摘要:

We analyze the oceanic “imprints” of atmospheric internal gravity waves detected in airborne synthetic aperture radar (SAR) images collected off the west coast of Vancouver Island, British Columbia, in June 1990. A sequence of SAR images taken over a period of 2.5 hours reveals that the waves traveled with group velocities of 3–6 m s−1toward the northwest, counter to the 10 m s−1surface winds. Wave crests were oriented normal to the coast and had lateral extents of several tens of kilometers. Observed wavelengths ranged from 1.2 to 2.3 km, while calculated wave periods ranged from 3 to 8 min. A comparison of the observed wave properties with values derived from a simple three‐layer density model of the troposphere suggest that the waves were low‐mode oscillations trapped within a 0.75‐km‐thick temperature inversion overlying a 1‐km‐thick weakly stratified marine boundary layer. Wave propagation was counter to the wind shear within the inversion layer. Generation of the internal wave groups appears to have occurred near the southern boundary of the stable marine air mass, where it bordered a convectively active air mass that was intruding nort

ISSN:0148-0227    

DOI:10.1029/92JC01178

年代:1992

数据来源: WILEY

摘要:

Altimetric data, climatological hydrological data, and numerical model results are compared over the tropical Atlantic Ocean between November 1986 and November 1988. All reproduce the seasonal cycle of the dynamic topography rather well, and the agreement is particularly good between altimetry and the primitive equation model. The study of the 1986–1988 period reveals interannual events evidenced by both the altimetry and the models, especially during spring 1988 in the Gulf of Guine

ISSN:0148-0227    

DOI:10.1029/92JC00818

年代:1992

数据来源: WILEY

摘要:

Several 5 to 10 m s−1westerly wind bursts of 10–15 days' duration occurred in the western equatorial Pacific during November 1989 to January 1990. The response to these wind bursts was characterized by a 400‐ to 600‐km‐wide eastward jet in the upper 100–150 m along the equator between 135°E and the date line. Flow in this jet accelerated to speeds of over 100 cm s−1within 1 week after the onset of westerly winds in November 1989 in association with supertyphoon Irma. In addition, a 20 to 40 cm s−1westward counterflow developed between 2°N and 2°S below the surface jet separating it from the eastward flow of the Equatorial Undercurrent in the thermocline. Changes in surface layer zonal volume transports in the western Pacific due to westerly wind bursts were 25–56 Sv based on comparison of three shipboard velocity transects in November and December 1989. Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer, the Equatorial Undercurrent decelerated to less than 20 cm s−1(i.e., less than half its speed before the onset of westerlies) by early December 1989. Westerly winds also produced a strong surface convergence in both the meridional and zonal directions west of the date line. In late November 1989, this convergence was associated with downwelling of 2–3 m d−1along the equator, and a 20‐ to 30‐m depression of the thermocline. In addition, sea level rose by 10 dyn cm from mid‐November to mid‐December between 1°S and 3°N, 165°E. Sea surface temperature dropped over a large region by 0.2°–0.5°C during episodes of high westerly winds, and some episodes of high winds were associated with 0.2°–0.3°C sea surface temperature inversions supported by salt‐stratified buoyancy gradients in the upper 100 m. The near‐surface salinity balance was dominated by lateral advection when relatively fresh water from north of a salinity front centered near 3°N was advected southward by Ekman drift in late November 1989. Displacement of this front past a current meter mooring at 0°, 165°E, appeared as a local drop of 1 psu in surface layer salinity in 10 days. These results are discussed in the context of a swing in the climate system towards El Ni

ISSN:0148-0227    

DOI:10.1029/92JC01197

年代:1992

数据来源: WILEY

摘要:

The reflection of interannual Rossby waves over a 2.7‐year period (i.e., November 1986 to August 1989) is examined in the western tropical Pacific utilizing altimetric sea level observations from the Geosat Exact Repeat Mission. In 1987 this reflection process generates upwelling Kelvin waves that tended to limit the growth of the 1986–1987 El Niño in the eastern equatorial Pacific. Two‐dimensional autocorrelation and spectral analyses verify the existence of westward traveling Rossby waves in both the equatorial and off‐equatorial domains, and eastward propagating Kelvin waves in the equatorial domain. Extended empirical orthogonal function (EEOF) analysis characterizes the time‐space evolutionary behavior of the Rossby wave reflection process, extending from the El Niño in 1986–1987 toward the anti‐El Niño in 1988–1989. Both the Philippines Archipelago in the northern hemisphere and the Solomon Archipelago/Bismarck Archipelago/New Guinea complex in the southern hemisphere appear to reflect incident Rossby waves, with maximum amplitude near 8°N and 8°S, generating Kelvin waves in the equatorial wave guide that conduct the anomalous signal eastward along the equator. This apparent reflection process is examined in terms of the linear theory, with sea level phase information in the northern hemisphere at the western boundary (130°W) expanded in terms of the Kelvin and symmetric Rossby meridional wave modes. These modes are formed as weighted sums of parabolic cylinder functions. This procedure, operating under the constraint that the reflection process occurred according to linear theory, determines the percent contribution that each incident Rossby wave mode amplitude has upon the amplitude of the reflected Kelvin wave. This percentage changed little over the evolution of the EEOF pattern. The amplitude of the first‐mode Rossby wave (i.e., mode 2) accounts for most of the Kelvin wave amplitude (i.e., 70–80%), with the higher‐mode Rossby wave amplitudes (i.e., 4, 6, and 8) contributing less (i.e., 20%, −10%, and 10%, respectively), interacting both constructively and

ISSN:0148-0227    

DOI:10.1029/92JC01441

年代:1992

数据来源: WILEY

摘要:

Hydrographic observations in an area immediately northeast of Taiwan in April 1989 indicate an on‐shelf intrusion of Kuroshio water across a sharply curved continental shelf break. It appears that a part of the Kuroshio on the cyclonic side overran the shelf break and penetrated northward as a shallow surface current. The remainder of the Kuroshio, presumably affected by the shoaling topography, largely turned and ran along the shelf break to the northeast. Between the two, the flow was weak and disorganized. Conservation of potential vorticity and constancy of the Bernoulli function in an analytical, reduced‐gravity model of the incidence of a baroclinic current upon a step shelf lead to an on‐shelf flow field that is compatible to the observed hydrographic distributions. In particular, the integrated balance of along‐step momentum yields an expression of the angle of intrusion in terms of the incidence angle and of the ratio of step depth to the depth of the upper layer of the ocean. In addition, the transport of the intrusion is shown to be equal to the product of the depth ratio squared and the incident transport. Calculations for the condition of the April survey yield results in agreement with the obse

ISSN:0148-0227    

DOI:10.1029/92JC01401

年代:1992

数据来源: WILEY

摘要:

A suite of observations from July 2–3, 1988, is used to describe the spatial structure and temporal evolution over the tidal period of currents and hydrography across the northern side of Georges Bank in summer under light winds. The data set includes moored current and hydrographic observations at four cross‐bank positions, fast response thermistor chain observations at two of the sites, a conductivity‐temperature‐depth section, 10 repeated sections over the tidal period using a towed Batfish and ship‐mounted acoustic Doppler current profiler, and two surface drifter trajectories. The observations provide a detailed description of previously identified features such as the strong semidiurnal tidal currents, an internal tide, a tidal front, and a residual current jet and also reveal a hierarchy of energetic smaller‐scale structures. These include an internal hydraulic jump during off‐bank tidal flow and subsequent internal waves propagating onto the bank and also a surface convergence in the frontal zone. The physical oceanographic regime on the northern side of Georges Bank during spring‐fall can be conceptualized as a hybrid of a stratified shelf break with strong tidal advection and a tidal(mixing) front. Key factors to the regime are the strong tidal currents and abrupt topographic variation over the bank's side. The result is a nonlinear and baroclinic tide‐topography interaction at the bank edge and a frontal zone with strong variability over the tidal period associated with tidal advection and large‐amplitude internal waves. The along‐bank transport in the residual jet is estimated to be 0.91 Sv, indicating that it may be the largest summertime transport feature on the northwestern Atlantic shelf between Cape Hatter

ISSN:0148-0227    

DOI:10.1029/92JC01342

年代:1992

数据来源: WILEY

摘要:

Anticyclonic eddy X91 was observed (by remote sensing) from May 1991 to February 1992 in the southern Bay of Biscay. Two drogued Argos buoys deployed in the center in September showed that particle separation rates in the core of the eddy at a depth of 100 m were of the order of 100 m/day. Similar values were found for the mean radial velocity component of the buoys with respect to the eddy center. The center of the eddy core (∼10‐km radius) was in near‐solid‐body rotation with a period approaching 3.5 days and relative vorticity of −0.4ƒ. Deep expendable bathythermograph profiles (to 2000 m) in September gave an eddy diameter of about 100 km with geostrophic velocities falling to half values at a depth of 800 m. The geostrophic azimuthal transport was ∼10 Sv; the kinetic energy was ∼1.4 × 1014J. X91 was resurveyed in December, and the azimuthal currents extended to greater depths (∼1 km). The transport was still ∼10 Sv but the kinetic energy was reduced to ∼75% of the September value. Drogued buoys showed a slower (∼10%) rotation period in December. X91 is a good example of a coherent vortex structure, and the structural similarity between X91 and F90a (the slope water oceanic eddy observed in 1990) suggests th

ISSN:0148-0227    

DOI:10.1029/92JC01181

年代:1992

数据来源: WILEY

摘要:

Data from two cruises in the western Bering Sea are presented and examined. During spring 1990, conditions were typical, and the Kamchatka Current had a transport of ∼11 Sv. On the other hand, during fall 1990, this flow had weakened to ∼6 Sv, although a strong flow near the Komandorski Islands occurred. Various eddylike features were common during both cruises and were particularly developed in fall. Although some aspects of the flow patterns may be seasonal in nature, the major weakening of transport probably represents an interannual cha

ISSN:0148-0227    

DOI:10.1029/92JC01196

年代:1992

数据来源: WILEY

摘要:

Recently it was reported that sea ice extents in the northern hemisphere showed a very slight but statistically significant decrease over the 8.8‐year period of the Nimbus 7 scanning multichannel microwave radiometer (SMMR) data set. In this paper the same SMMR data are used to reveal spatial patterns in increasing and decreasing sea ice coverage. Specifically, the length of the ice season is mapped for each full year of the SMMR data set (1979–1986), and the trends over the 8 years in these ice season lengths are also mapped. These trends show considerable spatial coherence, with a shortening in the sea ice season apparent in much of the eastern hemisphere of the north polar ice cover, particularly in the Sea of Okhotsk, the Barents Sea, and the Kara Sea, and a lengthening of the sea ice season apparent in much of the western hemisphere of the north polar ice cover, particularly in Davis Strait, the Labrador Sea, and the Beaufort

ISSN:0148-0227    

DOI:10.1029/92JC01367

年代:1992

数据来源: WILEY

摘要:

Nearshore variability of the thermocline depth and associated baroclinic currents at Sydney, New South Wales are analyzed for the locally and remotely forced components. Summer and winter data acquired in 65 m of water 3.5 km from the coast include thermistor string and current meter measurements. Cross‐spectral analyses of the data are interpreted with reference to the free and forced modes of a simple two‐layer frictional shelf model. Subinertial frequency (∼ 40 day‐21 hour period) baroclinic variability appears to be locally forced, in contrast with the barotropic variability which is principally remotely forced and acts as one of the local forcing agents of the baroclinic variability through bottom Ekman layer dynamics. The local wind stress appears to be the more important forcing term, despite having less weather band variance than the bottom stress. Strong winds at Sydney are usually from the south and hence lead to downwelling, while bottom stress is responsible for the rarer upwelling events. Near‐inertial variability is substantially inhibited at the study site except during winter when the internal Rossby radius becomes comparable to the distance from the coast. Superinertial frequency (to 6‐hour period) baroclinic variability is dominated by standing internal Poincaré waves, the principal energy source of which is presumed to be tidal currents over the outer shelf. The existence of a standing wave pattern implies that little dissipation occurs upon reflection

ISSN:0148-0227    

DOI:10.1029/92JC01294

年代:1992

数据来源: WILEY