Residual Mean Circulation and Temperature Changes during the Evolution of Stratospheric Sudden Warming Revealed in MERRA

Abstract. Residual mean circulation and temperature changes during the evolution of major stratospheric sudden warming (SSW) are investigated by composite analyses of 22 SSW events from 1979 to 2012 during the Northern Hemisphere winter (November–March) using four reanalysis data sets (MERRA, ERA-Interim, NCEP-NCAR, and JRA-55). The SSW events are classified as Type-1 or Type-2 based on the relative amplitude of planetary waves with zonal wavenumbers 1 and 2. The residual mean circulation induced by each forcing term in the Transformed Eulerian mean (TEM) momentum equation and the temperature advection associated with the circulation are calculated for both types of SSW, based on the generalized downward control principle using the MERRA data set. When ‘Lag = 0’ is defined as the day on which the wind reversal occurred at 60° N and 10 hPa, strong poleward and downward motion exists at Lag = −8 and Lag = −1 for Type-1 and at Lag = −3 for Type-2, which is induced primarily by the Eliassen–Palm flux divergence forcing (EPD). The poleward and downward motion is stronger for Type-2 than for Type-1. Gravity wave drag (GWD) produces a smaller contribution to the residual circulation than EPD. During the warming phase (at Lag = −2), strong temperature advection by the EPD induces primarily polar stratospheric warming. On the other hand, during the temperature recovery phase (at Lag = +2), anomalous negative temperature advection and diabatic cooling produce negative temperature tendency anomalies. Structures in the temperature tendency and temperature advection calculated using the MERRA data set are similar to those calculated using the ERA-Interim data set.