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.
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