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The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Weekly to Monthly Variability

S. P. HayesPacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington

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M. J. McPhadenPacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington

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J. M. WallaceDepartment of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

Temporal correlations between near-equatorial surface wind and sea-surface temperatures (SST) at 11°W in the eastern Pacific Ocean are investigated using data from an array of moored sensors between 5°N and 5°S. The signature of tropical instability waves with periods of 20–30 days is apparent in time series of SST and both the meridional and zonal wind components. Results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional SST gradient (equatorial front) normally located just north of the equator. Perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds. Evidence of the air-sea coupling is stronger in time series of the meridional gradients of wind and SST than between time series of the variables themselves. The meridional differencing serves as a high-pass filter in the space domain, which removes planetary-scale wind fluctuations that are unrelated to the local SST perturbations. The wind fluctuations observed in association with tropical instability waves are on the order of 1–2 m s−1.

These results indicate that SST variability on weekly to monthly time scales forces perturbations in the surface wind field. It is suggested that the principal coupling mechanism in this region is the modification of the atmospheric boundary layer stratification. Over the equatorial cold SST tongue the vertical wind shear within the lowest 100 m of the atmosphere is strong and the surface winds are conspicuously weak. As the air flows northward across the equatorial front the boundary layer becomes destabilized, momentum is mixed downward, and the surface winds increase.

Abstract

Temporal correlations between near-equatorial surface wind and sea-surface temperatures (SST) at 11°W in the eastern Pacific Ocean are investigated using data from an array of moored sensors between 5°N and 5°S. The signature of tropical instability waves with periods of 20–30 days is apparent in time series of SST and both the meridional and zonal wind components. Results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional SST gradient (equatorial front) normally located just north of the equator. Perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds. Evidence of the air-sea coupling is stronger in time series of the meridional gradients of wind and SST than between time series of the variables themselves. The meridional differencing serves as a high-pass filter in the space domain, which removes planetary-scale wind fluctuations that are unrelated to the local SST perturbations. The wind fluctuations observed in association with tropical instability waves are on the order of 1–2 m s−1.

These results indicate that SST variability on weekly to monthly time scales forces perturbations in the surface wind field. It is suggested that the principal coupling mechanism in this region is the modification of the atmospheric boundary layer stratification. Over the equatorial cold SST tongue the vertical wind shear within the lowest 100 m of the atmosphere is strong and the surface winds are conspicuously weak. As the air flows northward across the equatorial front the boundary layer becomes destabilized, momentum is mixed downward, and the surface winds increase.

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