What Is the Role of Air-Sea Interaction in the Intra-Seasonal Fluctuation of the Monsoon Trough over the Western North Pacific?

We investigated the role of air-sea interaction in the intra-seasonal fluctuation of the monsoon trough (MT) over the Western North Pacific (WNP) during boreal summer by comparing a global non-hydrostatic atmospheric numerical model “NICAM” and its ocean coupled version “NICOCO.” We conducted a series of 10-member ensemble experiments from 10 July to 1 September 2020, with NICOCO, NICAM forced by sea surface temperature (SST) that only fluctuates along the climatological seasonal cycle (NICAM-A), and NICAM forced by daily-mean SST simulated in the NICOCO experiments (NICAM-N). All models simulated small eastward extension of the MT, characteristic under a La Niña condition. NICAM-N slightly overestimated MT activities. Compared to NICOCO, NICAM-A significantly underestimated the northward propagation speed of the convective envelope associated with the MT. This difference was attributed to variations in SST fluctuations. Clouds associated with the MT reflect downward shortwave radiation, reducing ocean heating. Additionally, when convective activities intensify over the WNP, cyclonic circulations associated with convections accelerate (decelerate) the background westerly/southwesterly on the southern (northern) side of convections, enhancing (reducing) latent heat release. Consequently, NICOCO simulated dominant negative net downward surface heat flux and resultant negative SST anomaly on the southern side, whereas NICAM-A did not simulate such fluctuations since the effect of the atmosphere to the ocean is not considered. The meridionally asymmetric SST anomaly structure simulated in NICOCO likely supports the northward propagation of convections. These results suggest that air-sea interaction is important in predicting the fluctuation of the MT and convections over the WNP.