Current and Thermal Variations to Westerly Wind Bursts in the Equatorial Pacific Ocean

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el niño, equatorial undercurrent, kelvin wave, la niña, rossby wave, south equatorial current, westerly wind burst

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The observed upper ocean zonal current, 20°C isotherm depth, and zonal wind stress, calculated from the European Centre for Medium Range Weather Forecasts (ECMWF) model output, were examined in the equatorial Pacific during the period of frequent Westerly Wind Bursts (WWBs) in the period October-April from 1988 (La Niña year) to 1992 (El Niño year). WWBs increased in strength and number, shifting eastwardly towards El Niño, and causing the frequent reversal of surface current, and its vertical gradient within the western basin. Similar features were also observed in the west-central basin, but only during the El Niño event. The reversal of surface zonal current in the central and eastern basins was generally accompanied by surfacing of the Equatorial Undercurrent (EUC), though the vertical gradient of zonal current velocity in the upper ocean never reversed. Fluctuations in upper ocean zonal currents in response to the WWBs were largest in the western basin, while fluctuations in 20°C isotherm depths were largest in the central and eastern basins. Discernible peaks in ocean responses due to WWBs were modulated in their propagation away from the forced region. Since WWBs occurred in succession at various locations, ocean responses to each individual WWB were interfered with by other WWB events. A simple analytical model was therefore devised to describe the temporal and spatial evolution of the interference patterns in relation to various locations and WWB events. Pattern evolutions for both upper ocean zonal current and thermal structures were further modulated by easterly wind fluctuations east of the dateline. Tracing individual wave modes using peak-to-peak comparisons is therefore a subjective process.

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Terrestrial, Atmospheric and Oceanic Sciences, v. 15, issue 2, p. 151-177