Evaluation of the lead-lag relationship between SST and precipitation in a coupled reanalysis using TAO-TRITON data

Abstract:

The Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA) has developed a coupled atmosphere-ocean data assimilation system, MRI-CDA1, based on the coupled atmosphere-ocean general circulation model and separated atmosphere and ocean analysis routines adopted in JMA’s operational weather and climate prediction systems. In this study, we evaluate the sea surface temperature (SST)-precipitation relationship reproduced in a coupled reanalysis experiment using MRI-CDA1 through comparison with TAO-TRITON data and other observation-based datasets. Variations of the lead-lag correlation between SST and precipitation on both the weather and subseasonal time scales are better reproduced in the coupled reanalysis experiment although their amplitudes are larger than those for TAO-TRITON data. These improvements are mainly brought by adjustment of the SST field to the atmospheric forcing through the coupled model physics, and the precipitation field is not changed in a manner improving the SST-precipitation relationship. We found that the negative-to-positive transition of precipitation is delayed from the positive peak of SST due to downward heat propagation in the ocean. Comparison with TAO-TRITON data indicates that the downward propagation of heat signals is too fast in the coupled reanalysis experiment, resulting in smaller lags of transitions of precipitation behind SST peaks. This result suggests that we need to pay attention to near-surface ocean process as well as the contribution of sea surface heat flux in determining SST for further development of coupled data assimilation systems. We expect that enhanced observation in the oceanic surface mixed layer by the tropical Pacific mooring buoy array planned by the Topical Pacific Observing System 2020 (TPOS2020) project will provide valuable information for that purpose.