FY 2003

Barrier layer formation during westerly wind bursts

Cronin, M.F., and M.J. McPhaden

J. Geophys. Res., 107(C12), 8020, doi: 10.1029/2001JC001171 (2002)

Barrier layers between the base of a shallow halocline and the top of the thermocline are a common feature of the western Pacific warm pool. In this paper, we investigate barrier layer formation and erosion processes associated with two westerly wind bursts (WWB). WWBs are typically associated with increased rainfall, but increased wind stirring and convective mixing from surface cooling can cause the fresh water to mix down to the top of the thermocline and thereby erode preexisting or newly formed barrier layers. However, not all WWBs lead to barrier layer erosion. In this study, we show that a WWB in November 1989 was associated with the formation of an extremely (~100 m) thick barrier layer. During this event, zonal and meridional advection of surface fresh water from the west and north were the dominant processes responsible for the thick barrier layer, while rainfall was a secondary process. Likewise, during the Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment, a relatively thick barrier layer formed near 0°, 160°E and 0°, 165°E following the October 1992 WWB. Zonal convergence associated with this WWB caused a zonal salinity gradient to intensify. The surface-intensified wind-driven jet then tilted the zonal salinity gradient into the vertical, thus generating a shallow halocline above the top of the thermocline. In both events, feedbacks appear to have occurred between formation of stratification and formation of vertical shear. Shear formed through a depth-dependent pressure gradient associated with the salinity gradient and through trapping of wind-forced momentum above the developing stratification. The increased sheared flow then led to further surface-intensified fresh water advection and stratification. These examples illustrate how, in the presence of zonal and meridional salinity gradients, the equatorial ocean