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Michael A. Alfultis
and
Peter Cornillon

Abstract

The North Atlantic Subtropical Mode Water (STMW) layer was identified based on its temperature, large thickness, and small temperature gradient. Comparisons between this method and identifying the STMW layer using a density-based (i.e., potential vorticity) criteria indicate that this method successfully identifies the STMW layer as the remnant of the previous winter's convective mixing. By using this temperature-based characterization of the STMW layer, this method was able to develop a climatology using the large number of expendable bathythermographs (XBTs) deployed between 1968 and 1988, and contained in the World Ocean Atlas 1994 historical hydrographic database. From this climatology, the STMW layer that is the remnant of the previous winter's convective activity is typically found between 175 and 450 m, has an average temperature near 18°C, and has a mean temperature gradient of 0.5°C (100 m)−1. Comparisons of the STMW temperature, thickness, and temperature gradient characteristics in this climatology agree with other observations of the North Atlantic STMW layer.

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Kenneth S. Casey
and
Peter Cornillon

Abstract

The purpose of this study is to present a satellite-derived sea surface temperature (SST) climatology based on Pathfinder Advanced Very High Resolution Radiometer (AVHRR) data and to evaluate it and several other climatologies for their usefulness in the determination of SST trends. The method of evaluation uses two long-term observational collections of in situ SST measurements: the 1994 World Ocean Atlas (WOA94) and the Comprehensive Ocean–Atmosphere Data Set (COADS). Each of the SST climatologies being evaluated is subtracted from each raw SST observation in WOA94 and COADS to produce several separate long-term anomaly datasets. The anomaly dataset with the smallest standard deviation is assumed to identify the climatology best able to represent the spatial and seasonal SST variability and therefore be most capable of reducing the uncertainty in SST trend determinations.

The satellite SST climatology was created at a resolution of 9.28 km using both day and night satellite fields generated with the version 4 AVHRR Pathfinder algorithm and cloud-masking procedures, plus an erosion filter that provides additional cloud masking in the vicinity of cloud edges. Using the statistical comparison method, the performance of this “Pathfinder + erosion” climatology is compared with the performances of the WOA94 1° in situ climatology, the Reynolds satellite and in situ blended 1° analysis, version 2.2 of the blended 1° Global Sea-Ice and Sea Surface Temperature (GISST) climatology, and the in situ 5° Global Ocean Surface Temperature Atlas (GOSTA).

The standard deviation of the anomalies produced using the raw WOA94 in situ observations and the reference SST climatologies indicate that the 9.28-km Pathfinder + erosion climatology is more representative of spatial and seasonal SST variability than the traditional in situ and blended SST climatologies. For the anomalies created from the raw COADS observations, the Pathfinder + erosion climatology is also found to minimize variance more than the other climatologies. In both cases, the 5° GOSTA climatology exhibits the largest anomaly standard deviations.

Regional characteristics of the climatologies are also examined by binning the anomalies by climatological temperature classes and latitudinal bands. Generally, the Pathfinder + erosion climatology yields lower anomaly variances in the mid- and high latitudes and the Southern Hemisphere, but larger variances than the 1° climatologies in the warm, Northern Hemisphere low-latitude regions.

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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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Avijit Gangopadhyay
,
Peter Cornillon
, and
D. Randolph Watts

Abstract

The Parsons–Veronis model, based on a two-layer wind-driven ocean, predicts the latitude at which the western boundary current separates from the western boundary. It has been tested on the Gulf Stream using both satellite and in situ observations. The hypothesis attributes the difference in the thermocline depth from the eastern to the western side of the ocean and the corresponding northward geostrophic transport (with closed northern end) to the southward Ekman transport integrated across the basin. Twelve years (1977–88) of satellite sea surface temperature data and wind data [from the Fleet Numerical Oceanography Center (FNOC) wind database] have been used for this study. The satellite-derived Gulf Stream northern edges were used to determine the latitudes of separation (i.e., crossing the 2000-m isobath into deep water).

Parsons' model is sensitive to two “free” parameters, namely, the reduced gravity and the thermocline depth on the eastern side of the basin. Based on available CTD data and previous current meter studies, these free parameters are selected to establish a representative two-layer model for the midlatitude North Atlantic. When the Ekman drift is integrated over several years, the predicted separation latitude variability agrees with observations with unit slope within 95% confidence limits. The relevant time scale of integration is on the order of 3 years, somewhat less than the estimated time for long baroclinic planetary waves to cross the Atlantic. For this limited dataset, little improvement in the prediction is found for a larger number of years of averaging. More detailed and long-term investigation of this hypothesis should be made in future in context of other western boundary currents.

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Richard Everson
,
Peter Cornillon
,
Lawrence Sirovich
, and
Andrew Webber

Abstract

The empirical orthogonal function decomposition is used to analyze time records of AVHRR sea surfacetemperature observations of the western North Atlantic from 32.9° to 43.6°N, 62.7° to 76.3°W. A manuallydeclouded dataset covering the spring of 1985 is analyzed. The majority (80%) of the variance about the meanis accounted for by an empirical eigenfunction, which is identified with seasonal warming. This eigenfunctionshows that the shelf water, excluding Georges Bank, warms the most rapidly; the surface water of the Gulf ofMaine warms a little less rapidly and the Gulf Stream and Sargasso Sea surface water warm the least rapidly.The SST of the Gulf Stream is also shown to behave more like that at 30°N than like Sargasso Sea waterimmediately to its south (∼35°N). The second EOF is found to be a small correction to the general warmingrate described by the first EOF. The third and fourth EOFs are determined primarily by meander propagation.Observations with partial cloud cover from the period 1985 to 1991 are also analyzed. Again, the dominanteffect is identified as seasonal warming.

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Qingtao Song
,
Tetsu Hara
,
Peter Cornillon
, and
Carl A. Friehe

Abstract

Simulations, made with the fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5), of the response of the marine atmospheric boundary layer (MABL) as air moves over a sharp SST front are compared with observations made during the Frontal Air–Sea Interaction Experiment (FASINEX) in the North Atlantic subtropical convergence zone. The purpose of undertaking these comparisons was to evaluate the performance of MM5 in the vicinity of an SST front and to determine which of the planetary boundary layer (PBL) parameterizations available best represents MABL processes. FASINEX provides an ideal dataset for this work in that it contains detailed measurements for scenarios at the two extremes: wind blowing from warm to cold water normal to a 2°C SST front and the converse, wind blowing from cold to warm water.

For the wind blowing from warm to cold water, there is a pronounced modification of the near-surface wind field over the front, in both model results and aircraft observations. The decrease of near-surface wind speed and stress is due to a stable internal boundary layer (IBL) induced by the SST front, restricting exchange of mass and momentum between the surface and upper part of the MABL. For the cold-to-warm case, the relatively strong vertical mixing through the entire MABL over warm water dampens the response of the near-surface winds and surface stress to the SST front. The properties observed by the aircraft are simulated quite well in both cases, suggesting that MM5 captures the appropriate boundary layer physics at the mesoscale or regional scale.

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