Recent Wintertime Cooling in the Upper Troposphere and Lower Stratosphere of the Arctic

Abstract As Arctic surface temperatures exhibit amplified warming, a warming center in the upper troposphere and lower stratosphere (UTLS) is also notable, which potentially plays a crucial role in polar‐to‐mid‐latitude interactions. While the surface warming has accelerated recently, UTLS temperatures, however, have declined since 2001, as demonstrated by multiple reanalysis data sets. This shift in trends provides a unique opportunity to understand the factors influencing Arctic UTLS temperature changes. Further analysis indicates that the descending component of the Brewer‐Dobson (BD) circulation over the Arctic was strengthened before 2000 but weakened thereafter. Simultaneously, Arctic lower stratospheric water vapor showed a decreasing trend before 2000, followed by an increasing trend. The weakening of the BD circulation led to reduced dynamical heating, and the increased water vapor resulted in enhanced radiative cooling, which together contributed to the cooling of the Arctic UTLS after 2000.