Modulation of equatorial Pacific sea surface temperature response to westerly wind events by the oceanic background state

Equatorial Pacific Westerly Wind Events (WWEs) impact ENSO evolution through their local and remote oceanic response. This response depends upon the WWE properties (duration, intensity, fetch...) but also on the underlying oceanic state. Oceanic simulations with an identical idealised western Pacific WWE applied every 3months on seasonally and interannually varying oceanic conditions over the 1980-2012 period allow characterizing and understanding the modulation of the WWE response by the oceanic background state. These simulations reveal that the amplitude of the Sea Surface Temperature (SST) response, which can vary by one order of magnitude, is far more sensitive to the oceanic background conditions than the dynamical response to WWEs. The amplitude of the surface-flux driven cooling in the western Pacific is strongly modulated by zonal advection, through interannual variations in the background SST zonal gradient. The amplitude of the warming at the warm pool eastern edge is controlled by horizontal advection, and varies as a function of the zonal SST gradient and distance between the WWE and warm pool eastern edge. The amplitude of the eastern Pacific warming varies as a function of the background thermocline depth and local winds. Overall, only the amplitude of the WWE-driven western Pacific cooling can be clearly related to the phase of ENSO, while the WWE driven SST response in the central and eastern Pacific is more diverse and less easily related to large-scale properties. The implications of these findings for ENSO predictability are discussed