摘要:

World ocean simulations are used to investigate the pathways feeding the Indonesian throughflow as a function of depth, including the role of the global thermohaline (“conveyor belt”) circulation. The simulations use a horizontal resolution of 1/2° for each variable and the vertical resolution ranges from 1.5‐layer reduced gravity to six layers with realistic bottom topography. They are forced by theHellerman and Rosenstein[1983] monthly wind stress climatology. Contrary to the classical theory ofStommel and Arons[1960], the Naval Research Laboratory model shows the Antarctic Circumpolar Current (ACC) region as the main region of abyssal to upper ocean water upwelling which compensates for the deep water formation in the far North Atlantic, a result corroborated by recent observational evidence [Toggweiler and Samuels, 1993]. We examine the contribution of the global conveyor belt circulation to the throughflow by systematically varying the model dynamics (e.g., by disabling the far North Atlantic ports which parameterize deep water formation in that region). The model simulations show a global conveyor belt circulation contribution of 5.7 Sv to the throughflow, a contribution provided mainly by wind‐driven upwelling in the Indo‐Pacific ACC region. This is due to a cooperative interaction between the thermohaline and wind‐driven circulations. The thermohaline circulation makes the throughflow more surface trapped and less subject to topographic blocking in the Indonesian passageways, while the wind‐driven circulation provides the Indonesian throughflow pathway for the thermohaline flow upwelled in the ACC region. Mean layer transport fields, cross‐layer mass transfer fields, and Lagrangian tracers are used to identify pathways feeding the Pacific to Indian Ocean throughflow via Indonesia. Starting from the ACC, Sverdrup flow shows a circuitous route that is northward in the eastern South Pacific, then westward in the South Equatorial Current (SEC). The SEC retroflects into the North Equatorial Countercurrent (NECC) followed by cyclonic flow around the Northern Tropical Gyre and into the North Equatorial Current (NEC), then into the Mindanao Current, the Sulawesi Sea, the Makassar Strait, and the Indian Ocean. The depth‐integrated pathways from nonlinear simulations show the retroflection from the SEC into the NECC as a secondary route and retroflection into the Equatorial Undercurrent (EUC) as the primary route. The EUC connects with the NECC by westward and then northward flow on the northside of the EUC. The pathways as a function of depth can be presented in three layers: a surface layer, the layer containing the EUC, and layers below the EUC. In the top layer the EUC to NECC connection is via upwelling from the EUC in the central and east‐central equatorial Pacific. Some of this upwelled water is returned to the EUC layer via downwelling at midlatitudes where it feeds into the NEC or SEC. Very little water in the South Pacific EUC layer passes into the Indian Ocean without upwelling into the surface layer first. While the pathways in the top two layers are complex and strongly coupled and enter the Indonesian Archipelago from the northern hemisphere, below the EUC layer a very direct Pacific to Indian Ocean route is found: SEC → Sulawesi

ISSN:0148-0227    

DOI:10.1029/96JC03602

年代:1997

数据来源: WILEY

摘要:

The mean state of the transport field of the subtropical gyre of the South Indian Ocean has been derived for the upper 1000 m from selected historical hydrographic data. The subtropical gyre in the southwestern Indian Ocean is stronger than the flow in the other two oceans of the southern hemisphere. Most of the water in the South Indian gyre recirculates in the western and central parts of the basin. In the upper 1000 m the eastward transport of the South Indian Ocean Current starts with 60 Sv in the region southeast of South Africa. Between the longitudes of 40° and 50°E about 20 Sv of the 60 Sv recirculates in a southwest Indian subgyre. Another major diversion northward occurs between 60° and 70°E. At 90°E the remaining 20 Sv of the eastward flow splits up, 10 Sv going north to join the westward flow and only 10 Sv continuing in a northeastward direction to move northward near Australia. Near Australia, there is indication of the poleward flowing Leeuwin Current with a transport of 5 Sv. In the central tropical Indian Ocean between 10°S and 20°S, about 15 Sv flows to the west. The western boundary current of this subtropical gyre consists of the Agulhas Current along the east coast of southern Africa. Its mean flow is composed of 25 Sv from east of Madagascar and 35 Sv from recirculation in the southwest Indian subgyre south of Madagascar, with only 5 Sv being contributed from the Mozambique Channel. A net southward transport of 10 Sv results for the upper 1000 m of the South Indian Ocean. In contrast to the triangular shape of the subtropical gyre in the South Atlantic, probably caused by the cross‐equatorial flow into the North Atlantic, the area influenced by the subtropical gyre in the South Indian Ocean is more rec

ISSN:0148-0227    

DOI:10.1029/96JC03455

年代:1997

数据来源: WILEY

摘要:

A simplified form of the motional induction equation is used to calculate the dominant three‐dimensional (3‐D) electromagnetic (EM) fields generated by a specified steady 3‐D global ocean circulation. The EM calculations require, at most, vertical integrations and do not require running a global 3‐D model. Two cases for ocean bottom conductivity are considered: an electrically insulating ocean bottom and a high‐conductance sediment layer. The approximations are discussed, and the solutions are plotted for various depth levels. Many aspects of the dominant ocean‐generated EM fields (particularly the electric currents near the sea surface and the magnetic fields) are shown to be insensitive to ocean bottom conductivity. Other aspects (particularly the horizontal electric field in shallow water) are very sensitive. We perform a global integration to estimate the role of the “nonlocal” electric currents. We find that the importance in including these nonlocal currents when making EM field estimates is the same or less than that for including a model for the bottom conductance. Hence the simple EM estimates from one‐dimensional integrations are not improved in globally integrated models until these models include a realistic model for bo

ISSN:0148-0227    

DOI:10.1029/96JC03545

年代:1997

数据来源: WILEY

摘要:

Five research cruises were conducted over the continental shelf and slope near the Farallon Islands, California, in February, May, August, and October/November 1991 and February 1992. The observations consisted of shipboard hydrographic and acoustic Doppler current profiler data and moored current meter measurements. Water mass anomalies were calculated for each cruise by subtracting seasonal means based on historical data. In general, the maximum anomalies were observed subsurface in the 100‐to 150‐m range. In May 1991, equatorward, upwelling favorable winds elevated the thermocline resulting in cold, salty anomalies nearshore, with cold, fresh anomalies offshore associated with the advection of Pacific Subarctic Water into the region from the north. Warm, fresh anomalies and a strongly depressed thermocline were observed during the February 1992 cruise. A combination of coastal sea level and wind stress data and output from the Los Alamos National Laboratory parallel ocean program model was used to explain the cause of these anomalies. The February 1992 anomalies were shown to be due to both the deepening of the Aleutian low in the North Pacific associated with the 1991–1993 El Nino/Southern Oscillation event in the equatorial Pacific and poleward propagating intraseasonal coastal trapped Kelvin waves also arising from this event. The anomalous poleward wind forcing produced onshore flow, deepening of the thermocline, and downwelling at progressively southward locations. The “downwelling” Kelvin waves propagated northward with the two signals meeting somewhere near the cruise region. Both the model and the coastal sea level data showed the phase speed of the waves to slow by about 50% after passing the Gulf of California. This may be due to the scattering of energy from the fastest baroclinic mode into a slower mode. The strongest wave signal in the equatorial Pacific did not necessarily produce the strongest anomalies off central C

ISSN:0148-0227    

DOI:10.1029/96JC03050

年代:1997

数据来源: WILEY

摘要:

Along‐track sea level anomalies derived from Geosat altimeter data from November 1986 to November 1988 are assimilated by Kalman filtering into a wind‐forced second‐baroclinic vertical mode linear model of the tropical Atlantic Ocean. To save computer time, the filter is degraded, mostly by fixing the error covariance matrix of the estimate once the filter has reached its asymptotic behavior. Geosat altimeter data have been processed using improved corrections. The sea surface height variability signal is extracted using the classical along‐track technique, relative to a complete reference cycle, and using only tracks longer than 2200 km. This processing has preserved oceanic signals both on large scales (above 1000 km) and on the mesoscale (around 200 km). Sea level anomalies predicted at Principe Island are close to in situ tide gage data, though some differences can be partly related to tidal or orbit error corrections. Oceanographic signals are analyzed from two different sets of fields: one issued from anisotropic space‐time objective analysis of Geosat data and the other from the model assimilation. The latter appears as an interesting method to extract low‐frequency and propagating signals. Along the equator, eastward propagating features are consistent with Kelvin waves correlated with zonal wind stress anomalies. Upwelling in the Gulf of Guinea is 1 month earlier in 1987 than in 1988. After elimination of the annual and semiannual signals by harmonic analysis, the residual signal over the whole tropical basin, decomposed into complex empirical orthogonal functions, is found dominated by variations between the 2 years, equatorial and tropical signals being ant

ISSN:0148-0227    

DOI:10.1029/96JC03245

年代:1997

数据来源: WILEY

摘要:

A numerical sigma‐coordinate ocean circulation model is used to investigate tidally forced flow near Fieberling Guyot. The aim is to reproduce the observed currents and to identify the dominant physical mechanisms that lead to the complex three‐dimensional flow fields at this tall and steep seamount in the northeast Pacific. Our very high resolution simulation (with 500 m horizontal and less than 20 m vertical grid spacing in the vicinity of the seamount summit) was performed with the newest version of the terrain‐following sigma‐coordinate primitive equation model (SPEM). As in previous more idealized studies, the seamount was placed in the center of a uniformly rotating periodic channel and forced by diurnal barotropic tides. The exponential initial stratification, the tidal forcing amplitude, and its orientation were chosen based on observations from the Fieberling area. Three major characteristics of the flow observed at Fieberling Guyot are reproduced both qualitatively and quantitatively: diurnal currents reach about 20 cm s−1, a twentyfold amplification relative to the tidal flow away from the seamount; the time‐mean anticyclonic flow speeds are close to the observed 10 cm s−1maximum azimuthal velocity; and the spatial structure of this vortex shows a maximum at about 6 km radius between 400 and 600 m depth, clearly lifted off the bottom. The time‐mean flows are found to be maintained by diurnal waves of a mixed type: the net motion shows characteristics of both bottom‐intensified seamount trapped waves and vertically propagating vortex trapped waves. While the former are mainly responsible for setting up a time‐mean anticyclonic flow along the upper flanks at the bottom, the latter are needed to redistribute the momentum and time‐mean density, thereby reproducing the observed structure with a flow maximum off the bottom. An analysis of the energy, momentum, and density fluxes shows that rectification depends critically on the eddy fluxes that balance the time‐mean downwelling over the seamount summit and upper flanks. The results of sensitivity and parameter studies are utilized to further interpret the role of individual physical mechanisms and the time

ISSN:0148-0227    

DOI:10.1029/96JC03414

年代:1997

数据来源: WILEY

摘要:

Turbulent convection from a localized circular top surface into a rotating, homogeneous layer has been investigated in a cylindrical laboratory tank. The initial conditions for the experiments were selected so that the aspect ratioR/H≫1 and a three‐dimensional turbulent layer penetrated with speedu≈(0.6±0.1) (B0h)1/3and then spread radially in the form of a gravity front upon reaching the bottom of the tank (hereB0is the buoyancy flux,his the local depth,His the total depth, andRis the radius of the source). This front later underwent a baroclinic instability generating mesoscale vortices with maximum densityg′≈(10±1) (B0R)2/3/H. Measurements and theoretical arguments have enabled us to scale the number of vorticesN≈(1.5±0.3) (R0,R)−2/3(R/H)−3/5, their mean diameterD/R≈8 ((R0,R)2/3, swirl velocity ν≈(B0R)1/3, and relative voracity ƒ′/ƒ≈0.5 (here (R0,R) = (B0ƒ3R2)1/2is a Rossby number based onR, and ƒ is the Coriolis parameter). An application of the present study to deep convective events observed in the Golfe du Lions compares favora

ISSN:0148-0227    

DOI:10.1029/96JC03546

年代:1997

数据来源: WILEY

摘要:

Nonlinear effects produced during the scattering of a barotropic shelf wave (BSW) by a spatially varying mean current are studied using a primitive equation numerical model. Both the BSW phase and the mean current propagate in the same (positive) direction along shelf/slope topography which is uniform everywhere except for a localized topographic irregularity, e.g., a submarine canyon. The mean current is specified at the upstream boundary and adjusts to the topography, closely following isobaths through the model domain. The incident BSW signal is then introduced at the upstream boundary either as a harmonic wave or as a pulse of finite duration. The BSW signal scatters its energy into other available wave modes when it encounters the topographic irregularity. The scattered wave field is dominated by evanescent modes which are trapped at the topographic irregularity and appear as intense mesoscale flows between the coast and the mean current. Nonlinear dynamics transform these large‐amplitude evanescent modes into persistent eddy‐like features on the shelf. The nonlinear interaction is much stronger when the current on the shelf associated with the BSW is opposite to the mean current direction (i.e., negative), so anticyclonic eddies are preferentially generated at the topographic irregularity. For a harmonic BSW, an anticyclonic eddy periodically appears when the negative current phase passes and disappears when the positive current phase passes. A BSW pulse with negative velocity at the coast produces a strong anticyclonic eddy which persists, after the pulse has passed, for a time period substantially longer than the pulse duration. A pulse with positive velocity at the coast does not generate any persistent features on the shelf. The anticyclonic eddies produce mass exchange between the shelf and the mean current and could contribute significantly to cross‐shelf exchange on continental sh

ISSN:0148-0227    

DOI:10.1029/96JC03452

年代:1997

数据来源: WILEY

摘要:

Patterns of wind stress and heat flux between the atmosphere and the ocean over the Southern California Bight are described based on observations from buoys and ships. During the winter, the wind stress is spatially homogeneous and temporally variable, with strong events corresponding to low‐pressure systems sweeping through the area. During the summer, spatial patterns are more persistent, with large gradients. Inshore of a line running approximately between Point Conception and Ensenada, Mexico, winds are weak. Offshore wind speeds are comparable in magnitude to those found over the continental shelf north of Point Conception. The boundary is the location of maximum wind stress curl, and the spatial resolution afforded by California Cooperative Fisheries Investigation (CalCOFI) observations suggests maximum wind stress curls over 3 times larger than the values proposed byNelson[1977]. Net heat flux estimates derived from the CalCOFI measurements are somewhat larger than the values proposed byNelson and Husby[1983], due to differences in latent heat flux estimates. Possible mechanisms responsible for the spring‐summer spatial structure in the wind and the relationship between these gradients and the properties of the underlying ocean are discus

ISSN:0148-0227    

DOI:10.1029/96JC02801

年代:1997

数据来源: WILEY

摘要:

This paper presents an analysis on the space/time statistical thermal structure in the Yellow Sea from the Navy's Master Observation Oceanography Data Set during 1929–1991. This analysis is for the establishment of an Optimum Thermal Interpolation System of the Yellow Sea (a shallow sea), for the assimilation of observational data into coastal σ coordinate ocean prediction models (e.g., the Princeton Ocean Model), and for the design of an optimum observational network. After quality control the data set consists of 35,658 profiles. Sea surface temperatures at 50% and 80% water depths are presented here as representing the thermal structure of surface, middepth, and near‐bottom layers. In the Yellow Sea shelf the temporal and spatial signals fluctuate according to the Asian monsoon. Variation of surface forcing from winter to summer monsoon season causes the change of the thermal structure, including the decorrelation scales. Our computation shows that the seasonal variation of the surface horizontal decorrelation scale is around 90 km from 158 km in winter to 251 km in summer and the seasonal variation of the surface temporal decorrelation scale is around 2.4 days from 14.7 days in winter to 12.3 days in summer. The temporal decorrelation scale increases with depth in both summer (evident) and winter (slight). The near‐bottom water (σ=0.8) has the longest temporal scale in summer, which could be directly related to the existence of the Yellow Sea Cold Water throughout the summer in the middle of the Yellow Sea. The temporal and spatial decorrelation scales obtained in this study are useful for running optimum interpolation models and for designing an optimum observational network. The minimum sampling density required to detect thermal variability in the Yellow Sea shelf would be 50–80 km and 4–6 day intervals per temperature measurement with the knowledge that the subsurface features will also be adequat

ISSN:0148-0227    

DOI:10.1029/96JC03428

年代:1997

数据来源: WILEY