Coupling-to-ocean extends the predictability of tropical intraseasonal oscillation

The possible impacts of different sea surface temperature (SST) configurations on the predictability of boreal-summer Tropical Intra-Seasonal 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 that excludes 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 fully interactive ocean-atmosphere coupling generates the best SST boundary condition, which 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 daily SST from the coupled forecast 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). Coupling the atmospheric model to a slab mixed-layer ocean achieves a predictability of 25 days.

The boreal-summer mean easterly shear and the overturning Hadley circulation in the northern Indo-western Pacific sector act as “amplifiers” for the SST feedback to the convection of the TISO. The positive SST anomalies in the northern Indo-western Pacific Oceans initiate 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. The overturning Hadley 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.