Stratospheric Circulation and its Changes in FGOALS-s2

Abstract

The present study focuses on the performance of FGOALS-s2 and its atmospheric component SAMIL in simulating winter stratospheric circulation and its variability. The results demonstrate that the winter climatology can be reproduced well, although a “cold pole” bias remains; the dominant and recurrent stratospheric oscillation mode has been captured effectively in a perpetual January run of SAMIL, including the oscillation timescale and the systematic meridional and vertical propagation of circulation anomalies accompanying the occurrence of the oscillation events. Diagnosis of changes in the winter stratospheric circulation in CMIP5 scenarios indicates that FGOALS-s2 can capture the general features of the long-term changes that occurred during 1950–2000, including the global stratospheric cooling and the strengthening of the westerly polar jet, although the simulated polar vortex (jet) is much cooler (stronger) and the projected changes are generally weaker than those in the observation. To the increase in Green House Gases (GHGs) effect in HISTORICAL, RCP4.5 and RCP8.5 scenarios, the stratospheric response is averagely steady, exhibiting an increasing stratospheric cooling and a strengthening and equatorward extending polar jet. Correspondingly, the timeseries of the leading oscillation mode exhibits a clear positive trend in each scenario. However, the positive trend of the leading Polar Vortex Oscillation (PVO) and the strengthening of the polar jet are not accompanied by decreases in planetary wave activities, implying that the positive trend of PVO and the cooling trend of the polar stratosphere were likely caused by the radiation cooling effect induced by increases in GHGs.