Abstract
The Indochina Peninsula (ICP) is dominated by a tropical monsoon climate, and changes of the prevailing winds here have a marked impact on moisture and aerosol transport. September to early October is within the monsoon retreat period that is preceded by a change in the zonal wind from westerly to easterly. Here, we show the long‐term change of the monsoon retreat date and its related climate factors. Climatologically, the zonal wind is primarily controlled by the meridional land–sea temperature gradient through the geostrophic balance. A simple physical model suggests that a 1 K increase in this gradient is associated with a zonal wind speed change of 2.17 m·s −1 , and this estimate is consistent with reanalyses. Over the most recent 40 years there has been a positive trend of 0.107 K·decade −1 in the gradient, and linear regression shows that the average rate of delay of the summer monsoon retreat date over the ICP associated with trend in land–sea contrast is 23.5 day·K −1 . The mixing ratio gradient also makes a contribution to the monsoon retreat but its significance is lower than the land–sea temperature contrast. The ICP is located in a transitional zone bounded by three sub‐regions of Asia‐Pacific monsoon system; the delayed ICP monsoon retreat is most consistent with changes of the East Asian‐Western North Pacific monsoon.