Impacts of the SSTs over Different Regions on the Troposphere-to-stratosphere Transport over the Tropical Western Pacific in the Boreal Winter

The strong upward motion in the lower tropical tropopause layer (TTL) over the tropical western Pacific (TWP) and the large-scale tropical upwelling of the Brewer-Dobson circulation (BDC) in boreal winter form a critical pathway for tropospheric trace gases entering the stratosphere. An index, called the TSTI, is defined using the combination of the upward motion in the lower TTL and the tropical upwelling in the lower stratosphere to reflect the troposphere-to-stratosphere transport (TST) over the TWP in boreal winter (NDJFM). The TSTIs derived from JRA55 and CMIP6-AMIP historical simulations show significantly increasing trends in the past decades. The impacts of the changes in the SSTs over 9 different regions on the TSTI in NDJFM during 1958–2017 are further investigated with WACCM4 numeric simulations. The results show that the warmed sea surface temperatures (SSTs) over the TWP make a significant contribution to the increasing trends of the upward motion over the TWP and the tropical upwelling in the lower stratosphere, resulting in an enhanced TSTI over the TWP. The changes in SSTs over other regions may singly impact the upward motion in the lower TTL over the TWP or the tropical upwelling in the lower stratosphere, but their overall impacts on the TSTI over the TWP are weak. The WACCM4 simulations further suggest that the increased TST over the TWP could not lead to a wetter lower stratosphere since the enhanced deep convection accompanied by the increased TST over the TWP induces a decreased tropopause temperature and hence a drier lower stratosphere.