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Coupling-to-ocean extends the predictability of tropical intraseasonal oscillation
PAPER WITHDRAWN
Xiouhua (Joshua) Fu, Univ. of Hawaii, Honolulu, HI
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.
Session 1, Coupled ocean-atmosphere interactions and their contribution to climate variability on all time scales (Part I)
Monday, 20 August 2007, 9:00 AM-10:30 AM, Broadway-Weidler-Halsey
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