摘要:

Geostrophic currents and transports in the boundary currents near Madagascar are described for a section at 23°S off the east coast and at 12°S to the northeast of Cape Amber. These results are based on conductivity‐temperature‐depth and expendable bathythermograph sections made during cruises of theMarion Dufresnein 1984, 1985, and 1986, supplemented by those historical data that were sufficiently deep. The reference levels used are 1100 dbar at 12°S and 1170 dbar at 23°S, based on the results from 11 months of current meter records obtained in 1984–1985. Comparisons are made between the geostrophic current profiles and direct observations, both from moored current meters and from acoustic Doppler profiling in the Upper 200 m. The mean geostrophic transports above the chosen reference levels are 29.6 Sv northwestward off Cape Amber out to 115 km offshore and 20.6 Sv southward at 23°S out to 110 km offshore. The directly recorded currents show no detectable seasonal signal below 200 m: above that level other historical data suggest a seasonal amplitude in transport of approximately ±2 Sv at 12°S and ±0

ISSN:0148-0227    

DOI:10.1029/JC093iC05p04951

年代:1988

数据来源: WILEY

摘要:

Moored current measurements of 11‐month duration were carried out in the boundary currents east of Madagascar, near 12°S at Cape Amber where the mean current flows northwestward and near 23°S where the mean current flows approximately southward. Transports derived from the moored current measurements in the depth range 150–1100 m compare reasonably well with those derived from ship sections by Swallow et al. (this issue). At 12°S, very energetic boundary current transport variations occur in the 40‐ to 55‐day‐period band, contributing about 40% to the total transport variance, while at 23°S the 40‐ to 55‐day‐period band fluctuations contribute only 15% to the total transport variance. The fluctuations near 12°S do not seem to be caused by local wind forcing, which does not show an energy peak in this period band. A significant annual cycle cannot be detected in the moored current and transport time series despite significant variation of wind forcing over the subtropical Indian Ocean. A comparison of the observations is carried out with two different numerical Indian Ocean models, both forced by the seasonally varying winds of Hellerman and Rosenstein (1983). A reduced‐gravity model gives mean boundary current transports which compare well with the observations and also shows a negligible seasonal cycle. The multilayer Geophysical Fluid Dynamics Laboratory model also shows a small seasonal cycle. The observational evidence from the western subtropical Indian Ocean appears to be similar to that from the subtropical North Atlantic east of the Bahamas‐Antilles arc where also no significant seasonal boundary current response was detected, despite large annual variation of wind forcing over the ocean. The two observational situations and numerical model results for

ISSN:0148-0227    

DOI:10.1029/JC093iC05p04963

年代:1988

数据来源: WILEY

摘要:

Remote sensing is useful for studying certain Oceanographic problems only if the signal obtained from the sea surface contains information about subsurface structure. Historical bottle temperature data from the California Current were analyzed for surface manifestations of vertical structure and subsurface mesoscale structure. Results showed that surface temperature is useful in predicting thermocline strength over a large area south of Point Conception: the error of a regression estimate is 20–30% less than the error of the seasonal mean. However, surface temperature gives little useful information about mixed layer depth. Mesoscale patterns of temperature at the surface and at depth (caused by eddies, meanders and up welling) are coherent (r2>0.50) to a depth below the mixed layer only off central California and Point Conception and along the coast of Baja California. Coherence is most likely to extend below the mixed layer during summer, when the water column is strongly stratified and the mixed layer is most shallow. Thus some aspects of subsurface structure, within limited regions of the California Current, have surface manifestations potentially detectable by satellite sensor

ISSN:0148-0227    

DOI:10.1029/JC093iC05p04975

年代:1988

数据来源: WILEY

摘要:

When oceanic internal waves are forced by rapidly translating winds and wind stress is applied as a body force which is uniform within and vanishes outside a surface layer, their spectrum has a vertical wave number shape indistinguishable from the Garrett and Munk observational synthesis. Bandwidth and high‐wave number dependence are functions of the stress gradient layer thickness and vertical structure. The close correspondence between the predicted spectrum of forced waves and observed spectra suggests that the wave number shape of the near‐inertial part of the oceanic internal wave spectrum is a consequence of wind forcing. The same mechanism is consistent with observed equatorial wave number spectra. Conversely, if mid‐latitude near‐inertial internal waves and equatorial waves are directly wind forced, then their observed wave number spectra imply that the vertical gradient of stress due to wind is nearly a step function which cuts off sharply at a depth comparable to observed seasonal pycnocline

ISSN:0148-0227    

DOI:10.1029/JC093iC05p04985

年代:1988

数据来源: WILEY

摘要:

The Gulf of California, a narrow, semienclosed sea, is the only evaporative basin of the Pacific Ocean. As a result of evaporative forcing, salinities in the gulf are 1 to 2 ‰ higher than in the adjacent Pacific at the same latitude. This paper examines the thermohaline structure of the gulf and the means by which thermohaline exchange between the Pacific and the gulf occurs, over time scales of months to years. In addition to evaporative forcing, air‐sea heat fluxes and momentum fluxes are important to thermohaline circulation in the gulf. From observations presented here, it appears that the gulf gains heat from the atmosphere on an annual average, unlike the Mediterranean and Red seas, which have comparable evaporative forcing. As a result, outflow from the gulf tends to be less dense than inflow from the Pacific. Winds over the gulf change direction with season, blowing northward in summer and southward in winter. This same seasonal pattern appears in near‐surface transports averaged across the gulf. The thermohaline circulation, then, consists of outflow mostly between about 50 m and 250 m, inflow mostly between 250 m and 500 m, and a surface layer in which the direction of transport changes with seasonal changes in the large‐scale winds. Using hydrographic observations from a section across the central gulf, total transport in or out of the northern gulf is estimated to be 0.9 Sv, heat gain from the atmosphere is estimated to be 20 to 50 W m−2, and evaporation is estimated to be 0.95 m yr−1. These estimates are annual averages, based on cruises from several years. Seasonal variations in thermohaline structure in the gulf are also examined and found to dominate the variance in temperature and density in the top 500 m of the water column. Salinity has little seasonal variability but does exhibit more horizontal variablility than temperature or density. Major year‐to‐year variations in thermohaline structure may be attributable to El Niño‐Southern

ISSN:0148-0227    

DOI:10.1029/JC093iC05p04993

年代:1988

数据来源: WILEY

摘要:

The force of gravity singles out the vertical direction in a stably stratified fluid. Since buoyancy forces remove energy preferentially from the vertical component of velocity, there is a pervasive belief that turbulence in such systems may be significantly anisotropic, with horizontal length and velocity scales greatly exceeding their vertical counterparts. This paper examines the scaling of the equations of motion of a stratified fluid, assuming that the energy‐containing scales of the motion are significantly anisotropic and that nonlinear terms are important in the equations of motion (an assumption of the existence of “turbulence”). Relevant nondimensional parameters are found to be the vertical Reynolds numberRew≡wh/v, wherewandhare vertical velocity and length scales respectively, and the turbulence Froude numberFr≡u/Nl, whereuandlare the horizontal velocity and length scales, respectively, andNis the buoyancy frequency. Two determinate scalings are found. The first case, termed buoyancy‐affected highRewturbulence and characterized byRew≫ 1 andFr∼ 1, is necessarily isotropic. This scaling has its roots in the work of Dougherty (1961); its use is widespread in recent literature. It is important to recognize this as the appropriate scaling for isotropic turbulence in which the importance of buoyancy forces is in limiting the vertical (hence through isotropy, also the horizontal) scale of the energy‐containing eddies which irreversibly transfer energy to smaller scales, while noting that in a stratified fluid there are other possible modes of motion which can transfer energy reversibly (linear waves) or to larger scales (vortical modes). The second, termed buoyancy‐affected lowRewturbulence and characterized byRew∼ 1 andFr∼ 1, is truly anisotropic, with the possibility ofu≫wandh≪1. The field observations of Gargett et al. (1984) and the laboratory observations of Stillinger et al. (1983) and Itsweire et al. (1986) are examined in the context of these scalings. It is concluded that the field observations are characteristic of the buoyancy‐affected highRewregime, while the laboratory regime may be buoyancy‐affected lowRewthroughout its reported evolution. A critical test for the existence of

ISSN:0148-0227    

DOI:10.1029/JC093iC05p05021

年代:1988

数据来源: WILEY

摘要:

Coastal density currents induced by buoyancy supply along an upper coast are studied in a three‐dimensional numerical model. Scaling analysis of the governing equations reveals that when the vertical eddy diffusivity is small, the horizontal Ekman numberEh=Ahƒ/(g'Qe)2/3is the crucial parameter determining the structures of the current, whereAhis the horizontal eddy viscosity,Qeis the discharge rate per unit length along the coast,g' is the reduced gravity constant, and ƒ is the Coriolis parameter. Based on numerical experiments withAhandQevaried as external parameters, a criticalEh(Ehc) is found between 0.354 and 0.566. The density current develops unstably below this value. In cases whereEh

ISSN:0148-0227    

DOI:10.1029/JC093iC05p05037

年代:1988

数据来源: WILEY

摘要:

Extraordinary interannual variability in ice cover, air and sea surface temperatures (SST), and surface winds in the eastern Bering Sea have been observed over recent years. To investigate the causes of this interannual variability, long‐term (20–30 years) time series of air, ocean, and ice parameters from the Bering Sea were cross‐correlated with the southern oscillation index (SOI), an index of El Nino‐Southern Oscillation (ENSO) events in the tropical southern hemisphere, as well as with an index of Pacific/North American (PNA) events in the north Pacific. Five to thirty percent of the interannual variability (linear regression with various smoothing) in the Bering Sea data sets, with the exception of surface winds, is explained by the SOI when the Bering Sea data lags the SOI. For comparison, 29–52% of the variability in the SST off South America can be explained by the SOI. The signs of the correlations all suggest that warming in the Bering Sea follows negative anomalies in the SOI, that is, ENSO events. Positive anomalies in the SOI, which tend to precede El Nino events, were found to precede cooling in the Bering Sea. Significant correlation persists for 18–20 months. Higher‐order polynomial regressions between the SOI and Bering Sea data can explain up to 40% of the Bering Sea variability. The mechanism for the connection between ENSO events and Bering Sea interannual variability appears to be of atmospheric nature and is associated with the winter position and intensity of the Aleutian low. The Aleutian low is intensified and eastward of normal in association with El Nino, i.e., warm events but is weaker and westward of normal during cool events. It is the regional winds associated with the variable position of the Aleutian low that cause the regional warming and cooling events. This seesaw in the Aleutian low is also used to explain the out‐of‐phase ice conditions between the Bering Sea and the Sea of Okhotsk. PNA events are also associated with an intensified Aleutian low as well as 700 mbar ridging over northwestern North America causing southerly flow over Alaska. However, while correlation between ENSO events and PNA was significant, the correlation between the Bering Sea and PNA was marginal except for surface winds. Here surface winds from the north were significantly correlated with the PNA. This apparent inconsistency, as well as the lack of correlation of surface winds with the SOI, is explained by the lack of preferred site of the Aleutian low during ENSO events. This lack of preferred site helps explain why the major El Nino event of 1982–1983 had little apparent effec

ISSN:0148-0227    

DOI:10.1029/JC093iC05p05051

年代:1988

数据来源: WILEY

摘要:

Wind stress curl patterns over the north Pacific, between 3° and 55°N, are calculated from the Comprehensive Ocean‐Atmosphere Data Set (COADS). The mean wind stress curl pattern consists of a basin‐wide band of negative curl south of about 30°N and a basin‐wide band of positive curl to the north. Most of the variance lies in the northern half of the domain, associated with variability in the Aleutian low during winter. The dominant modes of variability are associated with the changes in position and intensity of the Aleutian low and the north Pacific (subtropical) high, each of which extends across the basin, the former in winter and the latter in summer. The seasonal signal accounts for about 40% of the variability in the region of the Aleutian low, about 30% of the variability in the northeast of the basin, and about 60% of the variability in the southwest of the basin between 20° and 30°N. The seasonal signal and first three nonseasonal empirical orthogonal functions (eofs) account for about 60% of the total variability in the mid‐latitude interior Pacific. The first eof represents variations in intensity of the Aleutian low, the second eof represents changes in intensity of the subtropical high and in the latitudinal position of the Aleutian low, and the third eof represents changes in the longitudinal position and orientation of the low. These three eofs together account for about 30% of the nonseasonal variability. On an interannual time scale, a negative (positive) wind stress curl anomaly in winter in the northeast often coincides with a positive (negative) SST anomaly in that region, although the relationship is far from perfect because of the importance of other mechanisms which influence SST variability. There is no distinct relationship between El Nino‐southern oscillation events and wind stress curl anomalies, although weak El Ninos seem to coincide with a positive wind stress curl anomaly across the central

ISSN:0148-0227    

DOI:10.1029/JC093iC05p05069

年代:1988

数据来源: WILEY

摘要:

A two‐level model with a flat bottom on a β‐plane is employed to examine wind‐ and buoyancy‐driven coastal circulation on the western boundary of the ocean. The model of the ocean interior receives wind stress through the surface Ekman layer, which is purely determined from wind stress. Buoyancy flux is given to the upper level. Momentum equations are linear, and geostrophy in cross‐shore momentum is assumed. Density equations retain horizontal and vertical advection terms as well as horizontal diffusion. This cost‐effective three‐dimensional model is capable of a case study of a seasonal time‐scale simulation with various parameters varied. Oceanic responses to wind stress and buoyancy flux occur only along the coast to right of the forcing area facing offshore. A barotropic component merging to the western boundary is responsible for elimination of the undercurrent in the wind‐driven flow in most cases. The widths of the barotropic and baroclinic components are proportional to (horizontal viscosity)1/3and (horizontal diffusion)1/2, respectively. Thus the undercurrent exists with a small diffusion coefficient. A wind‐driven current in a stratified system is insensitive to bottom friction, while a homogeneous system is sensitive to the friction. A buoyancy flux produces a baroclinic eddylike feature in the forcing area plus a baroclinic coastal flow. A significant barotropic component is produced by bottom friction in the buoyancy‐driven case, increasing (reducing) the upper (lower) level flow. Model sensitivity to assumed interactions between the Ekman layer and upper level and vertical finite‐difference scheme is examined. The flow field is insensitive to the assumptions and scheme, while vertical str

ISSN:0148-0227    

DOI:10.1029/JC093iC05p05078

年代:1988

数据来源: WILEY