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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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Peter Cornillon
and
Randolph Watts

Abstract

The northern edge of the Gulf Stream off Cape Hatteras, North Carolina was located in 155 AVHRR-derived maps of sea surface temperature (SST) using five different methods. One method was subjective location of the northern edge by an analyst; the other four involved objective location of the edge by computer using various statistics of the SST field. Specifically, the quantities considered were: maximum SST gradient (calculated over a 3 × 3 pixel box), maximum SST (on a pixel-by-pixel basis), maximum variance (calculated over a 7 × 7 pixel box), and change in the skewness of the SST distribution (calculated over a 5 × 5 pixel box). The resulting locations were compared with the location of the 15°C isotherm at 200 m (T 15) determined from inverted echo sounders (IESs) moored on the sea floor. The best method, which yielded the smallest rms difference from the IES-derived T 15, was the subjective one; the surface front was located 9.0 km shoreward of T 15 with a rms difference of 14.3 km. The best objective technique used the skew of the SST distribution: Each pixel in the image was replaced by the skew of the distribution of the twenty-five SST values obtained from a 5 × 5 pixel square centered on that pixel. The skew changes sign when a step in the SST data, such as the Gulf Stream northern edge, is crossed. The Gulf Stream northern edge located in the skew images was found to be 14.0 km shoreward of T 15 in the mean with a rms difference of 18.2 km. In general, the more spatial information used, the better the estimate.

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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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Kenneth S. Casey
and
Peter Cornillon

Abstract

Individual sea surface temperature (SST) anomalies are calculated using a satellite-based climatology and observations from the World Ocean Atlas 1994 (WOA94) and the Comprehensive Ocean–Atmosphere Data Set (COADS) to characterize global and regional changes in ocean surface temperature since 1942. For each of these datasets, anomaly trends are computed using a new method that groups individual anomalies into climatological temperature classes. These temperature class anomaly trends are compared with trends estimated using a technique representative of previous studies based on 5° latitude–longitude bins.

Global linear trends in the data-rich period between 1960 and 1990 calculated from the WOA94 data are found to be 0.14° ± 0.04°C decade−1 for the temperature class approach and 0.13° ± 0.04°C decade−1 for the 5° bin approach. The corresponding results for the COADS data are 0.10° ± 0.03°C and 0.09° ± 0.03°C decade−1. These trends are not statistically different at the 95% confidence level. Additionally, they agree closely with both SST and land–air temperature trends estimated from results reported by the Intergovernmental Panel on Climate Change. The similarity between the COADS trends and the trends calculated from the WOA94 dataset provides confirmation of previous SST trend studies, which are based almost exclusively on volunteer observing ship datasets like COADS.

Regional linear trends reveal a nonuniformity in the SST rates between 1945–70 and 1970–95. Intensified warming during the later period is observed in the eastern equatorial Pacific, the North Atlantic subtropical convergence, and in the vicinity of the Kuroshio extension. Also, despite close agreement globally, localized differences between COADS and WOA94 trends are observed.

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Jean-François Cayula
and
Peter Cornillon

Abstract

An algorithm to detect fronts in satellite-derived sea surface temperature fields is presented. Although edge detection is the main focus, the problem of cloud detection is also addressed since unidentified clouds can lead to erroneous edge detection. The algorithm relies on a combination of methods and it operates at the picture, the window, and the local level. The resulting edge detection is not based on the absolute strength of the front, but on the relative strength depending on the context thus, making the edge detection temperature-scale invariant. The performance of this algorithm is shown to be superior to that of simpler algorithms commonly used to locate edges in satellite-derived SST images. This evaluation was performed through a careful comparison between the location of the fronts obtained by applying the various methods to the SST images and the in situ measures of the Gulf Stream position.

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Jean-François Cayula
and
Peter Cornillon

Abstract

This paper presents an approach based on the analysis of an image sequence to detect temperature fronts in a sea surface temperature image. The multi-image edge detection algorithm starts by applying a single-image edge detection algorithm to the sequence of images under study. Next, fronts or portions of fronts, which were detected in neighboring images by the single-image algorithm and which match features in the current image, are identified as persistent. The coordinates of these persistent fronts are then passed to the single-image edge detection algorithm so that additional fronts can be detected. The performance of the multi-image edge detection algorithm, of various single-image algorithms, and of a human expert are evaluated on a set of 98 images. For that purpose, the location of the fronts obtained by applying various methods to the SST images is compared to the in situ measures of the Gulf Stream position. With respect to both quality and the number of detected edges, the multi-image edge detection algorithm is the only automated method that achieves results comparable to those obtained by a human expert.

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David S. Ullman
and
Peter C. Cornillon

Abstract

Sea surface temperature (SST) fronts detected in Advanced Very High Resolution Radiometer (AVHRR) data using automated edge-detection algorithms were compared to fronts found in continuous measurements of SST made aboard a ship of opportunity. Two histograms (a single-image and a multi-image method) and one gradient algorithm were tested for the occurrence of two types of errors: (a) the detection of false fronts and (b) the failure to detect fronts observed in the in situ data. False front error rates were lower for the histogram methods (27%–28%) than for the gradient method (45%). Considering only AVHRR fronts for which the SST gradient along the ship track was greater than 0.1°C km−1, error rates drop to 14% for the histogram methods and 29% for the gradient method. Missed front error rates were lower using the gradient method (16%) than the histogram methods (30%). This error rate drops significantly for the histogram methods (5%–10%) if fronts associated with small-scale SST features (<10 km) are omitted from the comparison. These results suggest that frontal climatologies developed from the application of automated edge-detection methods to long time series of AVHRR images provide acceptably accurate statistics on front occurrence.

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