Impact of Indo-Pacific warm pool Hadley circulation on the seasonal forecast performance for summer precipitation over the western North Pacific

Abstract

The western North Pacific summer climate affects the densely populated East Asian countries, while seasonal forecasting over these regions remains challenging for dynamical models. This study evaluates the effect of the spring Indo-Pacific warm pool (IPWP) Hadley circulation on the forecast performance for summer western North Pacific (WNP) precipitation in the Met Office Global Seasonal Forecast System (GloSea5). GloSea5 skillfully predicts IPWP Hadley circulation, but has moderate forecast skill for summer WNP precipitation. Compared to observations, the significant relationship between the spring IPWP Hadley circulation and the summer WNP precipitation is overestimated in most hindcast members and in the ensemble mean. Furthermore, we confirmed that the forecast ensemble members with a stronger such regional circulation–precipitation relationship have better forecast performance for the summer WNP precipitation, suggesting the importance of such a relationship for the seasonal forecast of WNP precipitation. These results also imply the need to further investigate other important factors for the WNP precipitation, the effects of which may be suppressed by the overestimated regional circulation–precipitation relationship in GloSea5.