Contributions of Upper Ocean Variability over the Western North Pacific to ENSO

ENSO variability is linked to wind stress variations and oceanic heat content in the tropical Pacific. We examined the changes in the relationship of upper ocean stratification-ENSO, and recent changes in warm pool SST and its linkage with ocean stratification. The western Pacific warm pool is conventionally defined as an area where SST is higher than 28°C. Increased SST enhances the vertical stratification at the upper level. The warm pool SST variability is highly correlated with the change in maximum buoyancy frequency which is captured by the wind stress variations projecting onto the first baroclinic mode over the warm pool region. The variability of warm pool SST is related to wind stress forcing over the western Pacific. Ocean dynamics in the warm-pool act on the eastward expansion of warm SST through equatorial wave-induced zonal advection, and it also plays a key role to maintain the warm-pool SST through the poleward Ekman transport at the surface layer. Anomalous easterly (westerly) trade winds force the thermocline depth. The trade winds also induce the Ekman upwelling associated the Coriolis effects, which brings the cold water to surface layer in the eastern Pacific. Warmer SST may change surface winds, which in turn change ocean currents connecting the western tropical Pacific Ocean with the eastern tropical Pacific Ocean.

Acknowledgement: This study was funded by the Korea Ministry of Environment (MOE) as ‘‘Climate Change Correspondence Program’’.