SST Bias over the Northwest Pacific in Oceanic Data Assimilation Experiments with the Interdecadal Climate Prediction System IAP DecPreS and Its Impacts on the Asian Summer Monsoon Simulation

Based on the oceanic data assimilation experiments (the merge of the Ensemble Optimal Interpolation and Incremental Analysis Update, hereafter EnOI-IAU) using the near-term climate prediction system of the Institute of Atmospheric Physics (IAP DecPreS), the authors first evaluate the model skill for annual cycle simulation of sea surface temperature (SST) over the Northwest Pacific. By comparing with the historical simulation without oceanic data assimilation, the authors investigate the impacts of differences in simulated summer SST over Northwest Pacific on the Asian summer monsoon simulation in the framework with ocean-atmosphere coupling. The results show that the spatial distribution of SST over the Northwest Pacific is generally captured by the EnOI-IAU experiment and the cold SST biases throughout the year in the historical simulation are significantly reduced. According to the mixed-layer heat budget analysis, oceanic processes including the assimilation process are primarily responsible for the improvement in the simulation of SST. During the summer, low-level westerlies associated with the Indian summer monsoon in the EnOI-IAU experiment are stronger and shifted eastward compared with observations. Meanwhile, excessive precipitation is found over the western equatorial Pacific while dry biases are seen over the Indian Ocean region. The moisture budget analysis demonstrates that the anomalous moisture convergence due to changes in circulation plays a crucial role in the simulated rainfall changes over the Asian monsoon region. Compared with the historical simulation, warmed summer SST over the Northwest Pacific in the EnOI-IAU experiment enhances local convection and induces anomalous local ascending motions, which are favorable for the development of anomalous low-level westerly flow and off-equatorial cyclonic circulation anomalies. The low-level westerly anomalies induce low-level divergence anomalies over the Indian Ocean and partly reduce the corresponding wet biases in the historical simulation. The cyclonic circulation anomalies over the Northwest Pacific, on the one hand, enhance low-level westerlies associated with the Indian summer monsoon, which transport more water vapor form the Arabian Sea to the western Pacific warm pool and increase precipitation over this region. On the other hand, the enhanced low-level westerlies reduce the overly strong low-level southerly winds over southern China in historical simulation, and subsequently improve the simulation of low-level southwesterly winds over East Asia.