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Peter Cornillon

Abstract

It is commonly held belief that meandering of the Gulf Stream increases dramatically downstream of the New England Seamount chain. In fact this appears not to be the case. The envelope of Gulf Stream northern edges derived from 30 months of satellite data remains constant from 65° to 58°W. Only the mean of the integrated path length normalized by the mean path length appears to increase downstream of the seamounts, but at very nearly the same rate as upstream of them.

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Tong Lee
and
Peter Cornillon

Abstract

Positions of the Gulf Stream path from 74° to 45°W were obtained from satellite infrared images for the period of April 1982–December 1989. The propagation of meanders between 74° and 70°W was studied through spectral analysis in wavenumber-frequency space, empirical orthogonal function analysis in time and frequency domains, and direct measurements of individual meander properties. Progressive meanders are found to have a broad range of periods from days to years, and wavelengths from about 200 to 1100 km. Good agreement is found between the satellite and Inverted Echo Sounder data for short-period (<80 days) progressive propagation. Retrogressive meanders with wavelengths longer than 1100 km are found to coexist with progressive ones at periods longer than 4 months. The empirical dispersion relation is in qualitative agreement with the linear prediction of a recent equivalent-barotropic,β-plane thin-jet model, the comparison also suggests that topographic β may need to be considered in order to account for the magnitudes of observed retrogressive phase speeds. Amplitude dependence of propagation is observed. with the phase speed decreasing as the amplitude increases. Standing meanders are observed at periods when both progressive and retrogressive propagation are present; their wavelengths fall between those of oppositely traveling meanders. These standing meanders are responsible for the standing wave pattern of the path envelope between Cape Hatteras and 69°W It is argued that they are formed by near-stationary meanders of a similar wavelength but different amplitudes propagating in opposite directions as a result of the combined amplitude-dependent and β effect.

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Tong Lee
and
Peter Cornillon

Abstract

Analysis of the Gulf Stream path between 75° and 60°W indicates that the spectral signature of propagating and standing meanders is qualitatively similar to that observed for the upstream region 74°–70°W. Progressive, retrogressive,and standing meanders coexist at periods of several months and longer.

The amplitude-dependent dispersion relation obtained for the region 75°–45°W demonstrates the decrease of phase speed as the amplitude increases; the dependence of phase speed on amplitude is found to be stronger than that on wavelength. The average phase speed decreases with downstream distance primarily due to the downstream increase of meander amplitude. Consequently, a relation between phase speed and wavelength for the region west of 70°W, averaged over all amplitudes, is not uniformly valid for a larger domain. Furthermore, downstream propagating meander troughs are steeper and travel more slowly than meander crests. The average stationary wavelength, 700–800 km for 75°–60°W, is much shorter than that predicted based on an equivalent barotropic,,β-plane thin-jet model.

The most energetic meanders have a period of 46 days and a wavelength of 427 km. The period of the fastest-growing meanders is approximately 40 days, close to the period of the most energetic meanders. The wavelength of the fastest-growing meanders, about 350 km, is shorter than the wavelength of the most energetic meanders.

The New England Seamounts do not have a significant effect on the most energetic meanders. However, meanders having periods either shorter or longer than the period of the most energetic meanders are affected by the seamounts. For long-period meanders, their lateral excursions seem to be constrained by the seamounts.

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Vladimir Osychny
and
Peter Cornillon

Abstract

This study uses satellite observations of sea surface height (SSH) to detect westward-propagating anomalies, presumably baroclinic Rossby waves, in the North Atlantic and to estimate their period, wavelength, amplitude, and phase speed. Detection involved a nonlinear fit of the theoretical dispersion relation for Rossby waves to the time–longitude spectrum at a given latitude. Estimates of period, wavelength, and phase speed resulted directly from the detection process. Based on these, a filter was designed and applied to extract the Rossby wave signal from the data. This allowed a mapping of the spatial variability of the Rossby wave amplitude for the North Atlantic. Results showed the familiar larger speed of observed Rossby waves relative to that expected from theory, with the largest differences occurring at shorter periods. The data also show that the dominant Rossby waves, those with periods that are less than annual, propagated with almost uniform speed in the western part of the North Atlantic between 30° and 40°N. In agreement with previous studies, the amplitude of the Rossby wave field was higher in the western part of the North Atlantic than in the eastern part. This is often attributed to the influence of the Mid-Atlantic Ridge. By contrast, this study, through an analysis of the wave spatial structure, suggests that the source of the baroclinic Rossby waves at midlatitudes in the western North Atlantic is located southeast of the Grand Banks where the Gulf Stream and the deep western boundary current interact with the Newfoundland Ridge. The spatial structure of the waves in the eastern North Atlantic is consistent with the formation of these waves along the basin's eastern boundary.

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Michael A. Alfultis
and
Peter Cornillon

Abstract

Subtropical mode waters (STWMs) are water masses formed in winter by convective mixing on the equatorward side of western boundary currents in the subtropical gyres. After the return of the seasonal stratification in spring, it is found at the stratification minimum between the seasonal and main pycnoclines. By characterizing STMW primarily at the density gradient minimum, previous studies were limited in their ability to describe STMW properties over large temporal and spatial scales. Rather than using a density-based characterization, the North Atlantic STMW layer was identified here by its much smaller temperature gradient relative to the more stratified seasonal and main thermocline, its temperature, and its large thickness. By using this temperature-based characterization, this study was able to develop a climatology using the large number of XBTs deployed between 1968 and 1988 and contained in the World Ocean Atlas 1994 historical hydrographic database and to use this climatology to examine STMW properties on large spatial and long temporal scales. Three different characterizations were used to assess the degree of convective renewal of the STMW layer during the 1968–88 winters. Two characterizations were based on comparing the winter mixed layer properties to the STMW layer properties in the previous fall, while the third characterization involved comparing the temperature gradient through the STMW layer in the spring to the STMW layer temperature gradient in the previous fall. Based on these characterizations, there was considerable spatial and temporal variability in the renewal of the STMW layer's vertical homogeneity from 1968 to 1988. Basinwide renewal occurred in 1969, 1970, 1977, 1978, 1981, and 1985, with more localized renewal, usually east of 55°W, in the other years. While STMW is nearly vertically homogeneous immediately after renewal, the temperature gradient through the layer increases with time following renewal. The annual rate of increase in the temperature gradient in the year following renewal is ∼5–6 (× 10−4°C per 100 m per day), while the interannual rate of increase is ∼2.0 × 10−4°C per 100 m per day following winters with no renewal of the STMW layer.

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Peter Cornillon
,
Tong Lee
, and
Gregory Fall

Abstract

Eight years of Gulf Stream data were examined to determine the percentage of meander crests that give rise to warm core rings. Because of cloud cover in many of the satellite images it was not possible to associate a specific meander crest with each warm core ring formed, nor was it possible to follow each meander crest from the point at which its amplitude was first detectable to the point at which it left the study area, was absorbed by another meander, or split to form two meanders. Despite these problems, a lower bound of 0.24 could be placed on the probability that a meander crest detaches to form a warm core ring. This was obtained by considering all disturbances in the path of the Gulf Stream that could be tracked over a several day period. If consideration is restricted to disturbances with a length scale the size of warm core rings or larger, the probability of formation increases to 0.40. The authors argue that this is a more interesting number in that the wavelength of the meanders observed rarely (less than 20% of the time) increased by more than 50% of their initial value; hence only ring-scale meander crests can result in warm core rings, of these more than 40% do. Finally, it could be argued that the numbers for troughs forming cold core rings are similar.

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George R. Halliwell Jr.
and
Peter Cornillon

Abstract

We describe properties of large-scale fluctuations in the wind field represented by two analyses (FNOC and NOAA ATOLL), and in the SST field represented by maps derived from 5-day composite AVHRR images, within an 11° longitude by 10° latitude domain during FASINEX (January through June 1986). FNOC and ATOLL wind and wind stress time series were highly correlated with each other (<0.8) over the entire domain, and they were also highly correlated with time series measured at the FASINEX site ≥0.89. At periods of 2–13 days, clockwise →τ energy exceeded anticlockwise energy by an order of magnitude, while the energy was nearly partitioned equally at periods of 13–40 days. Mean ATOLL winds were everywhere more eastward than mean FNOC winds, probably in part because the ATOLL (FNOC) analyses are intended to represent 850 mb (19.5 m) winds. The 850 mb winds are expected to be more eastward due to the thermal wind balance with the mean southward air temperature gradient. The SST maps represented large-scale variability with reasonable accuracy, although a mean positive SST bias of about 0.54.6°C resulted from the image compositing procedure. This bias did not seriously affect the ability to detect either large-scale SST features or frontal zones in the 5-day maps. A single frontal zone up to 3° of latitude wide, which crossed the domain in a predominantly zonal direction, was the dominant large-scale frontal feature in the SST maps. Perturbations in the strength of this frontal zone shifted westward at several kilometers per day.

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George R. Halliwell Jr.
and
Peter Cornillon

Abstract

We analyzed the influence of wind-deriven horizontal heat advection on the large-scale [O(1000) km wavelength] variability of both the upper-ocean mixed-layer heat content and the subtropical frontal zone (SFZ) within an 11° by 10° domain in the western North Atlantic Ocean during FASINEX (January through June 1986). By estimating heat advection due to both Ekman transport and interior geostrophic (Sverdrup minus Ekman) transport from a slab mixed layer heat balance equation using satellite-derived sea surface temperature (Ts ) and wind analysis maps, it was found that these processes could not account for the observed variability in either beat content or the SFZ. The annual cycle of surface vertical heat flux had the dominant influence on the heat content. Even when the average heat balance was analyzed during a 4-month time interval when the net influence of the annual cycle was nearly zero (mid-January to mid-May 1986), westward-propagating Ts spatial anomaly features with peak-to-peak scales of several hundred kilometers apparently had the dominant influence on heat content. The influence of Ekman transport appeared to become marginally detectable only when terms in the heat equation were zonally averaged across the entire analysis domain, apparently reducing the influence of the propagating anomaly features. Ekman transport did act to maintain the SFZ during the 4-month interval, and thus may have been ultimately responsible for its existence, but the large-amplitude variability in heat content and the SFZ driven by other processes made this impossible to prove conclusively in the FASINEX region.

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Peter Cornillon
,
Richard Weyer
, and
Glenn Flierl

Abstract

Five warm Core rings were examined during short time intervals using thermal infrared satellite imagery. A total of 12 pairs of observations of these rings were made, all east of 72°W. The observations concentrated on the translational velocity of the rings and the mean velocity of the surrounding slope water. The mean translational velocity of the rings was found to be 8.5 ± 3.1 cm s−1 at 282° from north. The mean velocity of the surrounding slope water, determined from a combination of in situ observations with a depth weighting based on theoretical arguments, was 5.2 ± 0.3 cm s&−1 at 258°. The difference, i.e., the velocity of the ring relative to the slope water, was 4.6 ± 3.0 cm s−1 at 208°, or, in component form, these rings were found to move relative to the surrounding slope water with a mean northward component of 2.8 ± 1.7 cm s−1 and a mean westward component of 3.2 ± 2,3 cm s−1. The observations presented here are in disagreement with previous observations of ring displacements which show a general southerly trend, as well as with most theoretical analyses of eddy propagation which show either westward or southwestward velocities. Previous observational studies, however, examined warm core rings over long periods of time, periods during which the rings invariably interacted with the continental shelf, with the Gulf Stream and/or with other rings. Because the continental shelf constrains rings to move generally in a southwesterly direction it is not surprising that estimates derived over long periods show a southerly trend. Only rings free of such interactions were considered in this study. This resulted in the observation intervals being short (12–36 h) and in a limited number of observations (12), hence in a relatively high uncertainty in the estimates. Nevertheless we note that 8 of the 12 observations showed a northward component. Finally, previous observational results dealt only with the rings' absolute velocities, not their relative velocities.

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Daniel L. Codiga
and
Peter Cornillon

Abstract

Interpretation of sea surface height anomaly (SSHA) and wind forcing of first baroclinic mode Rossby waves is considered using linear inviscid long-wave dynamics for both the standard and surface-intensified vertical mode in a continuously stratified rest-state ocean. The ratio between SSHA variance and vertically integrated energy of waves is proportional to 1) a dimensionless ratio characterizing the surface intensification of the pressure eigenfunction, 2) the squared internal gravity wave speed, and 3) the inverse of the water depth. Geographic variations in stratification and bathymetry can therefore cause geographically varying SSHA variance even for spatially uniform wave energy. The ratio between SSHA variance and wave energy across the North Atlantic shows important spatial variations based on eigensolutions for the standard vertical mode determined numerically using climatological hydrography. The surface-intensified mode result is similar, though the ratio is generally slightly larger and less sensitive to depth variations. Results are applied to the propagating annual-frequency portion of TOPEX altimeter SSHA in the North Atlantic. SSHA variance at 35° in the western half of the basin increases by ∼63% over that in the east, but the associated change in inferred first-mode baroclinic Rossby wave energy is a substantially smaller increase of ∼26% (∼34%) for the standard (surface intensified) mode. This is mainly associated with increases to vertical mode surface intensification and squared internal gravity wave speed in the west due to stronger stratification above the pycnocline. The wind-forced wave equation for SSHA has a dimensionless coefficient of Ekman pumping that is proportional to the ratio between SSHA variance and wave energy, implying similar geographic variation in efficiency of wind excitation of Rossby wave SSHA.

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