Changes in winter stratospheric circulation in CMIP5 scenarios simulated by the climate system model FGOALS-s2

Diagnosis of changes in the winter stratospheric circulation in the Fifth Coupled Model Intercomparison Project (CMIP5) scenarios simulated by the Flexible Global Ocean-Atmosphere-Land System model, second version spectrum (FGOALS-s2), indicates that the model can generally reproduce the present climatology of the stratosphere and can capture the general features of its long-term changes during 1950-2000, including the global stratospheric cooling and the strengthening of the westerly polar jet, though the simulated polar vortex is much cooler, the jet is much stronger, and the projected changes are generally weaker than those revealed by observation data. With the increase in greenhouse gases (GHGs) effect in the historical simulation from 1850 to 2005 (called the HISTORICAL run) and the two future projections for Representative Concentration Pathways (called the RCP4.5 and RCP8.5 scenarios) from 2006 to 2100, the stratospheric response was generally steady, with an increasing stratospheric cooling and a strengthening polar jet extending equatorward. Correspondingly, the leading oscillation mode, defined as the Polar Vortex Oscillation (PVO), exhibited a clear positive trend in each scenario, confirming the steady strengthening of the polar vortex. However, the positive trend of the PVO and the strengthening of the polar jet were not accompanied by decreased planetary-wave dynamical heating, suggesting that the cause of the positive PVO trend and the polar stratospheric cooling trend is probably the radiation cooling effect due to increase in GHGs. Nevertheless, without the long-term linear trend, the temporal variations of the wave dynamic heating, the PVO, and the polar stratospheric temperature are still closely coupled in the interannual and decadal time scales.