摘要:

The Genesis of Atlantic Lows Experiment (GALE) was conducted during early 1986 to investigate the rapidly developing, extratropical winter cyclones that travel across eastern North America and out over the northwestern Atlantic Ocean. Until only recently, these storms have been difficult to predict accurately [Sanders, 1986a,b], and they are well known for the devastating weather they sometimes bring to the eastern seaboard [Bosart, 1981;Kocin and Uccellini, 1984]. Such storms frequently produce “crippling” ice, heavy snow, and gale force winds, and they may batter the east coast from the Carolinas northward, often causing property damage and loss of life. The strong winds that accompany such a storm, as well as the cold, dry, continental air that often flows out over the Atlantic during the cold air outbreak stage of the storm's passage, are responsible for some of the most vigorous air‐sea exchanges of momentum, heat, and moisture in this part of the world [Budyko, 1974;Bunker and Worthington,

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10685

年代:1989

数据来源: WILEY

摘要:

Data collected during winter from a set of offshore buoys and coastal meteorological stations on the continental shelf of the southeastern United States were analyzed as part of a meteorological and oceanographic study of the genesis of winter cyclones (Project GALE). The analyses were designed to describe the temporal and spatial variability of wind stress and heat flux over the shelf. While there were some episodes of strong spatial variations in wind stress, the wind field, on the whole, was remarkably uniform. Largest gradients were associated with times immediately preceding outbreaks of cold continental air over the shelf. Maximum heat fluxes occurred during these cold air outbreaks and reached as high as 1400 W/m2on the outer shelf near the Gulf Stream. The largest wind stress values occurred in the cross‐shelf component and reached 0.7 Pa. The cross‐shelf wind was highly correlated with the geostrophic wind, but there were significant ageostrophic deviations in the alongshelf component. These deviations were correlated with fluctuations in sea‐air temperature differences. The fluctuations in heat flux are thought to modulate the alongshelf component by inducing added vorticity in the planetary boundary layer over the scale width of the continental

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10686

年代:1989

数据来源: WILEY

摘要:

Continental shelf waters are particularly responsive to winter storm events mainly because of their shallow depths. Those of the southeastern United States (the South Atlantic Bight (SAB)) are especially responsive because they are broad and shallow. Also, the Gulf Stream serves as a continual source of warm water at the outer boundary. Thus the SAB receives strong meteorological (wind stress and heat loss) and oceanographic (advective) forcing. During the Genesis of Atlantic Lows Experiment (GALE) the response of shelf waters to winter storm events and Gulf Stream forcing was observed. The mean conditions showed a mixed water column with areas of stratification near the coast and at the shelf break. The nearshore area was stratified only during weak offshore winds, and the shelf break area was stratified during southward winds with accompanying onshore Ekman flow. On the inner shelf, advective buoyancy flux was similar in value to heat flux buoyancy and the buoyancy equivalent of wind mixing. Over the shelf break the advective buoyancy flux was 4 times the other forms of buoyancy flux and controlled the observed potential energy variability. A simple box model heat budget used to separate the effect of Gulf Stream eddies and meanders, and Ekman flow and air‐sea heat exchange on the shelf heat content showed that the observed heat content variability was caused by intrusion of Gulf Stream water. The intrusions may be caused either by onshore Ekman flow during southward winds or Gulf Stream meander event

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10699

年代:1989

数据来源: WILEY

摘要:

The South Carolina shelf water response to strong winter atmospheric forcing and Gulf Stream variations over the slope is discussed with the aid of a comprehensive set of atmospheric and oceanic observations made during project GALE (Genesis of Atlantic Lows Experiment). A mixed response is observed to wind and Gulf Stream forcing. Subtidal variability of inner and middle shelf waters is primarily a barotropic, forced response to coherent, synoptic scale, alongshore winds. This response is not trapped over the shelf, as has been found for other wide, non‐equatorial shelves, but is also significant at the shelf edge, possibly because of the comparable widths of the shelf and slope. The outer shelf also shows a significant Gulf Stream influence. The Gulf Stream appears to have two preferred position modes, either onshore, with the stream flowing over the Charleston bump and along the shelf break, or offshore, when the stream can be located up to 100 km east of the shelf break; transition between the two states is rapid. Subtidal variability in the outer shelf is dominated by weekly period frontal eddies and meanders when the stream is in an onshore mode, and by persistent southward flow when the stream is offshore. This southward flow is produced by enlarged cyclonic eddies with upwelled centers located over the slope between the offshore Stream and the shelf edge, sometimes referred to as the Charleston gyre. Positive mean alongshore sea level slopes at the shelf edge, due to the persistence of the cyclonic gyres, are geostrophically balanced by mean onshore flow, which transports upwelled waters onto the shelf, enhancing the biological productivity of the area relative to other regions of the South Atlantic Bigh

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10715

年代:1989

数据来源: WILEY

摘要:

Aircraft, buoy and satellite measurements have been used to study the wintertime air‐sea interaction processes across the Gulf Stream during January 25–30, 1986. The turbulent flux regime in the marine atmospheric boundary layer exhibited considerable spatial and temporal variability during this 6‐day period, which was related to both the evolution of the synoptic scale atmospheric conditions and the sea surface temperature (SST) field. During the pre‐storm conditions prior to January 25, the spatial structure of the SST field played an important role in generating a shallow atmospheric frontal zone along the Gulf Stream front by causing differential heating of the marine atmospheric boundary layer over the stream versus over the cooler shelf waters. As this front moved shoreward on January 25, the warm, moist, maritime air flowing northwestward behind the front induced moderate ocean‐to‐atmosphere heat fluxes (∼300 W m−2total heat flux measured over the core of the Gulf Stream). The subsequent outbreak of eastward flowing cold, dry, continental air over the ocean on January 27 and 28 generated high total heat fluxes (∼1060 W m−2over the core of the Stream), as did a second, somewhat weaker outbreak which followed on January 30 (∼680 W2over the core of the Stream). During each of these outbreaks, with air flowing from land out over the continental shelf, Gulf Stream and Sargasso Sea waters, the SST field again affected the spatial structure of the flux fields. The near‐surface fluxes of both sensible and latent heat were found to be relatively low over the cool continental shelf waters, while higher fluxes were seen over the Gulf Stream and Sargasso Sea. Similar spatial structure was seen in the near‐surface momentum flux values, but relative changes were typically smaller from one location to another on a particular day. The most noticeable responses of the Gulf Stream to these surface fluxes were the deepening of its mixed layer and a loss of upper layer heat; however, no direct current observations were made in the stream, so velocity changes may not be assessed. An average mixed layer deepening of about 35 m was observed in the stream, and the upper layer heat loss was estimated to be 3.2×1013J m−1alongstream, an amount sufficient to decrease the average mixed layer temperature by 0.62°C. No path changes in the stream could be attributed to the atmospheric forcing of this period, since there was a large offshore movement of the stream in the region of the Charleston bump at this time due to other processes. Any path changes that may have been associated with the atmospheric forcing would have been mas

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10755

年代:1989

数据来源: WILEY

摘要:

This study examines the atmospheric response of the deep ocean using 200 instrument years of moored current measurements from across the breadth of the mid‐latitude North Pacific. We have found evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35°N. This modulation is often in phase with the local atmospheric forcing function. At frequencies between 0.1 and 0.01 cpd, correlations between the local wind stress curl and the deep currents have been found. For a few locations the local forced response is parallel to the potential vorticity gradient (∇f/H) and consistent in amplitude with a local topographic Sverdrup balance. Nonlocal forcing in the form of flow along isolines of potential vorticity has been estimated, and it is comparable to the observed flow at some locations, but the modeled flow is not correlated with the observations. Throughout the mid‐latitude North Pacific the bottom slope tends to enhance the β effect. This suggests that topography narrows the available bandwidth for forced barotropic Rossby waves, facilitating a quasi‐steady topographic Sverdrup response over most of the basin. However, the enhanced β effect reduces the magnitude of the ocean's response to wind forcing. Nonlocally forced Sverdrup currents (flow along isolines off/H) are the dominant component, but are only of the order of 1 cm s−1. Locally forced Sverdrup currents (flow across isolines off/H) are typically of the order of 0.1 cm s−1rms and are not commonly measured. Hence these wind‐forced variations are a background level signal in the mid‐latitude eddy kinetic energy. In a few isolated areas the bottom slope reduces the β effect. In these regions the local Sverdrup response is amplified an

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10773

年代:1989

数据来源: WILEY

摘要:

Relatively cool and fresh water, originating from the Middle Atlantic Bight continental shelf, has often been found along the northwestern margin of the Gulf Stream. In this paper we present measurements of temperature, salinity, and velocity within subsurface filaments of entrained shelf water adjacent to the Gulf Stream. The data indicate that vertical mixing within and near these filaments is effected by double‐diffusive processes and shear‐induced turbulence. The measurements also reveal a complex velocity and hydrographic structure within the filaments and show that the character of individual filaments differed considerably. For example, the measured mean flow within two filaments was much slower than the current in the adjacent Gulf Stream, whereas a third filament appeared to be incorporated within the high‐velocity region of the Gulf Stream. The most thoroughly sampled band of entrained shelf water exhibited temperature inversions and contained a slowly moving intrusion of warm water, presumably detached from the Gulf Stream. The estimated transport of shelf water within this band was roughly 7.2×104m3s−1, comparable with the rate at which water is transported over the southern Middle Atlantic Bight continental shelf as estimated by a previous study. However, the transport of individual entrained shelf water filaments may vary considerably because of the significant variation in their currents and hydrographic com

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10791

年代:1989

数据来源: WILEY

摘要:

Mean monthly mixed layer depth (MLD), sea surface temperature (SST), and surface current climatologies are generated for the tropical Indian Ocean. In addition, surface meteorological climatologies are produced for those variables which could influence the evolution of the MLD and SST fields. Only the MLD climatology is described in detail, as climatologies for the other variables have appeared previously in the literature. The sum of the annual and semiannual harmonics account for greater than 75% of the energy in the MLD time series over most of the basin. The amplitude of the annual signal is greater than 20 m between 10°S and 25°S, with deepest MLDs observed during the southern hemisphere winter. The south central Arabian Sea, between the equator and 10°N, and the northern Arabian Sea are also regions of larger annual harmonic amplitude (>15 m). The amplitude of the semiannual harmonic is largest in the central Arabian Sea (>25 m). Deepest MLDs are observed there during the height of the two monsoon seasons. Correlation coefficients are computed between MLD and SST and several other oceanographic and meteorological variables to explore possible causal relationships. Net energy flux through the sea surface can account for 75% of the variance in the SST and MLD time series over most of the region south of the equator. Large coefficients are also observed in the northwestern Arabian Sea. Correlations between SST and MLD and surface currents are in general small throughout the region, with maxima observed in the central Arabian Sea, in the vicinity of the South Equatorial Current and in the extreme eastern equatorial Indian Ocean. These correlations will be examined in more detail in part 2 of this study in which simple models of mixed layer dynamics are employ

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10801

年代:1989

数据来源: WILEY

摘要:

The two principal fronts within the Antarctic Circumpolar Current (ACC), the Subantarctic Front (SAF) and the Polar Front (PF), are investigated with respect to their spatial patterns and relations to deep water masses from the northern Drake Passage to the western and southern Argentine Basin. Observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters. East of Drake Passage, the width of the Polar Frontal Zone (PFZ) increases rapidly when the SAF turns sharply northward as a part of the Falkland (Malvinas) Current, while the PF remains oriented mainly toward the northeast. Found within this region of the PFZ are mesoscale eddies, the upper layers of which consist of Subantarctic Mode Water from the Pacific. The contrasts in upper layer water properties that identify the SAF can be traced the entire length of the Falkland Current to the Brazil‐Falkland confluence zone and then southward to the southern Argentine Basin. There the SAF turns eastward and later, but perhaps not always, merges with the PF north of Ewing Bank to form a single, intense current core strongly influenced by bottom topography. At times this eastward current loops southward through a gap in the Falkland Ridge into the Georgia Basin, thereby supplying relatively warm and salty Subantarctic water to the Antarctic Zone. East of the gap, the two fronts become separated and are once again distinct features in the vicinity of the Islas Orcadas Rise. At depth, within Drake Passage and the western Scotia Sea, the SAF and PF are not lateral boundaries between distinct water masses, but are instead identified by enhanced vertical displacements of property isopleths. With the northward turn of the SAF east of Drake Passage, a thick layer of Circumpolar Deep Water (CDW) is advected over the Falkland Plateau into the Argentine Basin. There it is joined by waters entering the Argentine Basin via a deep spreading route through the Georgia Basin: denser CDW, deep water from the Weddell Sea, and episodically, deep water from the southeastern Pacific Ocean. Together, these waters form a northward flow off the Patagonian shelf that extends from the sea surface to the bottom, most of which turns back toward the south with the Falkland Current return. Also flowing south, along the seaward side of the Falkland Current return well away from the western boundary, is the main core of North Atlantic Deep Water (NADW). Detatched masses of NADW are observed within the CDW at the western boundary as well as within the ACC south of the SAF in the south central Argentine Basi

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10817

年代:1989

数据来源: WILEY

摘要:

Repeated ADCP sections without acoustic bottom reference and based on the directly observed athwartship velocity component have been worked across the surface water mass of the Faroe Bank Channel. A pronounced inhomogeneity of the current field was observed at depths of around 300 m. This anomaly could be correlated to the hydrographic observations which were undertaken throughout the experiment, since the pertinent segment of the temperature‐salinity (T‐S) relationship showed the presence of a water mass with characteristics deviating from those normally encountered in this zone. The outcome of a set of extended dynamic calculations furthermore showed that over this range of depths the observed velocity structure could not be accounted for solely in terms of geostrophic mot

ISSN:0148-0227    

DOI:10.1029/JC094iC08p10839

年代:1989

数据来源: WILEY