摘要:

During January and February 1989 the recirculation of the subtropical gyre in the eastern North Atlantic was surveyed with a three‐ship experiment. The analysis of hydrographic measurements and velocity data from a shipboard acoustic Doppler current profiler reveals the synoptic‐scale circulation patterns and water mass distributions in the Canary Basin. The geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation. The classical circulation scheme is shown by the stream function in the upper 200 m: the Azores, Canary, and North Equatorial currents. Unlike the deep‐penetrating Azores Current, the Canary Current and the North Equatorial Current are restricted to the upper 200 m. Both carry North Atlantic Central Water along the water mass boundary with South Atlantic Central Water. South Atlantic Central Water flows through the passage between the Cape Verde archipelago and Africa via narrow currents into the area north of 14.5°N. At the southern edge of the subtropical gyre we identify an eastward flow of Antarctic Intermediate Water between 700 and

ISSN:0148-0227    

DOI:10.1029/92JC01095

年代:1992

数据来源: WILEY

摘要:

A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August–October 1990. The wind direction reversed in mid‐September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind ch

ISSN:0148-0227    

DOI:10.1029/92JC01179

年代:1992

数据来源: WILEY

摘要:

Using a water‐mass‐based climatology of temperature and salinity, geostrophic transports derived from dynamic height differences across the width of the Kuroshio Extension are presented. The dynamic height differences are obtained by calculating dynamic heights in the warmer water mass south of the Kuroshio and the cooler water mass north of the Kuroshio. The mean baroclinic transport is examined between 141°E and 165°E relative to several pressure levels. The baroclinic transports are sensitive to the selection of the northern and southern boundaries. The baroclinic transports increase from 141°E to about 145°E and decline east of 145°E. The computed seasonal variation of the longitudinally averaged baroclinic transport shows a minimum during spring and a broad maximum from July to

ISSN:0148-0227    

DOI:10.1029/92JC01180

年代:1992

数据来源: WILEY

摘要:

The Point Sur transect (POST) has been occupied 6–8 times per year since 1988 to resolve the flow in the California Current system at seasonal and interannual time scales. The POST extends offshore, normal to bottom topography, along 36°20′N, to 123°01.7′W where it doglegs southwest along the California Cooperative Fisheries Investigation (CalCOFI) line 67. Hydrographic observations from seven cruises over 2 years have been used to study variations of alongshore geostrophic velocities and water mass characteristics within these time scales. The California Undercurrent was a prominent feature in six of the seven sections analyzed and was very weak during a period of strong equatorward wind stress. The position of the undercurrent core varied from 12 to 42 km from shore while its strength varied from less than 5 cm s−1to 35 cm s−1, with the maximum flow occurring in winter. The undercurrent (core) over the continental slope was found from 70 to 460 m depth throughout these seven cruises. The nature of the alongshore geostrophic velocities and the location and spatial extent of the California Undercurrent appear strongly related to specific wind events, both local and remote. Remote wind forcing from the south was believed to cause anomalous, strong poleward flow throughout the entire water column above 1000 dbar during a period of local equatorward

ISSN:0148-0227    

DOI:10.1029/92JC01094

年代:1992

数据来源: WILEY

摘要:

The propensity for an anticyclonic eddy (∼100‐ to 200‐km diameter) to develop west of Point Arena during the summer is documented with an analysis of satellite derived sea surface temperature (SST) data and a review of published historical hydrographic data. The satellite data span the period from 1981 through 1989 and the hydrographic data include CalCOFI surveys from 1949–1959 and a few more recent studies. A closed anticyclone was apparent in many of the historical dynamic height maps, while a meander pattern, lacking closed contours, characterized the coincident hydrographic 10‐m temperature maps. A conceptual model of an eddylike perturbation imbedded in a broader equatorward flow is adopted as a working hypothesis. In this simplistic model, the only distinction between an eddy (with closed contours) and a meander (without closed contours) is the relative strength of the eddylike perturbation. This led to a method to detect the eddies in the satellite SST data, which were dominated by meanderlike patterns: A linear trend was subtracted from each image to remove large‐scale SST gradients, leaving an expression of the residual eddylike perturbation. Composite images over select time periods were produced for analysis by computing spatial variance EOFs of the detrended data. With this approach, a warm‐core eddy was evident at Point Arena in summertime satellite SST data from 1981–1985 and 1987–1989 (formation of an eddy in 1986 remains unresolved because the readily available SST data were incomplete). The eddy was bounded on the inshore side by the jet and filament system studied during the 1987 and 1988 Coastal Transition Zone (CTZ) program. Recurrence of this eddy was apparently coupled with the recurring cold filament commonly observed at Point Arena during the summer. The coevolution of the eddy and meandering jet during 1988 was studied with sequential composite empirical orthogonal function (EOF) satellite SST images. These show that a deep anticyclonic meander was present near Cape Mendocino and directed the jet toward shore. In late July 1988 this meander pinched off a new eddy, and the subsequent jet flow was redirected southward and displaced farther offshore. These events account for the transition in the jet observed in the July CTZ hydrographic surveys. The length scale and position of the Point Arena eddy is consistent with a standing wave, phase locked to a prominent cape (in this case, Mendocino). The significance of the apparent splitting off of an eddy in 1988, as well as the apparent preponderance of anticyclones evident in these EOFs, is discussed relative to recent theoretical and numerical m

ISSN:0148-0227    

DOI:10.1029/92JC01131

年代:1992

数据来源: WILEY

摘要:

Quasi‐synoptic hydrographic data and satellite imagery are used to describe the circulation and the structural variability of the Black Sea with particular emphasis on the Turkish coast. The circulation is indicated to involve a variable cyclonic circulation with no apparent central locus and a well‐defined cyclonic “Rim Current” containing meanders and interacting eddy fields confined to the shelf slope. Interspersed between the coastal eddies are filaments and intense jets, often with dipole eddies at their termina. The extension of these features across the shelf‐slope into the central basin offshore waters implies important dynamical processes related to the shelf‐deep basin exchanges. These features are often steered by the topography and evolve continuously through the mixed baroclinic‐barotropic instability of th

ISSN:0148-0227    

DOI:10.1029/92JC00812

年代:1992

数据来源: WILEY

摘要:

We consider the connection between the wind field and the mesoscale circulation in the northern Greenland Sea, using over 50 record years of current observations to define the current structure in space and time. We find that east of the East Greenland Polar Front the motion is highly coherent in the vertical and that it scales in the horizontal as the internal Rossby radius. The structure depends very little on frequency. The East Greenland Current is dominated by smaller‐scale phenomena in both space and time, and the kinetic energy is low. The available energy in the wind field may be the largest in the world, especially over the Greenland and Iceland seas. The wind power shows a marked seasonal signal that east of the East Greenland Polar Front is paralleled by a similar signal in the fluctuating kinetic energy of the current, and the former leads the current fluctuations by 1–2 months. The mesoscale motions are probably dominated by nonlinear interactions, and the observed vertical and horizontal structure is in general agreement with predictions of the theory of quasi‐geostrophic turbulence forced by atmospheric fluctuations. We conclude that the majority of the mesoscale fluctuations in Fram Strait east of the East Greenland Polar Front are generated by the wind field. However, with few exceptions, the current field is not coherent with the curl of the wind stress, probably primarily owing to the isolating effect of the rough topography. The extent to which the wind field also generates mesoscale motions in the East Greenland Current is uncertain, although the eddy kinetic energy east of the East Greenland Polar Front is probably sufficient to account for conditions in the East Greenland Current by westward advection of e

ISSN:0148-0227    

DOI:10.1029/92JC01195

年代:1992

数据来源: WILEY

摘要:

The thickness distribution of sea ice is maintained by a balance of thermal and mechanical processes. Observations now exist that make it possible to quantify this balance and to test models of the individual physical processes. In particular, the observed distributions, used with fairly well established thermodynamic growth rates, give an estimate of the redistribution process that models the formation of pressure ridges. To highlight the main ideas, the evolution of the thickness distribution is recast as a Markov process forced by the mean velocity divergence and a measure of the random short term deformation. This model reproduces features of the observed thickness distribution such as the peak near 3 m, the mean thickness somewhat greater than 3 m, the long tail, and the variable shape of the thin side of the distribution. Analytical expressions for the mean and variance of the ice thickness are given, showing how they depend on the growth rates for thick and thin ice, the equilibrium thickness, the ice deformation, and the rule for building pressure ridges. These are sensitive to the mean divergence and the melt rate for thick ice. With large amounts of data now available for both the ice thickness and the ice motion, further progress is anticipated toward understanding the processes which shape the thickness distribution.

ISSN:0148-0227    

DOI:10.1029/92JC01199

年代:1992

数据来源: WILEY

摘要:

For a number of years during the 1980s, observations of the physical and chemical properties of seawater in the southeastern Beaufort Sea have been acquired in late winter. The most complete data set, from 1987, has been used in a comparison of winter and summer [Macdonald et al., 1989] water properties in the area. Most obvious is an increase in the salinity of surface waters in winter. The magnitude of this increase varies dramatically from year to year. Part of the increase is a consequence of brine rejection during the growth of sea ice, and part is associated with an intrusion over the shelf of a water mass of high nutrient and low oxygen concentrations which is a feature of the entire western Arctic Ocean. Principal component analysis was used to allow all five chemical tracers to be combined and viewed simultaneously. The properties of the upper 120 m are found to lie, to a close approximation, on a plane. This leads us to a simple interpretation based on a three‐component mixing model involving river runoff, water from the nutrient maximum, and an offshore near‐surface component. It is shown that the best fit plane occupied by arctic surface waters in the Beaufort Sea closely matches that defined by the influences of river inflow, of the freeze‐melt cycle, and of photosynthesis and respiration. However, the effects of freezing/melting and of river inflow cannot be clearly distinguished using the chosen suite of tracers. It has been determined that if the waters of the upper 250 m are to be represented in the same manner, a fourth end member is req

ISSN:0148-0227    

DOI:10.1029/92JC00842

年代:1992

数据来源: WILEY

摘要:

A free‐drifting buoy serving as an ice anchor and flotation sphere for a 125‐m instrumented mooring line was deployed in the Arctic pack ice by the R/VPolarsternin August 1987 at the northernmost point achieved by the Arktis IV expedition. Current measurements were made from this Arctic environmental drifting buoy (AEDB) using a 150‐kHz acoustic Doppler current profiler (ADCP) attached to the mooring line at a depth of 16 m below the flotation sphere with the transducers facing downward. Velocity profiles spanning depths between 35 and 327 m with 15‐m resolution were collected at half‐hour intervals over a period of 216 days while the buoy drifted from its starting point near 86°N, 22°E to about 70°N, 16°W. The full experimental record was separated into three sections corresponding to periods of the drift spent in the Nansen Basin, over the Yermak Plateau, and in the Greenland Sea. Velocity time series and spectra were computed, and results for the three analysis sections were compared. Spectra in the Nansen Basin were found to have energy levels significantly less than those typical for mid‐latitudes, roughly one third of the canonical Garrett‐Munk (GM) level, and a spectral slope between ƒ and 2.5ƒ that was significantly flatter than that expected from the GM model. Between 83°N and 81°N, as the buoy passed over the Yermak Plateau, internal wave energy increased to levels comparable to GM. Isolated, energetic near‐inertial wave packets with upward group velocity were observed in this portion of the record, presumably generated by interaction of the barotropic tide with the bottom topography of the plateau. Estimates of the upward energy flux from these near‐inertial packets were comparable to the downward fluxes from typical surface sources. Interpretation of the Greenland Sea section was complicated by ADCP sampling problems and a lack of Argos fixes, but the available data showed internal wave spectral levels that were intermediate between those of the Nansen Basin and Yermak Plateau and no evidence of upward‐propa

ISSN:0148-0227    

DOI:10.1029/92JC01098

年代:1992

数据来源: WILEY