摘要:

Offshore‐directed plumes of inshore water, marked by a shallow mixed layer, low salinity, and low temperature during winter, were observed south of the Hokitika Canyon on the South Island west coast, New Zealand. Hydrographic and satellite observations are presented which show that plume time scales were of the order of a few days, and across‐shelf length scales were approximately 50 km. The low‐salinity plumes were found to be associated with the southern edge of an offshore‐directed jet that followed the Hokitika Canyon bathymetry. Decay of the plumes was rapid and appears to be associated with mixing of plume water into offshore surface waters. The occurrence of the plumes is nonseasonal, and they do not appear to be associated with upwelling. Observations suggest that strong southward flow around the head of the Hokitika Canyon, perhaps as a consequence of coastal‐trapped wave activity, is a possible mechanism for plume formation. Biologically, the plumes were important because they stabilized the water column and prevented the mixing of phytoplankton below the photic zone. Although the plumes were found to transport productive nearshore waters offshore, the major impact of the plumes was to lower total water column chlorophyllain outer shelf regions. The relationship between the Hokitika plumes and the filaments and squirts identified off the Californian coast is discussed, and it is concluded that the plume dynamics resemble those of the squirts found inshore of the seasonal filaments off California. The results of this study indicate that squirts not only influence biological variability in shelf waters but are important mechanisms for the across‐shelf exchange of coastal and oce

ISSN:0148-0227    

DOI:10.1029/93JC01857

年代:1993

数据来源: WILEY

摘要:

The outflow of buoyant waters from major estuaries affects the dynamics of inner continental shelves profoundly as lateral density gradients force an alongshore current. Often the Coriolis force causes the outflow to remain trapped near the coast. We observed one such current, the Delaware Coastal Current, on the inner shelf near the Delaware Estuary on the eastern seaboard of the United States. The spatial variability along the shelf, however, suggests at least two dynamically distinct regions that we term source and plume regions. In the source region we find fronts, a current whose width scales well with the internal deformation radius, and a ratio of relative to planetary vorticity that reaches unity, that is, the Rossby number is O(1). As nonlinear inertial forces in the across‐shelf momentum balance are weak, we suggest that such forces contribute to the along‐shelf momentum balance only. Farther downstream in the plume region, we find much reduced lateral density gradients, a current much wider than the deformation radius, and relative vorticities that are much smaller than the planetary vorticity. From our observations we compute nondimensional dynamical parameters, with which we discuss our observations. The Burger, Rossby, and Ekman numbers for the Delaware Coastal Current suggest that most models of buoyancy‐driven coastal currents do not apply to this coastal

ISSN:0148-0227    

DOI:10.1029/93JC02112

年代:1993

数据来源: WILEY

摘要:

Examination of temperature and salinity data from the Scotian Shelf, Gulf of Maine, Gulf of St. Lawrence, and the adjacent continental slope has shown that the dominant low‐frequency event over the last 45 years was a cooling and subsurface freshening of the water masses from 1952 to 1967, followed by a rapid reversal of these trends. The largest temperature and salinity changes (1952–1967) were 4.6°C and 0.7, respectively, and occurred at about 100 m over the slope. Exchanges with shelf waters and vertical mixing gave rise to the surface manifestation of this variability. The westward transport of the Labrador Current was found to have similar variability, increasing from about 1 × 106m3s−1in the early 1950s to about 4 × 106m3s−1in the mid‐1960s. A simple model that accounts for this variation of transport and has a constant entrainment of North Atlantic water indicates that changes of the westward flow of the Labrador Current could contribute significantly to theT

ISSN:0148-0227    

DOI:10.1029/93JC02191

年代:1993

数据来源: WILEY

摘要:

Satellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual surface heat flux for the area north of Guaymas and Santa Rosalia is estimated to be +74 W m−2for the 1984–1985 time period. This is comparable to the +20–50 W m−2previously obtained from heat and freshwater transport estimates made with hydrographic surveys from different years and months. The spatial distribution of the net surface heat flux shows a net gain of heat over the whole northern gulf. Except for a local maximum near San Esteban Island, the largest heat gain (+110–120 W m−2) occurs in the Ballenas and Salsipuedes channels, where strong tidal mixing produces anomalously cold sea surface temperatures (SSTs) over much of the year. The lowest heat gain occurs in the Guaymas Basin (+40–50 W m−2), where SSTs are consistently warmer. In the relatively shallow northern basin the net surface heat flux is fairly uniform, with a net annual gain of approximately +70 W m−2. A local minimum in heat gain (approximately +60 W m−2) is observed over the shelf in the northwest, where spring and summer surface temperatures are particularly high. A similar minimum in heat gain over the shelf was observed in a separate study in which historical SSTs and 7 years (1979–1986) of meteorological data from Puerto Peñasco were used to estimate the net surface heat flux for the northern basin. In that study, however, the heat fluxes were higher, with a gain of +100 W m−2over the shelf and +114 W m−2in the northern basin. These larger values are directly attributable to the higher humidities in the 1979–1986 study compared to the 1984–1985 satellite study. Significant interannual variations in humidity appear to occur in the northern gulf, with relatively high humidities during El Niño years and low humidities during anti‐El Niño years. High humidities reduce evaporation and the associated latent heat loss, promoting a net annual heat gain. In the northern Gulf of California, however, tidal mixing appears to play a key role in the observed gain of heat. Deep mixing in the island region produces a persistent pool of cold water which is mixed horizontally by the large‐scale circulation, lowering surface temperatures over most of the northern gulf. These cold SSTs decrease evaporation by reducing the saturation vapor pressure of the overlying air. As a result, heat loss is substantially reduced, even when humidities are low. By removing heat from the surface, tidal mixing alters the time scale of air‐sea interaction and reduces or possibly even inhibits the formation of deep water masses via convection. Over climatological timescales, it may be tidal mixing that ultimately maintains the estuarinelike circulation in the northern Gulf of California, differentiating it from the Mediterranean and Red s

ISSN:0148-0227    

DOI:10.1029/93JC02190

年代:1993

数据来源: WILEY

摘要:

The Loop Current penetrated deep into the Gulf of Mexico in early 1989. After several eddy formation and reattachment cycles, a southwestward propagating anticyclonic eddy was formed during the summer. The Loop Current and subsequent eddy produced strong currents over widespread areas on the Louisiana continental slope, prompting a series of current measurement programs. Because of the lack of satellite infrared coverage in the Gulf of Mexico during the summer, the trajectories of 53 ARGOS tracked drifting buoys deployed in the Gulf of Mexico in 1989 were assembled to determine the synoptic history of the Loop Current and anticyclonic eddy during this event. Ten of the most critical summertime buoy trajectories are discussed here. In addition, the trajectories of three of the buoys simultaneously deployed at different radii in the eddy were chosen for analysis with a kinematic feature model. The model assumes the looping buoy motion is generated by an elliptical orbit around a translating eddy center. The buoy trajectories were used to determine time series of the feature model parameters, including eddy center, shape and circulation characteristics. The time series results indicate that the eddy actually propagated in a series of short sprints separated by longer stalls. During the sprints, the eddy propagated as an elliptical but otherwise symmetric solid body. During the stalls, significant asymmetries developed. The asymmetries are consistent with attachment to the Loop Current during the first stall, the effects of topographic Rossby wave dispersion during the second, and the effects of planetary Rossby wave dispersion during the third. The results provide both a framework for the interpretation of in situ observations and a detailed evolutionary history for numerical modelers.

ISSN:0148-0227    

DOI:10.1029/93JC02078

年代:1993

数据来源: WILEY

摘要:

Mesoscale fluctuations in the western tropical Atlantic are analyzed in Geosat altimetry sea surface height (SSH) and geostrophic velocity anomalies to investigate the role of eddies in the North Brazil Current (NBC) retroflection zone. The detachment of anticyclonic eddies from the NBC retroflection is observed during November through January, when the NBC retroflection into the North Equatorial Countercurrent (NECC) weakens and finally breaks down. These eddies are traced over more than 2 months between 50° and 60°W on their way toward the Caribbean, at average speeds of 15 cm s−1. In one case an apparent merger of two anticyclonic eddies occurs, one detached from the retroflection zone and one detached from the NECC. Cyclonic eddies are also observed but are generally less persistent. Mesoscale SSH variance just west of the retroflection increases by a factor of 2 from early summer to winter, mainly because of the anticyclonic eddies. Interhemispheric water mass transfer associated with the eddy flux out of the NBC retroflection may amount to an average transport of 3

ISSN:0148-0227    

DOI:10.1029/93JC01184

年代:1993

数据来源: WILEY

摘要:

To examine the long‐period variability of the Gulf Stream, sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 75°W and 60°W longitude) are used. Residuals, as derived from Geosat altimetry between November 1986 and December 1988, were gridded in space and time at a temporal resolution of 10 days and spatial resolution of 1/4°. Complex empirical orthogonal function (CEOF) analysis was applied to the data set to extract the spatially correlated signal with the original data sub sampled to 1/2°. In addition to determining the space‐time scales and propagation characteristics of the different modes, wavenumber‐frequency spectral techniques were used to separate the variability into propagating and stationary components. The CEOF technique applied to the data set indicated that the first four CEOF modes accounted for 60% of the variability and were found to be above the noise level 99% of the time. CEOF 1 was associated with westward propagation at 5 km/d at a wavelength of 2000 km and eastward propagation at 1–2 km/d centered at a 500‐km wavelength. This first CEOF is in good agreement with thin‐jet equivalent barotropic models which predict westward propagation for wavelengths greater than 1130 km. A deflection of the wavelike pattern at 65°W also indicates a possible topographic effect. A simple scaling of the effect of topography indicates that for length scales longer than the internal Rossby radius of deformation, the topographic term is at least of the same order of magnitude as the beta effect. The second CEOF was more broadbanded in wavenumber space, with eastward propagation occurring in a wavenumber‐frequency band between 300 and 1400 km and 0.5 and 2.0 cycles/yr. The third CEOF is similar in structure to the first, but with less energy. CEOF 4 was clearly identifiable with higher frequencies than the first three with westward propagation at 4 km/d. The spatial location of this mode along with the westward propagation indicates possible influences from eddy‐stream interactions. Thus topography, Rossby wave dynamics, and eddy‐stream interactions all appear to have a significant role in determining the space‐time scales and propagation properties of the long‐period response of

ISSN:0148-0227    

DOI:10.1029/93JC02002

年代:1993

数据来源: WILEY

摘要:

An attempt is made to find a steady state of a general circulation model consistent with error estimates of the North Atlantic hydrography during 1981–1985 and with wind and thermodynamic boundary conditions. The quadratic misfit of the model state from the data is minimized using the general circulation model together with its adjoint. A state which is steady within limits of estimated observational error is found, but no such state is simultaneously consistent with the observed hydrographic and surface flux fields. Model dynamics are able to sharpen gradients which were overly smooth in the mapped data, producing a meridional overturning cell with a maximum value of about 21 Sv. At equilibrium, the model must produce its own water masses and has a strong tendency to go toward wintertime conditions. The consequence is an estimated model surface temperature systematically lower than in the hydrography (which comes from all seasons). The western boundary current thermocline is also both colder and fresher than the mapped hydrography, and the overall meridional heat transport is low (about 0.6 × 1015W). The results suggest that the concept of a realistic steady state North Atlantic circulation has reached the end of its utility, that models with realistic property fluxes and divergences must have much higher resolution, and that the open‐ocean boundary conditions must be formulated as control varia

ISSN:0148-0227    

DOI:10.1029/93JC02159

年代:1993

数据来源: WILEY

摘要:

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well‐defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature>∼ 13.25°C, salinity>∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reache

ISSN:0148-0227    

DOI:10.1029/93JC02211

年代:1993

数据来源: WILEY

摘要:

Measurements made with satellite‐tracked buoys drogued in different layers between the sea surface and 30‐m depth under homogeneous winter conditions in the North Sea allow analysis of the Ekman currents under a large variety of wind conditions. The experiment lasted from November 20, 1991, until February 29, 1992. The first 4 weeks of this period, during which the buoys stayed close together, are used to determine the Ekman stresses. The total current field is a superposition of barotropic currents due to sea level variations and Ekman currents. The classical Ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth. This deflection is 10° near the sea surface and increases to approximately 50° in 25‐m depth. The relation between wind stress and the stress field in the interior of the water is given by a tensor, which describes the rotation and the variation of the stress with increasing depth. The concept of eddy viscosity is applicable, if a viscosity tensor is used to relate stress and vertical shear. The viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress. It shows maximum values in 15‐ to 20‐m depth and may be due to Langmuir circulation cells. Further studies are needed to determine the physics of this tensor. Its magnitude in the interior of the mixed layer exceeds 1000 cgs units. Consequently, Ekman currents are weak and may not be the dominant currents within the

ISSN:0148-0227    

DOI:10.1029/93JC01898

年代:1993

数据来源: WILEY