Simulation of long-term trends in Hadley circulation during boreal winter using an ocean data assimilation scheme with the coupled general circulation model FGOALS-s2

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In the Northern Hemisphere, an intensification of Hadley circulation has been observed during boreal winter since the 1950s. However, state-of-the-art climate models have limited skill in capturing the observed strengthening trend in Hadley circulation, as can be shown by an examination of historical climate simulations derived from several different climate models. Improving the ability of climate models to capture trends in Hadley circulation is a challenging issue facing the climate community. In this study, an improvement in model performance through constraining sea surface temperature (SST) biases was explored using an initialization run from the near-term climate prediction system DecPreS based on the coupled general-circulation model FGOALS-s2 developed by the Institute of Atmospheric Physics (IAP). The initialization run uses ocean data assimilation with a new assimilation scheme referred to as the Ensemble Optimal Interpolation–Incremental Analysis Update (EnOI-IAU) and applied to the FGOALS-s2 model. The objective of this study is to assess the performance of FGOALS-s2 in reproducing the observed intensification of long-term Hadley circulation during boreal winter when the modeled ocean temperatures are constrained by observational records. By performing a comparative study of the EnOI-IAU run and a conventional historical run of FGOALS-s2 from the Coupled Model Intercomparison Project Phase 5 (CMIP5), it was found that the EnOI-IAU run performed better than the historical run in simulating the observed long-term trend in Hadley circulation during boreal winter. The EnOI-IAU run accurately reproduced the significant strengthening of Hadley circulation in the range 5°–15°N evident in reanalysis data. In contrast, the historical run predicts a significant weakening of Hadley circulation during boreal winter. The improvement in model performance using the EnOI-IAU run is associated with reduced SST biases when the EnOI-IAU scheme is applied to the FGOALS-s2 model. Specifically, compared with the historical run, warm biases in the North Pacific are largely suppressed and areas of warming in the equatorial eastern Pacific are expanded in the EnOI-IAU run. Both the reduced warm biases and increased areas of warming contributed to an increase in SST meridional gradient over the North Pacific in the assimilation run. The increase in SST gradient then leads to a strengthening of Hadley circulation during boreal winter. Therefore, the application of the EnOI-IAU scheme is demonstrated to be an effective method to improve the capability of the FGOALS-s2 model in reproducing long-term trends in Hadley circulation during boreal winter.