Role of Air-Sea Interaction in the 30-60-Day Boreal Summer Intraseasonal Oscillation over the Western North Pacific

This study investigates the role of air-sea interaction in the 30-60-day boreal summer intraseasonal oscillation (BSISO) over the western North Pacific with daily outgoing longwave radiation (OLR), CFSR, and OAFlux datasets for 1985-2009. The BSISO events are identified with the first principal component of 30-60-day bandpass filtered OLR anomalies. Composite analysis of these events reveals that during the northward migration of BSISO, the convection can interact with underlying sea surface temperature (SST). A near-quadrature phase relationship exists between the convection and SST anomalies. An active (a suppressed) convection tends to induce a cold (warm) underlying SST anomaly by reducing (increasing) downward solar radiation but a warm SST anomaly in its northern (southern) portion by reducing near-surface wind and upward latent and sensible heat fluxes, resulting in a 10-day delayed maximized warm SST anomaly ahead of the active convection. In turn, this warm SST anomaly tends to increase upward surface sensible and latent heat fluxes via amplifying sea-air temperature and humidity differences. This oceanic feedback acts to heat, moisten, and destabilize the low-level atmosphere, favoring the trigger of shallow convection, which can further develop into deep convection. The maximum warm SST anomaly lies in the southern (northern) portion of the convectively suppressed (enhanced) area, which weakens the anomalous descending motion in the southern portion of convectively suppressed area and preconditions the boundary layer to promote convection development in the northern portion of convectively enhanced area. Such a spatial and temporal phase relationship between the convection and SST anomalies suggest that air-sea interaction can play a delayed negative feedback role in the BSISO cycle and provide an alternative mechanism responsible for its northward propagation.