Role of ocean-atmosphere interaction in intraseasonal variability of summer rainfall over the Indo-Northwest Pacific

The observed negative response of sea surface temperature (SST) to local summer rainfall peaks, with a lag of 5-10 days over the Indo-Northwest Pacific region, questions the accuracy of atmospheric models in simulating intraseasonal variability. An Atmospheric Model Intercomparison Project simulation forced by daily-varying SSTs (daily-AMIP) is used to evaluate the above question through comparison with its parallel coupled general circulation model (CGCM). In comparison to CGCM, daily-AMIP overestimates intraseasonal rainfall variability, especially over the South China Sea and the Northwest Pacific, consistent with differences in the mean state featuring a cyclonic low-level circulation pattern. Nevertheless, both CGCM and daily-AMIP demonstrate similar northward propagation of rainfall anomalies on the intraseasonal timescale, but daily-AMIP presents a larger amplitude, attributed to the absence of high-frequency ocean-atmosphere coupling and a stronger conversion of kinetic energy due to enhanced monsoon-trade winds confluence. The feedback of the ocean to the atmosphere is realized through surface heat flux exchange, and the near-quadrature relationship between intraseasonal SST and rainfall is considered evidence for ocean-atmosphere interaction. Implications for assessing the ability of AMIP to simulate intraseasonal variability over the Indo-Northwest Pacific region are discussed.