Toward Understanding the Diverse Impacts of Air-Sea Interactions on MJO Simulations

The role of air-sea interactions on MJO simulations has long been recognized. However, the reasons for the variation of the impacts of air-sea coupling among different models are still elusive. In this study, we used NCEP GFS under different cumulus schemes and SST conditions to explore this issue. We focused on the Oct-MJO and Nov-MJO during the DYNAMO IOP. We show that the effects of SST-feedback on MJO simulations not only vary between these two MJO events, but are also very sensitive to the specifics of cumulus schemes and intraseasonal SST forcing. The Oct-MJO basically reinvigorates over Indian Ocean in association with the arrival of global circumnavigating mode. The Nov-MJO is largely reinvigorated by a robust intraseasonal SST anomaly over Indian Ocean. We found that SST-feedback is crucial for the existence of the Nov-MJO, but has little effect on the Oct-MJO. This finding raises the possibility that the occurrence of some MJO events may be rooted in air-sea interactions. These results suggest that the diverse impacts of air-sea coupling on MJO simulations can be attributed to at least the following three potential causes: (i) diverse responses of ocean to individual MJO events in nature; (ii) diverse behaviors of various cumulus parameterizations; and (iii) diverse intraseasonal SST anomalies in coupled models. Either a too weak internal MJO mode in atmospheric models or a too weak intraseasonal SST anomaly in coupled models would lead to the underestimation of the impacts of air-sea coupling on MJO simulations.