Synoptic observations of the three‐dimensional structure and propagation of Gulf Stream meanders along the Carolina continental margin

Abstract

Simultaneously measured Eulerian currents and spatially extensive subsurface temperatures have provided a time series of eight synoptic, three‐dimensional views of the Gulf Stream frontal zone along the Carolina continental margin. Two large‐amplitude meanders were observed to progress through the study area between Charleston and Cape Hatteras during February 1979. Each meander had a vertically coherent, skewed wave‐like subsurface structure. The Eulerian velocity and temperature signatures produced by the meanders at the 250‐m level over the 390‐m isobath reflect this skewness. At a particular instrument, the in‐phase increases in temperature and downstream velocity associated with an approaching meander crest occurred during a longer time interval than did the more rapid decreases in these quantities following the crest's passage. Typically, the downstream velocity component at this level fluctuated from about −20 cm s−1 to near 100 cm s−1, while the cross‐stream component varied approximately ±25 cm s−1 about a near‐zero mean. For a particular meander, the maximum in the offshore velocity component led the downstream maximum in time in a manner typical of progressive wave motions; however, the lead time was always less than one quarter of a meander period implying that u and υ were not in quadrature, as would have been the case for stable waves. The two meanders were observed downstream of the area off Charleston where a seaward deflection of the stream is often found. Subsurface temperature data from February 10, 1979, show that on that date the degree of deflection was greatest near the surface, and that almost no deflection existed within the deeper reaches of the water column. According to later data, the deflection decreased as the meanders progressed alongshore away from the area, suggesting that the vertical structure of the deflection observed on the tenth may have been associated with the late stages of a meander passage. Filaments of warm Gulf Stream water extended southwestward ‘behind’ the crests of the two meanders. The filaments were relatively shallow features, extending from the surface to a depth of a few tens of meters. They were oriented essentially parallel to the bottom contours over the outer shelf and upper slope, and were separated from the main body of the Gulf Stream by cool water. The presence of the cool water between the stream and the filaments at the surface was due to upwelling of water from deep within or below the main stream. Peaks in the time series of vorticity components indicate that maximum cyclonic relative vorticity occurred behind the meander crests, in the leading portion of the trough near where a warm filament joined a meander crest. The meanders may have been initiated upstream of our study area, and then ‘amplified’ by the deflection process off Charleston. Energy flux calculations for the region off Onslow Bay indicate that meander kinetic energy was being converted to mean energy there. It seems likely that the deflection produces meander growth within the 100...

Estimates of gradient Richardson numbers from vertically smoothed data in the Gulf Stream region

“…These observations agree with the intensification of mixing observed within oceanic eddies that originated as Gulf Stream meanders (Miller and Evans, 1985). Meanders actually grow and propagate in very short times, with an intense frontogenetic phase that may last less than one inertial period (Bane et al, 1981). This is sufficiently fast that the associated flow may not have enough time to reach geostrophy.…”

“…At these scales we may expect that critical conditions will be attained whenever the Gulf Stream becomes sufficiently intensified, possibly related to some meander phases. The variability in the Stream's maximum velocity and structure (Rossby and Gottlieb, 1998;Rossby and Zhang, 2001) appears to be largely related to changes in its structure during different meander phases (Newton, 1978;Bane et al, 1981;Rodríguez-Santana et al, 1999). During these meanders the mean stratification (∂z/∂ρ) remains essentially unchanged, while the slope of the isopycnals is largely modifiedquite the opposite of what happens at small vertical scales.…”

“…During these meanders the mean stratification (∂z/∂ρ) remains essentially unchanged, while the slope of the isopycnals is largely modifiedquite the opposite of what happens at small vertical scales. Following Pelegrí and Csanady (1994), we may take this fact into consideration by writing the geostrophic gradient Richardson number (Equation 6) in isopycnic coordinates: , (6''') and using the thermal wind equation in isopycnic coordinates , (7') to obtain , (6''') Observations on the northern Blake Plateau (Bane et al, 1981;Rodríguez-Santana et al, 1999) have shown that the slope of the isopycnals (∂z/∂x) increases by a factor of four in a time period of only six days, as a response to the compression of the cross-stream horizontal coordinate. Under these conditions the gradient Richardson number would decrease by a factor of 16 and possibly lead to critical values.…”

Two Modes of Gulf Stream Variability Revealed in the Last Two Decades of Satellite Altimeter Data

“…GS meanders are affected by Rossby wave dynamics, which, in the absence of a background flow, forces them to propagate westward most of the time (Gangopadhyay et al 1992;Bower and Hogg 1992;Lee and Cornillon 1995;Taylor et al 1998;Nakamura and Chao 2000;Osychny and Cornillon 2004). At the same time, the GS exerts its influence over them too, advecting them eastward (Bane et al 1981;Watts and Johns 1982;Bower 1989;Savidge 2004). When these opposing influences balance, one might expect quasi-stationary meanders that have been observed sometimes during extended periods of almost a year (Nakamura and Chao 2000;Savidge 2004).…”

Physical Processes at the Shelf Edge in the Northwest Atlantic

“…(1981) and Bane et al (1981) have described the circulation of frontal eddies that produce warm salty tongues of Gulf Stream water that fold backwards along the inshore edge of the Stream to enclose a core of rich upwelled water. With cross-stream scales of 10 km, the eddies amplify as they propagate northward (at an average rate of 40 km/day) with maximum growth rates occurring just north of the "Charleston bump", a localized topographic irregularity that deflects the Gulf Stream seaward.…”