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Large-Scale SST Variability in the Western North Atlantic Subtropical Convergence Zone during FASINEX. Part I: Description of SST and Wind Stress Fields

George R. Halliwell Jr.Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island

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Peter CornillonGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island

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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.

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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