摘要:

Hydrographic data are presented from a near‐meridional section across the Agulhas Return Current, the Subtropical Front and the Antarctic Circumpolar Current (ACC) in the Southwest Indian sector of the Southern Ocean. The location and characteristics of the fronts are examined and the water masses are defined. The Agulhas Return Current emerges as a strong current as far east as 40°E which forms a marked front that is separate from the Subtropical Front, and can be distinguished by a separate water mass. Comparisons are made with a similar hydrographic section at the Greenwich Meridian and several notable differences are found. The Subantarctic Front at the Greenwich Meridian is a pronounced feature carrying a large proportion of the geostrophic transport of the ACC. On the Southwest Indian section, however, the Subantarctic Front is merged with the Polar Front at 48°S, where the flow associated with the ACC is concentrated. Geostrophic calculations show a large volume transport, 84 to 103 Sverdrup (1 Sv = 106m3s−1), associated with the Agulhas Return Current at 40°S. This is quite separate from the transport of the ACC which totals 138 Sv south of the Subantarctic Zone and the Crozet Plateau, or 150 Sv including flow in the Subantarctic Zone. The transport of the ACC out of the South Atlantic sector appears to be higher than the transport into the South Atlantic sector at Drake Passage, with the extra transport most probably carried by upper layer and intermediate waters. Eddies are seen to be an important feature of the Subantarctic Zone where they help to modify the water masses. Antarctic Intermediate Water appears in two forms, a saline (34.4) Indian Ocean variety and a fresh (34.2) Atlantic Ocean variety. The Atlantic variety is freshest at the Polar Front, and occupies the southern half of the Subantarctic Zone. Within the eddies, in the central Subantarctic Zone, there is interleaving between these two forms of Antarctic Intermediate Water. North Atlantic Deep Water is unable to cross the Southwest Indian Ridge but can pass south of the Crozet Plateau in the form of Circumpolar Deep Water, which is slightly more saline (34.75) than at Drake Passage (

ISSN:0148-0227    

DOI:10.1029/93JC00436

年代:1993

数据来源: WILEY

摘要:

Almost 3 years of Geosat data were analyzed to ascertain the characteristics of the sea surface height variability in the South Atlantic. The nature of the sea surface height field is shown to be nonstationary, heterogeneous, and anisotropic. In particular, the wavenumber spectral characteristics of the sea surface height field, such as spectral slopes, shape, breaks in spectral slopes, and energy density values, vary with latitude and longitude within the basin. The spectral slopes exhibit a spectral dependence approachingk−5in the high energy areas andk−3in the low energy areas. Along‐track characteristic wavelengths are found to decrease from north (500 km) to south (300 km) and from west to east. The (eddy available potential) energy exhibits maximum values in the highly energetic eddy shedding regions of the Malvinas‐Brazil confluence and the Agulhas leakage region. Mean frequency spectra, calculated from time series of sea surface height variability at crossover points, reveal evidence of annual and semiannual signatures with energy levels reaching 1000 cm2in the high‐energy western and eastern systems. These values represent 20% of the energy found for periods of 100 days or less, which is the time scale of the mesoscale variability. Baroclinic Rossby waves with periods of 400–500 days are found to propagate away from the eddy‐shedding region of the Agulhas leakage into the Atlantic. The phase (energy) of these planetary waves propagates toward the west‐southwest (west‐northwest) from their source near the southern tip of Africa. Correlation functions in space (both in the zonal and along‐track directions) and time lag corroborate these findings. Zonal and meridional wavenumbers were calculated using a simple wavenumber projection technique which takes into a count the anisotropic property of the wave field. The resultant wavelengths (approximately 250 km) are consistent with those calculated from the dispersion relation for baroclinic Rossby waves. These waves propagate with a group speed of 0.4 cm s−1and provide a net energy flux from the Agulhas leakage into the Atlantic toward the western boundary and th

ISSN:0148-0227    

DOI:10.1029/93JC00405

年代:1993

数据来源: WILEY

摘要:

We derived the sea level variability in the South Atlantic subtropical gyre between 15° and 35°S from 2 years of the Geosat exact repeat mission altimeter data. On the mesoscale, along‐track wavenumber spectra are those of nonlinear dynamics with a steeper slope and stronger energy along the eastern boundary. Frequency/wavenumber spectra reveal a significant semiannual signal at a wavelength of about 500 km. West of the Walvis ridge (5°E), this signal appears to correspond to semiannual Rossby waves of 2‐ to 3‐cm amplitude, propagating westward at 3 cm s−1, and showing latitude refraction effects. The pattern is very similar to that of annual Rossby waves as modeled by Reason et al. (1987). These waves are probably related to the semiannual wind component. However, understanding the exact mechanism for their excitation requires dynamical modeling. The large‐scale variability also shows a significant semiannual component (about 6‐cm amplitude and 15% of the yearly variance) in phase with climat

ISSN:0148-0227    

DOI:10.1029/93JC00456

年代:1993

数据来源: WILEY

摘要:

Surface winds from the Fleet Numerical Oceanography Center (FNOC) for 1979–1986 were used to compute the monthly mean Ekman up welling fields 〈we〉m, the seasonal and annual climatologies of 〈we〉m(〈we〉sand 〈we〉a, respectively), and the basin wide monthly mean vertical Ekman velocities 〈we〉mbin the North Atlantic Ocean (the analyses excluded the zone within ±5° of the equator). The annual climatology compares very well with the climatology of Leetmaa and Bunker (1978). The analyses show a pronounced minimum in 〈we〉mbduring 1981–1982. Additional analyses using 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts exhibit a similar pattern in 〈we〉mb. Further examination of the FNOC 〈we〉mfields shows that the primary source of the interannual signal was the region off NW Africa in the vicinity of the Guinea Dome. Other sectors of the basin experienced no significant interannual trends. Hydrographic data and sea surface temperature data from the NW Africa sector for 1981–1986 indicate a cooling trend beginning in late 1982, consistent with increased upwelling. Also, the fall and winter seasons' mixed layers at the center of the Guinea Dome were deeper in 1984 and 1985 than in previous years. Finally, the potential impact of large interannual variations in Ekman upwelling on b

ISSN:0148-0227    

DOI:10.1029/93JC00868

年代:1993

数据来源: WILEY

摘要:

The present paper examines the influence of evaporation fluxes over the Mediterranean Sea on water flows through the straits of Gibraltar and Sicily. Evaporation fluxes are estimated from European Centre for Medium‐Range Weather Forecasts atmospheric analyzed fields for the period 1981–1985. An aerological approach is followed to evaluate the net water exchanged between the sea and the atmosphere. The resulting Mediterranean Sea circulation is then deduced from water budget in the sea. Net evaporation is found to be the major forcing for the circulation through the straits. Mean flows through the Strait of Gibraltar are found to be slightly weaker than commonly accepted values but appear to be close to recent estimations. Temporal variations of net evaporation show a relatively important annual cycle which may induce significant annual cycle of water flows through the straits. For each month of the studied period, fluxes are separated into two parts: a steady state flux and transients. Transient atmospheric events generate a net water loss in the Mediterranean and may induce a net water inflow through Gibraltar. The latter flow is suggested to be related to Atlantic/Mediterranean water interface fluctuations. Water sinking is also investigated. It is mainly related to the mean component of atmospheric events in the eastern Mediterranean and to transient events in the western Mediterranean. Atmospheric pressure effect shows a significant variability, especially during winter, and may induce a slight water flow through the stra

ISSN:0148-0227    

DOI:10.1029/93JC00376

年代:1993

数据来源: WILEY

摘要:

In an area immediately northeast of Taiwan where in April 1989 an on‐shelf intrusion of the Kuroshio was observed, hydrographic observations from August 1991 indicate a blocking of the Kuroshio at the continental shelf break. It appears that the Kuroshio approaches the shelf break as a near‐bottom current and is deflected. The deflection apparently gives rise to a countercurrent along the shelf break which was observed by a buoy‐mounted acoustic Doppler current profiler deployed within a month of the hydrographic survey. The on‐shelf hydrographic distribution accompanying the deflection exhibits a field of near‐bottom eddies dominated by a particularly well organized large pool downstream and to the right of the approaching Kuroshio but well separated from the Kuroshio. The hydrographic properties of the pool are similar to those of the uplifted Kuroshio subsurface water observed in April 1989, suggesting that it may be the remnant of the Kuroshio subsurface water that intruded during the previous winter. A theory of the blocking is put forth in terms of the approach of a bottom current toward a step rise of the bottom in a two‐layered, reduced‐gravity ocean model in which the thickness of the current exceeds the rise in topography. The theory predicts a complete blocking that features a flow deflection in front of the step and the presence on the step of a preexisting region of high baroclinic pressure, similar to the observation. The theory also predicts approximately correctly the track of the isotherm that bounds the large pool on the side facing the on‐r

ISSN:0148-0227    

DOI:10.1029/93JC01075

年代:1993

数据来源: WILEY

摘要:

Due to topographic steering by the Crozet and Kerguelen Plateaus, the Antarctic Circumpolar Current (ACC) core in the Crozet Basin area is shifted to its northernmost position in the Southern Ocean, along the southern limb of the South Indian subtropical gyre. Here the jet‐like current is embedded within a narrow band (26.5 kg/m3) in the western half of the basin and heavier mode (γθ<26.7 kg/m3) in its eastern half. These different varieties of Crozet Basin Mode Water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone. The area north of the Kerguelen Plateau appears to be an important source of water mass modification. Antarctic Intermediate Water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher, oxygen‐richer, colder subantarctic water. Energetic eddy activities observed along the frontal zone likely enhance such cross‐frontal exchange of water masses. Equatorward spreading of the modified thermocline water masses (Mode Water, Antarctic Intermediate Water) occurs as part of the anticyclonic circulation in the South Indian subtropical gyre which is centered within the basin. Strong injection of North Indian Deep Water into the ACC south of Amsterdam Island is clearly evidenced for the first time. This deep oxygen minimum water centered at about 3000 m can be traced to the east of Madagascar, from where it spreads southward through the eastern part of the Crozet Basin. Due to the influence of this water mass, North Atlantic Deep Water characteristics of the Circumpolar Deep Water are significantly altered in Kerguelen‐Amsterdam passage. Our results indicate a cyclonic deep‐level circulation pattern in the Crozet Basin, with a northward flow (9 Sv) of Circumpolar Deep Water / Antarctic Bottom Water as a deep western boundary current and a partially compensating southward flow (3 Sv) of North Indian Deep Water through the eastern part of the basin. It is concluded that the Crozet Basin constitutes an important passage for the exchange of water masses between the Southern Ocea

ISSN:0148-0227    

DOI:10.1029/93JC00938

年代:1993

数据来源: WILEY

摘要:

Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October‐November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30‐min intervals with a range resolution of ±1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross‐correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10–15 cm) and heights (3–5 cm) are comparable to measurements made by divers. Ripple migration rates were 1–6 cm h−1directed offshore in the direction of the mean shore normal near‐bottom current and increased with increasing mean current speed. Evidence of megaripple‐like features (2–5 cm height; 1.5–3 m length) is found, also migrating in the direction of the mean shore normal near‐bottom drift, at speeds of about 20 cm h−1. Bedload transport rates in the onshore‐offshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large‐scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabe's (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest‐energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h−1. This erosion event was associated with a gene

ISSN:0148-0227    

DOI:10.1029/93JC00452

年代:1993

数据来源: WILEY

摘要:

An analytical, inviscid two dimensional model is used to examine the formation of surface‐to‐bottom density fronts over shelf slope topography. Previous studies have examined the geostrophic adjustment of a vertical front near the shelf break. However, our approach makes no such assumption about the initial density field, requiring only that all upper‐layer fluid originate over the shelf. The main limitation of the study is that the lower‐layer velocities are assumed to be negligible. The simple model demonstrates that geostrophy and potential vorticity conservation are sufficient conditions to ensure that a front will intersect the bottom close to the shelf break. Aspects of the theory have been compared with results from simple laboratory experiments. The experiments also reveal the influence of frontal processes such as instability and wave formation that are not included in the theoretical model. In particular, they provide clear confirmation of the stabilizing influence of steep topography below

ISSN:0148-0227    

DOI:10.1029/93JC00624

年代:1993

数据来源: WILEY

摘要:

Characteristics of springtime sea ice off East Antarctica were investigated during a cruise of the Australian National Antarctic Research Expedition in October through December 1988. The fractional coverage of the ocean surface, the ice thickness, and the snow cover thickness for each of several ice types were estimated hourly for the region near the ship. These observations were carried out continuously during the 4 weeks the ship was in the ice. Thin and young ice types were prevalent throughout the region, and the observations show a systematic increase in the total area‐weighted pack ice thickness (including open water area) from only 0.2 m within 50 km of the ice edge to 0.45 m close to the coast. Ice thickness averaged over the ice‐covered region only is also relatively thin, ranging from 0.35 m near the ice edge to 0.65 m in the interior. These values are probably typical of average winter thickness for the area. The average snow cover thickness on the ice increased from 0.05 m near the ice edge to 0.15 m in the interior. Average ice concentration increased from less than 6/10 near the ice edge to 8/10 in the interior. The ship‐observed concentrations were in good agreement with concentrations derived from passive microwave satellite imagery except in some regions of high concentration. In these regions the satellite‐derived concentrations were consistently lower than those estimated from the ship, possibly because of the inability of the satellite sensors to discriminate the appreciable percentage of very thin ice observed within the total area. Spectral albedo was measured for nilas, young grey ice, grey‐white ice, snow‐covered ice, and open water at wavelengths from 420 to 1000 nm. Allwave albedo was computed by using the spectral measurements together with estimates of near‐infrared albedo and modeled spectral solar flux. Area‐averaged albedos for the East Antarctic sea ice zone in spring were derived from representative allwave albedos together with the hourly observations of ice types. These area‐averaged surface albedos increased from about 0.35 at the ice edge to about 0.5 at 350 km from the edge, remaining at 0.5 to the coast of Antarctica. The low average albedo is in part due to the large fraction of open water within the pack, but extensive fractions of almost snow‐free thin ice also pla

ISSN:0148-0227    

DOI:10.1029/93JC00648

年代:1993

数据来源: WILEY