The surface-layer heat balance in the equatorial Pacific Ocean. Part II: Interannual variability

The surface-layer heat balance on interannual timescales in the equatorial Pacific has been examined in order to determine the processes responsible f or sea surface temperature (SST) variability associated with warm and cold phases of the ENSO cycle (El Nino and La Nina). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and veloci ty obtained from the Tropical Atmosphere Ocean buoy allay at four locations along the equator in the western (165 degreesE), central (170 degreesW), a nd eastern (140 degreesW and 110 degreesW) Pacific. A blended satellite/in situ SST product and surface heat fluxes based on the Comprehensive Ocean-A tmosphere Data Set are also used. Changes in heat storage, horizontal heal 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 hear, balance contribute to SST chan ge on interannual timescales, depending on location and rime period. Zonal advection is important everywhere, although relative to other processes, it is most significant in the central Pacific. The inferred vertical heat flu x out of the base of the mixed layer is likewise important everywhere, espe cially so in the eastern equatorial Pacific where the mean thermocline is s hallow. Meridional advection (primarily due to instability waves in this an alysis) 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 feedba ck, tending to damp SST anomalies created by ocean dynamical processes. The implications of these results for ENSO modeling and theory are discussed.