Sea Surface Temperature Feedback Extends the Predictability of Tropical Intraseasonal Oscillation

Abstract

Abstract The possible impacts of different sea surface temperature (SST) configurations on the predictability of the boreal summer tropical intraseasonal oscillation (TISO) are assessed with a series of ensemble forecasts. The five different lower boundary conditions examined in this study are, respectively, (i) the fully interactive ocean–atmosphere coupling, (ii) “smoothed” SST, which excludes the intraseasonal signal from sea surface forcing, (iii) damped persistent SST, (iv) coupling to a slab mixed-layer ocean, and (v) daily SST from the coupled forecast. The full atmosphere–ocean coupling generates an interactive SST that results in the highest TISO predictability of about 30 days over Southeast Asia. The atmosphere-only model is capable of reaching this predictability if the ensemble mean daily SST forecast by the coupled model is used as the lower boundary condition, which suggests that, in principle, the so-called tier-one and tier-two systems have the same predictability for the boreal summer TISO. The atmosphere-only model driven by either smoothed or damped persistent SSTs, however, has the lowest predictability (∼20 days). The atmospheric model coupled to a slab mixed-layer ocean achieves a predictability of 25 days. The positive SST anomalies in the northern Indo–western Pacific Oceans trigger convective disturbances by moistening and warming up the atmospheric boundary layer. The seasonal mean easterly shear intensifies the anomalous convection by enhancing the surface convergence. An overturning meridional circulation driven by the off-equatorial anomalous convection suppresses the near-equatorial convection and enhances the northward flows, which further intensify the off-equatorial surface convergence and the TISO-related convection. Thus, the boreal summer mean easterly shear and the overturning meridional circulation in the northern Indo–western Pacific sector act as “amplifiers” for the SST feedback to the convection of the TISO.