Delayed retreat of the summer monsoon over the Indochina Peninsula linked to surface warming trends

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 center dot 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 center dot 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 center dot 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.

Automated summary

The Indochina Peninsula experiences a tropical monsoon climate with a marked impact on moisture and aerosol transport. The zonal wind is primarily controlled by the meridional land-sea temperature gradient, with a positive trend in gradient in the past 40 years contributing to a delay in the summer monsoon retreat. The delayed monsoon retreat in the Indochina Peninsula is most consistent with changes of the East Asian-Western North Pacific monsoon.