INITIAL OBSERVATIONS OF THE SUBSURFACE STRUCTURE AND SHORT-TERM VARIABILITY OF THE SEAWARD DEFLECTION OF THE GULF STREAM OFF CHARLESTON, SOUTH CAROLINA.

A recurring seaward deflection of the surface layer of the Gulf Stream has been observed near 32 degree N latitude off the coast of the southeastern United States. It is suggested that ridge and trough bottom feature (the so-called 'Charleston bump') on the upper continental slope off the Georgia/South Carolina coast produces the deflection through a bottom steering effect. Present data indicate that the deflection is great enough to direct the Gulf Streams's shoreward surface thermal front to the east, and even south of east, about 70% of the time. Air-deployed expendable bathythermograph surveys have been made with sufficient coverage to provide several synoptic, three-dimensional views of the Gulf Stream's thermal frontal zone in the region between Savannah, Georgia, and Cape Hatteras, North Carolina. These views show the subsurface structure of the seaward deflection to exhibit large short-term variability. during wintertime conditions (February 1979) the greatest deflection ( greater than 090 degree true) of the near-surface front occurred at a time when the deeper front was more aligned (approximately 080 degree true) with local topography. 22 refs. These views show the subsurface structure of the seaward deflection to exhibit large short-term variability. During wintertime conditions (February 1979) the greatest deflection ( greater than 090 degree true) of the near-surface front occurred at a time when the deeper front was more aligned (approximately 080 degree true) with local topography. refs

Wintertime air-sea interaction processes across the Gulf Stream

Aircraft, buoy and satellite measurements have been used across the Gulf Stream during January 25-30, 1986. 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 (~300W m-2 total 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 (~1060W m-2 over the core of the Stream), as did a second, somewhat weaker outbreak which followed on January 30 (~680W m-2 over the core of the Stream)

Wind and Gulf Stream influences on along-shelf transport and off-shelf export at Cape Hatteras, North Carolina

Along-shelf transports across three cross-shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992-1994. The along-shelf convergence has been used to infer off-shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along-shelf transport variability is primarily wind-driven and highly correlated with sea level fluctuations at the coast. Both winds and along-shelf transport exhibit a near-annual period variability. Along-shelf transport is not well correlated with Gulf Stream offshore position. Along-shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along-shelf convergence by hours to a few days. Long-period variability of 14-16 months and 1-3 months is apparent in both Gulf Stream position and transport convergence. Variability in along-shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along-shelf transport convergence is that the Gulf Stream is directly influencing cross-shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along-shelf divergence and shoreward cross-shelf transport exist (∼10% of the time just north of Cape Hatteras and ∼34% of the time just south of Cape Hatteras during episodes of up to 3-8 days duration). Implied onshore flows of a few cm s-1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along-shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf

GULF STREAM MEANDERS OFF NORTH CAROLINA DURING WINTER AND SUMMER 1979.

Meanders produced most of the subtidal variability in the Gulf Stream off North Carolina during 1979. Recording instruments were moored in the lower half of the water column over the 200-m and 400-m isobaths for two periods of 4 months, one in the late winter and one in the late summer. In both seasons, the middepth current speed typically fluctuated between minus 50 cm s** minus **1 and plus 100 cm s** minus **1 about a 30 cm s** minus **1 downstream mean. The velocity, temperature, and salinity fluctuations had a prominent weekly time scale in the winter, caused by the meandering stream. In the summer the weekly time scale was less prominent within a generally energetic 3- to 10-day period band. In both seasons, the meandering currents were nearly in phase vertically, and the meanders propagated downstream at approximationly 40 km d** minus **1. Shallow, in-shore filaments of warm water, separated from the main stream by bands of cooler surface water, are often extruded from the Gulf Stream front during the shoreward-most phase (crest) of meanders

Comparison of the TIROS-N satellite and aircraft measurements of Gulf Stream surface temperatures.

A comparison is made between multi-channel infrared (3.7 and 11 micrometre) temperatures measured by the TIROS-N satellite and aircraft single channel radiometer and AXBT measurements over the Gulf Stream between Cape Hatteras and Savannah, Georgia, on November 27, 1979. After reducing the noise in the 3.7 micrometre TIROS-N data, a multi-channel method is used to estimate the sea surface temperatures. For a temperature band of 19 to 26oC, the estimated and AXBT measurements are in agreement within a standard error estimate of 0.5oC. A bias of 1.2oC was found between the aircraft radiometer and the AXBT measurements, and part of this bias is attributed to radiometer calibration errors

Gulf Stream meanders along the Continental Margin from the Florida Straits to Cape Hatteras

A rapid increase in the magnitude of Gulf Stream meanders downstream of a seaward deflection of the Stream off Charleston, South Carolina, has been indicated by an analysis of the shoreward surface thermal front of the Stream. The sixty four cases examined show that lateral movements of the Stream from Charleston to Cape Hatteras may be as great as 40 km from the mean, whereas upstream of Charleston the movements are generally less than 15 km in amplitude. This difference points to the importance of the deflection of the Stream by a bottom feature off Charleston in producing Gulf Stream meanders

Cyclogenesis in the deep ocean beneath the Gulf Stream 1. Description

One of the primary scientific results of the Synoptic Ocean Prediction (SYNOP) observational program was the discovery of strong cyclones in the deep ocean beneath the large amplitude Gulf Stream meander troughs that routinely form at about 68°W. These strong well-organized cyclones extend to at least 3500 m below the sea surface and are an important component of the overall dynamical variability of the Gulf Stream and adjacent deep waters. Typically, a small amplitude Gulf Stream meander "stalls" near 68°W and begins to amplify. As the amplitude of troughs in the Gulf Stream jet increases, the currents at 3500 m strengthen and turn, forming a cyclonic circulation pattern. During SYNOP, six well-defined instances of meander trough amplification and deep cyclogenesis occurred. The cyclones were characterized by strong swirl speeds (up to 0.5 m s-1) and were long-lived (typically lasting 6-9 weeks) frequent occurrences (present 35% of the time during the 25 month deployment period). The structure of the cyclones at 3500 m is characterized by increasing velocity from the cyclone center out to some radius of maximum velocity and decreasing velocity beyond that radius. This structure was robust over the lifetime of an event and from event to event. Cyclone radius and the radius to maximum velocity were consistently ≃ 130 km and ≃ 55 km, respectively. Evidence of cyclones at upper measurement levels and the low vertical shear values apparent in the deep water below the thermocline indicate that the cyclones extended throughout the entire water column; from the benthic boundary layer, through the thermocline, and to the ocean's surface

Cyclogenesis in the deep ocean beneath the Gulf Stream 2. Dynamics

Strong cyclones in the deep ocean beneath the Gulf Stream have been observed during the period June 13, 1988, to August 7, 1990, near 68°W, 37°N in data from the Synoptic Ocean Prediction (SYNOP) Experiment. These cyclones developed in association with the evolution of large amplitude, quasi-stationary meander troughs in the Gulf Stream. It is likely that baroclinic instability is responsible for cyclone spin-up. The dynamical analysis of these cyclones indicates the process is analogous with atmospheric cyclogenesis from the perspectives of divergence and conservation of potential vorticity, but not in terms of the density field evolution. Large positive vertical velocities in the thermocline over developing low pressure centers at 3500 m are consistent with convergence at depth and divergence in the upper ocean, and with stretching of the lower water column and shortening of the upper water column. The stretching of the lower water column accounts for the generation of positive relative vorticity there. However, the evolution of the density field in the oceanic case does not resemble the atmospheric case. In the atmosphere, density field adjustments in the air column above the low pressure center at the Earth's surface are in the correct sense to account for decreasing pressure there. In the ocean, density field adjustments in the water column fail to account for the developing low pressure centers, so sea surface height depressions must be responsible. These depressions must have an approximate magnitude of 0.5 m depth over a 250 km horizontal extent (the cyclone's diameter)

Air-sea interactions during the passage of a winter storm over the Gulf Stream: A three-dimensional coupled atmosphere-ocean model study

A three-dimensional, regional coupled atmosphere-ocean model with full physics is developed to study air-sea interactions during winter storms off the U. S. east coast. Because of the scarcity of open ocean observations, models such as this offer valuable opportunities to investigate how oceanic forcing drives atmospheric circulation and vice versa. The study presented here considers conditions of strong atmospheric forcing (high wind speeds) and strong oceanic forcing (significant sea surface temperature (SST) gradients). A simulated atmospheric cyclone evolves in a manner consistent with Eta reanalysis, and the simulated air-sea heat and momentum exchanges strongly affect the circulations in both the atmosphere and the ocean. For the simulated cyclone of 19-20 January 1998, maximum ocean-to-atmosphere heat fluxes first appear over the Gulf Stream in the South Atlantic Bight, and this results in rapid deepening of the cyclone off the Carolina coast. As the cyclone moves eastward, the heat flux maximum shifts into the region near Cape Hatteras and later northeast of Hatteras, where it enhances the wind locally. The oceanic response to the atmospheric forcing is closely related to the wind direction. Southerly and southwesterly winds tend to strengthen surface currents in the Gulf Stream, whereas northeasterly winds weaken the surface currents in the Gulf Stream and generate southwestward flows on the shelf. The oceanic feedback to the atmosphere moderates the cyclone strength. Compared with a simulation in which the oceanic model always passes the initial SST to the atmospheric model, the coupled simulation in which the oceanic model passes the evolving SST to the atmospheric model produces higher ocean-to-atmosphere heat flux near Gulf Stream meander troughs. This is due to wind-driven lateral shifts of the stream, which in turn enhance the local northeasterly winds. Away from the Gulf Stream the coupled simulation produces surface winds that are 5 ∼ 10% weaker. Differences in the surface ocean currents between these two experiments are significant on the shelf and in the open ocean

Gulf Stream path and thermocline structure near 74°W and 68°W

The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W moored observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime. The cross-stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form