摘要:

Possible source regions of the North Pacific Intermediate Water (NPIW) were investigated with synoptic conductivity‐temperature‐depth surveys. In the Okhotsk Sea Kuril Basin a pycnostad (referred to as the Okhotsk Sea Mode Water (OSMW)) was found in the density range of 26.6–27 σθ. The pycnostad is not seen in the Western Subarctic Gyre (WSAG); thus it is an indicative water peculiar to the Okhotsk Sea. The maximum thickness is at 26.7–26.9 σθ(centered at 26.8 σθ) which coincides with the NPIW density range in the North Pacific Subtropical Gyre. In the Oyashio area east of the southern Kuril Islands and Hokkaido, waters that have intermediate temperature, salinity, and potential vorticity between the OSMW and the WSAG water were distributed, suggesting a following transport route of the NPIW source water. The thick (thus low‐Q;Qis potential vorticity) and low‐salinity water mass outflowing from the Okhotsk Sea mixes with the relatively warm, saline, and high‐QWSAG water to form the Oyashio water. A coastal part of the Oyashio water flows southwestward along the east coast of the southern Kuril Islands and Hokkaido, retaining the pycnostad (Qminimum). A large part of the low‐QOyashio water flows further southward and merges with the Kuroshio Extension and then flows eastward, forming new NPIW. In this process a salinity minimum develops and theQminimum disappears because of the mixing with a saline Kuroshio water (old NPIW) without a pycnostad in the NPIW density range. These results suggest that the origin of NPIW is the water mass in the Okhotsk Sea (OSMW) and the density of the water mass primarily determines the NPIW density cente

ISSN:0148-0227    

DOI:10.1029/96JC02938

年代:1997

数据来源: WILEY

摘要:

Wind data, obtained from the model results of the European Centre for Medium Range Weather Forecasts (ECMWF), for the equatorial Pacific Ocean from 1985 to 1992, were employed to study the basin‐wide distributions for various constituents of zonal wind stress. Forcing the ocean by the individual constituent of zonal wind stress, the upper ocean thermal variation was investigated using a linear analytical ocean model. The interannual variation of zonal wind stress is largest in amplitude in the west‐central part of the basin and is dominated by an eastwardly propagating wave. The upper layer thickness perturbationhshows an evolving eastward propagation over the region where the interannual wind stress variation occurs, while to the east and west the region changes nearly in phase. The evolution ofhagrees well with the evolution of upper ocean heat content anomaly, estimated from the Tropical Atmosphere‐Ocean (TAO) moored array. The annual cycle of zonal wind stress can be described properly by a combination of annual and semiannual variations. The annual variation of zonal wind stress was found to propagate westward with relative maxima out of phase to the west and east of the west‐central Pacific. The semiannual variation is stationary and is largest in amplitude in the western basin. Forced by the two components of the annual cycle of zonal wind stress, the evolution ofhshows a westward propagation. This does not correlate well with the pattern of the evolution of the annual cycle of upper ocean heat content which emanates from the east‐central Pacific and shows both eastward and westward propagation. The seemingly anomalous eastward propagation is mainly related to the abrupt change during the relaxation/intensification period of the easterly wind stress. The abrupt change of the annual zonal wind stress variation in both time and space is, then, critical for the evolution of the pattern of the annual ocean

ISSN:0148-0227    

DOI:10.1029/96JC03030

年代:1997

数据来源: WILEY

摘要:

Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43‐year (1950–1992) record of SST [Smith et al, 1996] and other data derived from the Comprehensive Ocean‐Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions ofHoughton and Tourre[1992]. Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is correlated with Pacific El Niño‐Southern Oscillation (ENSO) variability in several regions. The major region affected is the North Atlantic area of NE trades west of 40°W along 10°N–20°N and extending into the Caribbean. There, about 50–80% of the anomalous SST variability is associated with the Pacific ENSO, with Atlantic warmings occurring 4–5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO‐related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in question, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North Atlantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the development of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Br

ISSN:0148-0227    

DOI:10.1029/96JC03296

年代:1997

数据来源: WILEY

摘要:

Fine‐scale velocity and density profile data with concurrent turbulent velocity and temperature dissipation estimates obtained above the flanks of Fieberling Guyot, a seamount in the eastern North Pacific Ocean, are examined for evidence of near‐bottom boundary mixing. Fine‐scale shear and strain spectral levels were elevated over the flanks of the seamount in a 500‐m‐thick stratified layer above the bottom. The velocity shear was horizontally isotropic, clockwise and counterclockwise‐with‐depth shear spectral levels were comparable, and no significant correlation between shear and strain was observed. Above the steepest bottom slopes near the seamount summit rim, excess vertical strain relative to shear was observed (as compared to the background internal wave field), suggesting the presence of high‐frequency internal waves. These signals may have been the product of wave reflections from the steep flanks of the seamount and/or wave generation from tidal currents flowing over the rough bottom. Associated with the enhanced shears and strains were more frequent occurrences of low 10‐m Richardson number events, increased overturning scales, and larger estimated turbulent eddy diffusivity relative to observations 15 km or more from the seamount. In particular, turbulent diffusivity estimates increased fromO(0.1×10−4m2s−1) in the ocean interior to 1–5×10−4m2s−1within 500 m vertically (1–3 km horizontally) of the seamount flank. A simple geometric scaling argument suggests that boundary mixing of this intensity has relevance to the large‐scale circulation at abyssal depths where a large fraction of the ocean waters is

ISSN:0148-0227    

DOI:10.1029/96JC03160

年代:1997

数据来源: WILEY

摘要:

The relation between sea level and barometric pressure and, specially, the validity of the inverted barometer (IB) approximation is examined over the global oceans, using nearly 2 years of TOPEX‐POSEIDON altimeter measurements. Both crossover differences and collinear differences between consecutive cycles are utilized in this study. Linear regressions between barometric pressure and sea level time series yield coefficients between 0.8 and 1 cm/mbar poleward of 20° and as low as 0.5 cm/mbar in the equatorial regions. Such deviations from the IB value of 1 cm/mbar can be due to the presence of data errors or to correlations between pressure and adjusted sea level (i.e., sea level corrected for IB effect). A simple error model for the pressure fields and a number of sensitivity tests are used to evaluate the changes in the regression coefficient possibly induced by data errors (pressure errors, altimeter measurements errors, and radial orbit errors). The combined (root‐mean‐square) effect of the different errors amounts to 0.8 mm/mbar poleward of 20° and 1.8 mm/mbar within 20° of the equator, in general smaller than the observed deviations from the IB value. Regression coefficients thus imply a correlation between adjusted sea level and pressure. Results from a shallow‐water, global ocean model forced by realistic wind and pressure fields corroborate this finding. The model is able to explain the observed coefficients, within measurement errors, with wind‐driven effects being most important in accounting for differences from the simple IB model. Pressure‐forced dynamical signals cause maximum deviations of only 1 mm/mbar. The analyses point to the general validity of the IB approximation over the deep oceans but also highlight the complex relation between sea level and barometric pressure resulting from correlations between various sea

ISSN:0148-0227    

DOI:10.1029/96JC02920

年代:1997

数据来源: WILEY

摘要:

The response of the Mediterranean mean sea level to atmospheric pressure forcing is analyzed using 3 years of TOPEX/POSEIDON data. Coherence analysis between mean sea level and atmospheric pressure shows a significant departure from a standard inverse barometer effect at frequencies higher than 30 days−1. At high frequencies the phase difference between sea level and pressure is about 100°, while it should be 180° for a perfect inverse barometer response. This result is in agreement with previous findings and confirms the role of the Straits of Gibraltar and Sicily in limiting the water exchange (and thus the response to atmospheric pressure forcing) at high frequencies. The response of the Mediterranean mean sea level is then investigated using theCandela[1991] analytical model which takes account of friction in the Straits of Gibraltar and Sicily. The model explains a large part of the variance in TOPEX/POSEIDON mean sea level variations (50% for the western basin and 38% for the eastern basin). Compared to an inverse barometer correction, it gives a smoother response with a phase delay at high frequencies. It also explains more variance in TOPEX/POSEIDON mean sea level variations (5 cm2and 7 cm2for the western and eastern basins, respectively). This demonstrates that this simple model provides an improved correction of atmospheric pressure effects in TOPEX/POSEIDON data. As the two corrections have an rms difference of 2–3 cm with maximum differences of up to 10 cm, the impact on the mapping of oceanic circulation is not negligible. This is exemplified through the comparison of sea level anomaly derived from the two correc

ISSN:0148-0227    

DOI:10.1029/96JC02777

年代:1997

数据来源: WILEY

摘要:

This process‐oriented study of the California Current system (CCS) uses a high‐resolution, multilevel, primitive equation ocean model on a β plane to isolate the response of that eastern boundary oceanic regime to temporal and spatially varying wind forcing. To study the generation, evolution, and maintenance of many of the observed features such as currents, meanders, and eddies in the CCS, the model is forced from rest with seasonal climatological winds. In response to the prevailing wind direction, surface equatorward currents develop, along with upwelling of cooler water along the coast and a poleward undercurrent. Baroclinic/barotropic instabilities in the equatorward surface current and poleward undercurrent result in the generation of meanders near the coast. As the meanders intensify, cold upwelling filaments develop along the coast and subsequently extend farther offshore. In time, the meanders form both cyclonic and anticyclonic eddies, which subsequently propagate farther offshore. Longer simulation times (∼3–4 years), in which a quasi‐equilibrium state for the CCS is reached, show a seasonal cycle in response to the wind forcing for the coastal currents, upwelling, and filaments. The meanders and eddies, however, can be quasi‐permanent as well as seasonal features. The quasi‐permanent features play a significant role in modifying coastal currents, upwelling, and filaments, which leads to large temporal and spatial variability in the CCS. In a sensitivity study, the results from several numerical experiments are used to examine the dependence of the generation of the currents, meanders, and eddies on the type of Coriolis parameterization, wind forcing, and coastline geometry. Both the meridional variability of ƒ(β plane) and the alongshore component of the wind stress are shown to be key ingredients for generating realistic vertical and horizontal structures for the cores of the surface equatorward and the subsurface poleward currents. With such structures the currents are baroclinically and barotropically unstable, resulting in the generation of meanders, filaments, and eddies. Irregularities in the coastline geometry are shown to be important for “anchoring” upwelling and filaments as well as for enhancing the growth of meanders and eddies. Cyclonic eddies tend to form in the vicinity of capes, while anticyclonic eddies tend to form in the coastal indentations between capes. The region off Cape Blanco is identified as the location where the coastal, equatorward flow off Oregon leaves the coast to develop a meandering jet off California. The results from these experiments support the hypothesis that wind forcing and coastline irregularities on a beta plane are important mechanisms for the generation of many of the observed

ISSN:0148-0227    

DOI:10.1029/96JC02803

年代:1997

数据来源: WILEY

摘要:

The seasonal‐mean circulation over the Scotian Shelf is studied numerically by computing mean and tidal current fields for winter, spring, and summer using a three‐dimensional nonlinear diagnostic model. The mean current fields are forced by seasonal‐mean baroclinic pressure gradients, tidal rectification, uniform wind stresses, and associated barotropic pressure gradients. A historical hydrographic database is used to determine the climatological mean baroclinic forcing. Upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along‐boundary elevation gradients or depth‐integrated normal velocities. The numerical solutions for nominal bimonthly periods (January–February, April–May, and July–August) reveal the dominant southwestward nearshore and shelf‐break flows of relatively cool and fresh shelf water from the Gulf of St. Lawrence and Newfoundland Shelf, with speeds up to about 20 cm/s. The seasonal intensification of the southwestward flows is reproduced by the model, with the transport increasing from 0.3 Sv in summer to 0.9 Sv in winter on the inner Halifax section. There are also pronounced topographic‐scale influences of submarine banks, basins, and cross‐shelf channels on the circulation, such as anticyclonic gyres over banks and cyclonic gyres over basins. Baroclinicity is the dominant forcing throughout the domain, but tidal rectification is comparable on the southwestern Scotian Shelf (e.g., about 0.2 Sv recirculating transport around Browns Bank for all the periods). The mean wind stress generates offshore surface drift in winter. The solutions are in approximate agreement with observed currents and transports over the Scotian Shelf, although there ar

ISSN:0148-0227    

DOI:10.1029/96JC03285

年代:1997

数据来源: WILEY

摘要:

New hydrographic data from the Texas‐Louisiana continental shelf were combined with data from older cruises covering significant portions of this shelf to produce spatial distributions of surface and bottom temperature and salinity as well as of surface geopotential anomaly relative to 70 dbar. These were used to calculate mean fields with their standard deviations for spring (May), summer (July–August), and fall (November). For each season, histograms were prepared of differences between properties in the individual fields and our seasonal mean values at each grid point in the individual fields. These histograms have highly tuned Gaussian distributions centered on zero differences, proving that a distribution selected randomly will likely be quite similar to the mean for the season in which the sample was made. The individual fields of salinity for summer and geopotential anomaly for spring are included for comparison with the mean fields. The mean fields, produced by adding a large data set to that used byCochrane and Kelly[1986], substantiate the bimodal annual patterns of circulation and property distributions over the inner shelf region described by them. Essentially, there is downcoast (directed from the Mississippi toward Brownsville) nearshore flow except during the summer months. That flow is driven by downcoast along‐shelf wind and enhanced by Mississippi‐Atchafalaya River discharge. In July and August the average wind has an upcoast component and the nearshore flow is reversed. Patterns and values of the standard deviations are used to infer causes and magnitudes of interannual variability, respectively. Three examples of anomalous property distributions are presented to illustrate the effects of the principal external forcing mechanisms affecting interannual variability on the Texas‐Louisiana shelf. These mechanisms are wind stress, Mississippi‐Atchafalaya River discharge, and mesoscale eddies in the offshore circulation near the shelf‐slope break. For each cruise examined, residuals of geopotential anomaly and surface salinity relative to the seasonal mean are examined in relation to departures of river discharge from the long‐term (64 year) average and an index of along‐shelf wind component appropriate to the times of the cruises. The residuals of geopotential anomaly were found to be significantly negatively correlated with those of surface salinity, with an intercept of approximately zero indicating that salinity plays the dominant role relative to temperature in year‐to‐year variability of the geopotential anomaly. Positive river discharge residuals were correlated with negative surface salinity residuals; enhanced downcoast wind resulted in negative surface salinity residuals; and enhanced upcoast wind resulted in positive surface salinity residuals. Most correlations were significant (different from zero) at the

ISSN:0148-0227    

DOI:10.1029/96JC03210

年代:1997

数据来源: WILEY

摘要:

Ocean gravity waves measured at the Harvest Platform off the coast of California were analyzed using tools derived from methods of nonlinear dynamics. We compare these with laboratory data of random surface wave fields. The Harvest data show clear signs of low‐dimensional dynamics in action and have an embedding dimension of six or seven. The largest Lyapunov exponent, a measure of future uncertainty, shows that the data are from a chaotic system and that predictions are limited to a horizon of about two major periods of the predominate wavelength. These results underscore the utility of nonlinear time domain methods for signal analysis and for extracting dynamical information not visible with linear method

ISSN:0148-0227    

DOI:10.1029/96JC02993

年代:1997

数据来源: WILEY