The Effect of Rainfall on the Surface Layer during a Westerly Wind Burst in the Western Equatorial Pacific

Abstract

Measurements of a fresh surface anomaly (fresh lens) produced by rainfall during a westerly wind burst have been analyzed. The measurements were made in December 1992 as part of the Coupled Ocean?Atmosphere Response Experiment in the western equatorial Pacific (2°S, 156°E). Measurements included radar estimates of rainfall, upper-ocean temperature (T), salinity (S), horizontal velocity, and microstructure. In situ observations of the fresh lens were made 5 to 7 hours after its formation. In the 5 hours after formation, the lens deepened to a depth of 40 m as indicated by its salinity anomaly. Salinity and temperature were highly correlated within the lens, consistent with its initial formation by cold rainfall. The T?S relation exhibited curvature, which can be explained by surface cooling and upper-ocean mixing subsequent to formation of the lens. The lens exhibited a horizontal velocity anomaly in the direction of wind, which extended down to a depth of 40 m. The horizontal velocity anomaly is consistent with momentum being trapped near the surface due to rain-induced stratification. Vertical velocity, estimated from the divergence of zonal velocity, showed downwelling at the leading edge of the lens and upwelling at the trailing edge. The magnitude of vertical velocity at a depth of 20 m is 20 m day?1. Richardson numbers within the lens were low (0.25 to 0.5), suggesting that turbulent mixing was governed by critical-Ri instability. Wavenumber spectra of T and S in the upper 20 m exhibit a ?5/3 range, which extends to wavenumbers below the range of local isotropy. Spectral levels were used to estimate turbulent dissipation rates of T and S, which were in turn used to estimate turbulent fluxes of heat and salt. Turbulent fluxes were also estimated from microstructure observations between depths of 10 and 60 m. Fluxes within the fresh lens were nearly uniform from 2 m to 35 m depth, then decreased to near zero at 45 m. The lifetime of fresh lenses during westerly wind bursts appears to be less than one day.