The Surface-Layer Heat Balance in the Equatorial Pacific Ocean. Part II: Interannual Variability

Abstract

The surface-layer heat balance on interannual timescales in the equatorial Pacific has been examined in order to determine the processes responsible for sea surface temperature (SST) variability associated with warm and cold phases of the ENSO cycle (El Niño and La Niña). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and velocity obtained from the Tropical Atmosphere Ocean buoy array at four locations along the equator in the western (165°E), central (170°W), and eastern (140°W and 110°W) Pacific. A blended satellite/in situ SST product and surface heat fluxes based on the Comprehensive Ocean?Atmosphere Data Set are also used. Changes in heat storage, horizontal heat advection, and heat fluxes at the surface are estimated directly from data; vertical fluxes of heat out of the base of the mixed layer are calculated as a residual. Results indicate that all terms in the heat balance contribute to SST change on interannual timescales, depending on location and time period. Zonal advection is important everywhere, although relative to other processes, it is most significant in the central Pacific. The inferred vertical heat flux out of the base of the mixed layer is likewise important everywhere, especially so in the eastern equatorial Pacific where the mean thermocline is shallow. Meridional advection (primarily due to instability waves in this analysis) is a negative feedback term on SST change in the eastern equatorial Pacific, tending to counteract the development of warm and cold anomalies. Likewise, the net surface heat flux generally represents a negative feedback, tending to damp SST anomalies created by ocean dynamical processes. The implications of these results for ENSO modeling and theory are discussed.