Dynamic Role of a Westerly Wind Burst in Triggering an Equatorial Pacific Warm Event

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Abstract
Dynamical impacts of a strong westerly wind burst (WWB) are studied using an ocean–atmosphere coupled simulation in which an intense westerly wind burst is introduced. The ocean response includes local and remote components with the development of a zonal surface jet and the appearance of positive sea surface temperature anomalies in the eastern Pacific. As observed during the triggering of the 1997/98 El Niño, remote warming results from the eastward propagation of a subsurface temperature anomaly generated in the western Pacific. A detailed heat budget helps to describe and understand the major mechanisms taking part to the formation of the subsurface temperature anomaly in the western Pacific, contributing to its eastward propagation and leading to the surface warming simulated in the eastern Pacific. Advective contributions are proved to be widely predominant in the heat budget. Further decomposition of advective terms into linear and nonlinear parts allows insights into the role of mean state and perturbation dynamics in the formation and evolution of the subsurface temperature anomaly. Vertical advection is shown to have a major role from the formation of the subsurface temperature anomaly in the western Pacific to its rise in the eastern Pacific: (i) linear vertical advection of the mean temperature by the Kelvin wave perturbation is the main process explaining the formation of a temperature anomaly in the western Pacific and its eastward propagation; (ii) nonlinear vertical advection acts to modify the vertical structure of the subsurface temperature anomaly; (iii) linear vertical upward advection of the temperature anomaly by the mean equatorial upwelling is shown to be the major process leading to the development of simulated surface warming in the eastern Pacific. Zonal advection is a second-order contribution but takes part in the propagation and evolution of the thermal pattern through linear and nonlinear processes. Meridional advection is a third-order contribution, only significant during the onset phase through a nonlinear contribution due to a WWB-induced Gill-type oceanic recirculation characterized by surface convergence and subsurface divergence. It is suggested that the previously mentioned mechanisms could be implied in the evolution of “subsurface El Niño events” in contrast with “surface El Niño events” on which numerous conceptual models, such as the delayed oscillator or the advective–reflective model, as well as theoretical unstable coupled modes have mainly focused.