摘要:

We investigate the seasonal sea surface height (SSH) variability on large spatial scales in the North Atlantic by using both a numerical simulation and in situ data. First, an ocean general circulation model is run with daily forcing from the European Centre for Medium‐Range Weather Forecasts reanalysis. We evaluate the different contributions to the seasonal SSH variability resulting from the surface heat fluxes, advection, salt content variability, deep ocean steric changes, and bottom pressure variability. These terms are compared with estimates from in situ data. North of 20°N, there is an approximate balance betweenhQ, the air‐sea heat flux induced changes in steric height, and SSH variability. The next important component is the advection (its contribution to the annual amplitude is of the order of 1 cm except near the western boundary); other contributions are found to be smaller. Between 10°N and 10°S the advection variability induced by the seasonal wind stress cycle is the primary source of SSH variability. We then compare the sea surface height annual harmonic from TOPEX/Poseidon altimetry with the steric effect from the heat flux and with model and/or in situ estimates of the other terms. In many areas north of 20°N the balance betweenhQand the altimetric SSH seasonal cycle is closed within the uncertainty limit of each of the terms of the SSH budget. However,hQand the SSH do not balance each other in the eastern North Atlantic, and the results are sensitive to the choice of the heat flux product, suggesting that significant errors, typically 20–40 W m−2for the seasonal cycle amplitude, are present in the meteorological model

ISSN:0148-0227    

DOI:10.1029/1999JC900296

年代:2000

数据来源: WILEY

摘要:

The National Centers for Environmental Prediction ocean reanalysis data for 1980–1995 have been analyzed along mean constant density surfaces (mean isopycnals) in order to describe better and understand the three‐dimensional space‐time evolution of the 1991–1992 El Niño event in the tropical Pacific. The major finding is of a well‐defined, shallow pathway of off‐equatorial temperature anomalies along the Pacific North Equatorial Countercurrent (NECC) associated with the onset phase of the event. This pathway originates from the western boundary off the equator in the Northern Hemisphere along 6°–10°N and then basically follows the mean circulation eastward and upward along the NECC path toward the central basin. Along this pathway, temperature anomalies show coherent phase relationships off and on the equator and in surface and subsurface layers, respectively. A sequence of events is described that lead to E1 Niño conditions in the tropical Pacific Ocean. Beginning in early 1989, a positive temperature anomaly progressed coherently eastward along the NECC path off the equator toward the central basin, finally making its way equatorward with the expected mean circulation in late 1990. As the thermocline shoals eastward and upward along the NECC, in due course the progressing subsurface anomaly outcropped near the date line off the equator and initiated and sustained a midbasin warm sea surface temperature (SST) anomaly during the period from late 1990 through early 1991. This SST anomaly induced westerly wind anomalies over the western tropical Pacific, favoring Ekman convergence onto the equator. These anomalous surface currents advected these initially subsurface‐produced SST anomalies equatorward. Subsequently, these SST and wind anomalies were coupled, which resulted in a large‐scale relaxation of the trade winds over the western and central tropical basin, generating eastward currents that transported water mass eastward along the equator. These results are markedly different from the delayed oscillator physics in that the major role can be played by advection and outcropping of off‐equatorial subsurface thermal anomalies along the shallow NECC pathway, not necessarily involving the western boundary reflection of

ISSN:0148-0227    

DOI:10.1029/1999JC900316

年代:2000

数据来源: WILEY

摘要:

Surface fluxes in the western equatorial Pacific warm pool region and their influence on sea surface temperature (SST) variability were investigated for the period November 15, 1992, to February 15, 1993, during the Intensive Observing Period of the Coupled Ocean‐Atmosphere Response Experiment (COARE). A blended flux data set was developed using a “reanalysis” of surface meteorology from the European Centre for Medium‐Range Weather Forecasts (ECMWF), turbulent flux components from the COARE bulk flux algorithm, and shortwave radiation and precipitation estimates from satellite remote sensing. Comparison with in situ fluxes from the center of the warm pool showed that the blended fluxes captured variability associated with the intraseasonal oscillation (ISO) while the fluxes produced by ECMWF did not. The influence of surface forcing on SST was assessed by comparing the observed SST tendency to that produced by a one‐dimensional model forced by the blended fluxes. On the ISO timescale, SST changes in a substantial portion of the warm pool were found to be dominated by local surface forcing, although significant contributions from other processes (e.g., horizontal advection) were not ruled out. The results emphasize the fact that high‐quality surface fluxes are essential to developing accurate predictions of SST variability in the warm

ISSN:0148-0227    

DOI:10.1029/1999JC900313

年代:2000

数据来源: WILEY

摘要:

The winter monsoon circulation in the northern inflow region of the Somali Current is discussed on the basis of an array of moored acoustic Doppler current profiler and current meter stations deployed during 1995–1996 and a ship survey carried out in January 1998. It is found that the westward inflow into the Somali Current regime occurs essentially south of 11°N and that this inflow bifurcates at the Somali coast, with the southward branch supplying the equatorward Somali Current and the northward one returning into the northwestern Arabian Sea. This northward branch partially supplies a shallow outflow through the Socotra Passage between the African continent and the banks of Socotra and partially feeds into eastward recirculation directly along the southern slopes of Socotra. Underneath this shallow surface flow, southwestward undercurrent flows are observed. Undercurrent inflow from the Gulf of Aden through the Socotra Passage occurs between 100 and 1000 m, with its current core at 700–800 m, and is clearly marked by the Red Sea Water (RSW) salinity maximum. The observations suggest that the maximum RSW inflow out of the Gulf of Aden occurs during the winter monsoon season and uses the Socotra Passage as its main route into the Indian Ocean. Westward undercurrent inflow into the Somali Current regime is also observed south of Socotra, but this flow lacks the RSW salinity maximum. Off the Arabian peninsula, eastward boundary flow is observed in the upper 800 m with a compensating westward flow to the south. The observed circulation pattern is qualitatively compared with recent high‐resolution numerical model studies and is found to be in basic agr

ISSN:0148-0227    

DOI:10.1029/1999JC900312

年代:2000

数据来源: WILEY

摘要:

Above the Izu‐Ogasawara Trench south of Japan, direct current measurements were made at 34°N from 1987 to 1996, and hydrographic observations were carried out at 34° and 30°N in 1995. The geostrophic shears calculated from the conductivity‐temperature‐depth data were consistent with the shears calculated from the current data. It is found that there are opposing currents that flow southward on the western flank and northward on the eastern flank along the isobaths. In the cross‐trench direction the magnitude of the mean velocity tended to increase with the distance from the deepest point of the trench and exceeded 10 cm s−1on the eastern flank. In the vertical direction the mean velocity increased with depth, but it decreased just above the bottom, probably because of friction. The southward transport above the western flank was estimated to be 5–8 Sv both at 34° and 30°N. However, the northward transport above the eastern flank increased from 5 Sv at 30°N to 22 Sv at 34°N, suggesting a large in

ISSN:0148-0227    

DOI:10.1029/1999JC900324

年代:2000

数据来源: WILEY

摘要:

Historical hydrographic data and a numerical plume model are used to investigate the initial transformation, dynamics, and spreading pathways of Red Sea and Persian Gulf outflow waters where they enter the Indian Ocean. The annual mean transport of these outflows is relatively small (<0.4 Sv), but they have a major impact on the hydrographic properties of the Indian Ocean at the thermocline level because of their high salinity. They are different from other outflows in that they flow over very shallow sills (depth<200 m) into a highly stratified upper ocean environment and they are located at relatively low latitudes (12°N and 26°N). Furthermore, the Red Sea outflow exhibits strong seasonal variability in transport. The four main results of this study are as follows. First, on the basis of observed temperature‐salinity (T‐S) characteristics of the outflow source and product waters we estimate that the Red Sea and Persian Gulf outflows are diluted by factors of ∼2.5 and 4, respectively, as they descend from sill depth to their depth of neutral buoyancy. The high‐dilution factor for the Persian Gulf outflow results from the combined effects of large initial density difference between the outflow source water and oceanic water and low outflow transport. Second, the combination of low latitude and low outflow transport (and associated low outflow thickness) results in Ekman numbers for both outflows that are O(1). This indicates that they should be thought of as frictional density currents modified by rotation rather than geostrophic density currents modified by friction. Third, different mixing histories along the two channels that direct Red Sea outflow water into the open ocean result in product waters with significantly different densities, which probably contributes to the multilayered structure of the Red Sea product waters. In both outflows, seasonal variations in source water and oceanic properties have some effect on the T‐S of the product waters, but they have only a minor impact on equilibrium depth. Fourth, product waters from both outflows are advected away from the sill region in narrow boundary currents, at least during part of the year. At other times, the product water appears more in isola

ISSN:0148-0227    

DOI:10.1029/1999JC900297

年代:2000

数据来源: WILEY

摘要:

Water mass distribution was studied by analyzing historical hydrographic data in the South China Sea. Despite considerable modification of characteristics as a result of mixing, waters of both salinity maximum and minimum of the North Pacific origin were traced on the density surfaces around 25.0 and 26.73 σθ, respectively. In the salinity maximum layer, property distribution suggests an intrusion into the South China Sea all year‐round through the Luzon Strait. The seasonal variation of the intrusion contains a pronounced semiannual signal, with greater strength in winter and summer than in spring and fall. From spring to fall, the intrusion water from the Pacific is narrowly confined in the continental slope south of China; only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. In the salinity minimum layer, water enters the South China Sea only in spring, when the intrusion in the salinity maximum layer is weakest. A combined use of the “island rule” with climatological data suggests a mean Luzon Strait transport of the order 4 Sv (1 Sv = 10

ISSN:0148-0227    

DOI:10.1029/1999JC900323

年代:2000

数据来源: WILEY

摘要:

Large solitary meanders form on the Agulhas Current at irregular intervals as it travels along the east coast of South Africa. These so‐called Natal pulses are thought to have a significant effect on the shedding of Agulhas rings downstream at the Agulhas Retroflection and thereby on the exchange of water properties between the Indian and the Atlantic Ocean. Data from the Geosat, ERS 1, and TOPEX/Poseidon satellite altimeters and Advanced Very High Resolution Radiometer (AVHRR) infrared imagery from the Pathfinder project are analyzed and show that this intuitive idea seems to be correct. Close to the coast, individual altimeter tracks are used to identify the cyclonic Natal pulses as depressions in the sea‐surface topography. Using different tracks, the pulses can then be followed from close to Durban to the Agulhas Bank. They show that each shedding of an Agulhas ring is preceded by the appearance of a Natal pulse close to Durban. A significant correlation is found with a time lag of 165 days. Interpolated topography maps are used to follow pulses along the Agulhas Bank to the ring‐shedding area. They indicate that sometimes pulses trigger ring shedding by themselves or by merging with Rossby wave‐like meanders in the Agulhas Return Current. Infrared imagery supports these interpre

ISSN:0148-0227    

DOI:10.1029/1999JC900196

年代:2000

数据来源: WILEY

摘要:

A two‐dimensional modified semi geostrophic model is used to study the evolution of oceanic fronts in the presence of vertical mixing. The parameterization used for mixing is an elevated constant value of vertical viscosity and friction; hence the Ekman layer acts as a surrogate mixed layer, with, however, vertical shear allowed everywhere. An initial condition representative of the observed fields in Frontal Air‐Sea Interaction Experiment (FASINEX) is used, and its modification in the presence of vertical mixing alone is investigated. Without external forcing an Ekman layer results because of the presence of vertical geostrophic shear at the surface. The maximum density gradient moves toward the dense side of the front driven by this flow. Convergence of Ekman flow results in downward bowing of the isopycnals beneath the surface expression of the front, a feature reminiscent of density sections taken during FASINEX. This feature is not evident when a barotropic convergence field is applied in the absence of mixing or in fully nonlinear simulations of the evolution of fronts with baroclinic instability. An analytic theory suggests that the maximum density gradient will increase over time when vertical mixing alone is present. In the presence of negative uniform wind stress in the direction opposite to the surface geostrophic flow the front moves toward the denser water, and the jet uniformly decreases in strength. In the presence of a positive uniform wind in the direction of the surface geostrophic flow the surface jet initially weakens but then strengthens again as the wind‐driven Ekman flow opposes the frictionally driven Ekman flow and the cross‐front density gradient in

ISSN:0148-0227    

DOI:10.1029/1999JC900336

年代:2000

数据来源: WILEY

摘要:

For the special hydrodynamic situation of unbounded homogeneous shear layers, turbulence closure models of Mellor‐Yamada type (MY) andk‐ε type are put into a single canonical form. For this situation we show that conventional versions of MY and variousk‐ε versions lack a proper steady state, and are unable to simulate the most basic properties of stratified shear flows exemplified in, for example, theRohr et al.[1988] experiments: exponential growth at sufficiently low gradient Richardson number (Rg), exponential decay at sufficiently largeRg, and a steady state in between. Proper choice of one special model parameter readily solves the problems. In the fairly general case of structural equilibrium (state of exponential evolution) in weakly to moderately stratified turbulence (Rg≲ 0.25), the ratio between the Thorpe scale (or Ellison scale) and the Ozmidov scale varies like the gradient Richardson number (Rg) to the power 3/4, and the ratio of the Thorpe scale to the buoyancy scale varies likeRg1/2. Length scales predicted by our current model are consistent with laboratory measurements ofRohr et al.[1988], with large‐eddy numerical simulations ofSchumann and Gerz[1995], and with microstructure measurements from the 1987 Tropic Heat Experiment in the equatorial Pacific byPeters e

ISSN:0148-0227    

DOI:10.1029/1999JC900329

年代:2000

数据来源: WILEY