Role of High-frequency Ocean-Atmosphere Interaction in Intraseasonal and Interannual Variability of Summer Rainfall over the Indo–Northwest Pacific with CESM2

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 and interannual 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) of CESM2. This ‘‘perfect model’’ approach sidesteps the issue of model biases that complicates the traditional skill metric based on the correlation between AMIP and observations. In comparison to CGCM, daily-AMIP overestimates intraseasonal and interannual 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. On the interannual timescale, despite weak local SST–rainfall correlation, the AMIP–CGCM rainfall correlation exceeds a 95% significance level over most of the Indo–western Pacific warm pool, indicating the importance of remote (e.g., El Niño in the equatorial Pacific) rather than local SST forcing. Indeed, the AMIP successfully reproduces large-scale modes of rainfall variability over the Indo–western Pacific region on the interannual timescale. Compared to the northwest Pacific east of the Philippines, the AMIP–CGCM rainfall correlation is low from the Bay of Bengal through the South China Sea, limited by internal variability of the atmosphere that is damped in CGCM by negative feedback from the ocean. Implications for assessing the ability of AMIP to simulate intraseasonal and interannual variability over the Indo-Northwest Pacific region are discussed.