moisture balance model: Climatology, interpentadal climate change, and coupling to an ocean general circulation model

Abstract: An atmospheric model incorporating energy and moisture balance equations is developed for use in process studies of the climate system. Given the sea surface temperature and specified surface wind field, the atmospheric model calculates the surface fields of air temperature, specific humidity, as well as heat and freshwater fluxes. The inclusion of the moisture balance in the atmospheric model allows the effects of latent heat transport to be included explicitly in the model. Under fixed climatological sea surface temperature (SST) and surface wind conditions, surface air temperatures, specific humidities, and surface fluxes are comparable to direct estimates. Precipitation compares less favorably with observations. As an extension to the climatological forcing case, we conduct a simple perturbation experiment in which the 1955–1959 pentad is compared to the 1970–1974 pentad by driving the model under the respective SST fields. The model exhibits a global air temperature decrease in the latter pentad of 0.27°C (comparable to direct estimates) with cooling in the northern hemisphere and warming in the southern hemisphere. Such large-scale cooling in our atmospheric model is driven by equivalent local changes in the prescribed SST fields, subsequently smoothed by atmospheric diffusion of heat. The interpentadal modeled differences are shown to be quite robust through model experiments using parameters representative of several different unrealistic climatologies. The resulting interpentadal difference fields change remarkably little even when the background state has changed dramatically. This emphasizes the almost linear response of the atmospheric model to the imposed SST changes. The atmospheric model is also coupled to an ocean general circulation model without the need for flux adjustments. This coupled climate model faithfully represents deep water formation in the North Atlantic and Southern Ocean, with upwelling throughout the Pacific and Indian Oceans. Water mass characteristics in the vertical compare very favorably with direct observations.

Circulation near submarine canyons: A modeling study

Abstract: Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantial cross-shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right-bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the canyon and weak exchange across the shelf break, while left-bounded flow creates upwelling at the head of the canyon and strong exchange between the ocean and shelf. In left-bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification limits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification, the surface temperature (density) is not modified at all by the flow interaction with the submarine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coastal flow.

Origins and variability of the Benguela Current

Abstract: 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.

Moored Observations of Western Boundary Current variability and thermohaline circulation at 26.5°N in the subtropical north Atlantic

Abstract: A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation. Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer,...

Future ocean uptake of CO2 : interaction between ocean circulation and biology

Abstract: We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition (Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric CO2. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO2. Running climate models and carbon cycle models separately seems to be justified.

Simulation of the Tropical Oceans with an Ocean GCM Coupled to an Atmospheric Mixed-Layer Model

Abstract: 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...

Rapid changes in ocean circulation and atmospheric radiocarbon

Abstract: A latitude-depth, coupled global ocean-ice-atmosphere model is extended to include a simple biosphere component. A physically reasonable adjustment of runoff into the North Atlantic is invoked to achieve a transient response to glacial meltwater perturbations, which closely resembles the Younger Dryas climate event. We then investigate the evolution of the isotopic ratio of atmospheric radiocarbon, Δ14C, due to the rapid changes of deep ocean circulation. When the North Atlantic branch of the conveyor belt circulation is interrupted, the oceanic uptake of radiocarbon is reduced, resulting in an increase of atmospheric Δ14C by about 35 ‰. The reduction of ventilation in the North Atlantic is partly compensated by an increase of the 14C ratios of the biosphere, the Southern Ocean, and the upper ocean above 1000 m depth. A plateau of the 14C year/calendar year relation can be generated at the time of the rapid reinitiation of deep ocean ventilation which begins coincident with the major temperature increase and lasts for about 60 years. It is hence significantly shorter than that found by analyzing tree rings during the termination of Younger Dryas (longer than 400 years). A sensitivity study reveals that the duration of the plateau depends strongly on the transient evolution of the gas exchange rate and can increase to 150–300 years if changes of pCO2 or sea ice coverage are taken into account.

Coastal ocean fronts and eddies imaged with ERS 1 synthetic aperture radar

Abstract: ERS 1 C band synthetic aperture radar (SAR) data were collected during the Norwegian Continental Shelf Experiment (NORCSEX) both in November 1991 during the ERS 1 commissioning phase and at different seasons in 1992 and 1993. Characteristic SAR image expressions are observed in relation to perturbation of the surface current- short wave interaction across the Norwegian Coastal Current front for winds less than 10-12 rn s -1. In situ measurements document the existence of alternating zones of convergence and divergence coexisting with a strong near-surface current shear of nearly 4f (where f is the Coriolis parameter) across a distance of a few kilometers. Under calm to moderate winds, i.e., 4-7 rn s -1, characteristic expressions of upper ocean circulation features also include the manifestation of eddies through the presence of surface film which damps the Bragg waves. Comparison of a near-coincident National Oceanic and Atmospheric Administration advanced very high resolution radiometer image and an ERS 1 SAR image supports the interpretation that surface current fronts are imaged by SAR. In combination with an SAR image simulation model, the relative quantitative importance of shear, convergence, and divergence along the front is examined. Although the model formulation is simple and the absolute magnitude of the perturbations is uncertain, the study shows that the SAR images can sometimes be used to interpret frontal dynamics, including growth and decay of meanders.

Global Ocean Circulation from Satellite Altimetry and High-Resolution Computer Simulation

Abstract: The sea surface elevation relative to the geoid, a dynamic boundary condition for the three-dimensional oceanic pressure field, is being determined over the global ocean every 10 days by a precision radar altimeter aboard the TOPEX/POSEIDON satellite. This is the most accurate altimeter data stream to date for the study of the ocean general circulation and its variability. The authors compare results from 2 years (October 1992-October 1994) of the satellite observations to computer simulations for the same period using a state-of-the-art ocean general circulation model driven by realistic winds from an atmospheric weather-prediction model. The average horizontal resolution of the model is 1/5° (varying from 30 km at the equator to 6 kin at the polar latitudes), the highest for a global simulation performed to date. Comparisons of the mean circulation, the mesoscale variability, the amplitude, and phase of the annual cycle, as well as intraseasonal and interannual changes show that the simulations...

The three-dimensional structure of an upper ocean vortex in the tropical Pacific Ocean

Abstract: IN the equatorial Pacific Ocean, easterly trade winds and the Earth's rotation combine to drive surface currents away from the Equator, thereby causing cold nutrient-rich subsurface water to upwell The front1 that forms between this upwelled water and warmer waters north of the Equator is sometimes visible as a spectacular "line in the sea"2 between 2° and 6° N Westward-propagating cusp-shaped disturbances observed along this front3 have been attributed to the effect of dynamical instabilities in the system of zonal equatorial currents4–11but the connection between these phenomena remains unclear Here we report extensive measurements from shipboard sensors, satellite and drifting buoys which reveal the three-dimensional structure of an anticyclonic eddy (or vortex) 500 km in diameter and centred at 4° N We suggest that cusp-shaped disturbances of the front are caused by trains of large-amplitude vortices, which are driven by instability of the mean zonal shear We show that these vortices not only play an important role in the meridional transport of heat, salt and momentum, but are also associated with regions of intense horizontal convergence along the front, where dramatic concentrations of marine life are observed

On the nature of decadal anomalies in North Atlantic sea surface temperature

Abstract: North Atlantic sea surface temperature data from the Comprehensive Ocean-Atmosphere Data Set were used to investigate the behavior of temperature anomalies on multiple-year timescales during the period 1948-1992. Monthly anomaly time series for each 2° square from the equator to 70°N were low-pass filtered at 4 years and normalized by the local standard deviation. Attention is focused on the extreme events, the upper and lower deciles, of the anomaly time series. A 45 -year sequence of January maps shows the already familiar phenomena of generally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The years 1978 through 1982 were largely devoid of persistent strong anomalies, but moderate cold conditions returned during 1983-1986. Warm conditions dominated the North Atlantic from 1987 onward. Within these thermal epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20° of latitude or longitude, but they range from barely detectable to spanning the width of the basin, the latter especially in lower latitudes. Most of the anomalies move long distances along certain preferred paths. These paths generally follow the routes of the subarctic and subtropical gyres. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approximate route of the North Atlantic Current. Midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast. The speed of these movements (1-3 km d -1 ) is generally less than the expected speed of the near-surface ocean circulation. Simple ideas about the effects of beta dynamics and air-sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies. The long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and propagation of features.