Impacts Of Sea Surface Tiemperature And Ozone Changes On The Stratospheric Climate Change

Using satellite observations, reanalysis datasets, Chemistry-Climate Models (CCMs) ensemble-mean datasets, combined with a state-of-the-art chemistry-climate model WACCM, the impacts of greenhouse gases (GHGs), sea surface temperature (SST), and stratospheric ozone on stratospheric climate change are thoroughly analyzed. We focused on the reponses of temperature, circulation, wave activity, stratosphere-troposphere exchange (STE), and the tracer gas. The possible mechanisms are also discussed. The main conclusions are summarized as follows:(1) Using a state-of-the-art chemistry-climate model WACCM, the temperature and circulation responses to the uniform and non-uniform increases in SST are analyzed. The results showed that the uniform and non-uniform increases in SST could intensify the subtropical westerly jets and significantly weaken the northern polar vortex. In the model runs, the global uniform SST increases produced a more significant impact on the southern stratosphere than the northern stratosphere, while SST meridional gradient increases produced a more significant impact on the northern stratosphere. The asymmetric responses of the northern and southern polar stratosphere to SST meridional gradient changes were found to be mainly due to different wave properties and propagations in the northern and southern stratosphere. Although SST increases may give rise to stronger waves, the effects of SST increases on the vertical propagation of planetary waves into the stratosphere vary with height and latitude and are sensitive to SST meridional gradient changes.(2) We also investigated the impacts of the stratospheric ozone depletion and the expected ozone recovery on the propagation of waves in December, January and February by using the WACCM. In the southern hemisphere (SH), the stratospheric ozone depletion leads to a cooler Antarctic stratosphere with stronger stratospheric circulation, while the stratospheric ozone recovery has the opposite effects. However, in the Northern Hemisphere (NH), the impacts of the stratospheric ozone depletion on polar stratosphere are not wholly opposite to that of the stratospheric ozone recovery. The stratospheric ozone recovery causes a cooling and strengthening Arctic polar vortex. This cooling is found to be dynamically induced via modulating the wave propagation by stratospheric ozone increases. The analysis of the wave refractive index and Eliassen-Palm flux in the NH indicates that the wave refraction in the stratosphere cannot fully explain wave flux changes in the Arctic stratosphere, and stratospheric ozone changes can cause changes in wave propagation in the northern mid-latitude troposphere and further affect wave fluxes in the NH stratosphere. Particularly interesting is that stratospheric ozone changes have opposite effects on the stationary and transient wave fluxes in the NH. In the SH, the radiative cooling (warming) caused by stratospheric ozone depletion (recovery) produces a larger (smaller) meridional temperature gradient in the mid-latitudes upper troposphere, accompanied by larger (smaller) zonal wind vertical shear and larger (smaller) vertical gradients of buoyancy frequency. Hence, there are more (fewer) transient waves propagating into the stratosphere. The dynamical warming (cooling) caused by stratospheric ozone decreases (increases) partly offsets their radiative cooling (warming).(3) The results of CCMs multi-model mean showed that the trends of Brewer-Dobson circulation (BDC) in the past (1979-2000) and in the future (2000-2050) are both increasing, but the increasing rate in the future is smaller than that in the past. The slowing of the enhancing trends in the tropical upwelling mainly occurs in JJA and DJF. The increasing of the enhancing trends in southern downwelling during the period of 2000-2050 are smaller than that during the period of 1979-2000, which are mainly contributed by the weakened southern downwelling in the future DJF. But the increasing of the enhancing trends in northern downwelling during the period of 2000-2050 are larger than that during the period of 1979-2000, which are mainly caused by the enhancing northern downwelling during the future JJA. The simulation results showed that both uniform and non-uniform SST increases accelerate the large-scale BDC, but the meridional gradient increases of SST between 60°S and 60°N resultes in younger mean age-of-air in the stratosphere and larger increase of tropical upwelling, with a much higher tropopause than that caused by the global uniform 1 K SST increase. The effect of GHG increases on the tropical upwelling were futher examined and the results indicated that GHG changes without corresponding SST changes have no significant effects on the tropical upwelling. Changes in tracer transport properties caused by GHG increases and decreases of the same magnitude are not in the same phase and amplitude. GHG decreases seem to have a more significant impact on the mean age-of-air and cross-tropopause mass flux than GHG increases of the same magnitude. In addition, the stratospheric ozone depletion leads to a stronger BDC, with an increase in tropical upward mass flux, a significant increase in the downward mass flux in the SH extratropical stratosphere. The stratospheric ozone recovery results in a weaker BDC with a decrease in tropical upward mass flux in the stratosphere and a significant decrease in downward mass flux in the NH extratropical stratosphere.(4) Using reanalysis datasets and satellite observations, the trends in zonal-mean tropical tropopause layer (TTL) since 1979s are studied. The results show that the zonal-mean tropical CPT has a lifting and cooling trend during 1979-2000, while a reducing and warming during 2000-2012. The main reason of this phenomenon is due to the different trends of ozone in the tropical low stratosphere during these two periods via changing the trends of temperature in the same regions. In addition, the CCMs multi-model mean results and MERRA reanalysis results show that the tropical CPT is lifting and cooling during the perid of 1979-2000 with a thinning trend, while that during 2000-2050 is still lifting and cooling with a larger thinning trend, but the lifting and cooling trend during 2000-2050 is smaller than that during the period 1979-2000.(5) Using the reanalysis datas and the WACCM4 results, the zonal structure of the TTL trends since the 1980s was investigated. The zonal asymmetry of the trends in TTL including the top of TTL, the bottom of TTL and the thickness of TTL are found:in the tropical central and eastern Pacific (CEP) regions, the TTL’s top is warming and lifting, the TTL’s bottom is also warming but sinking, which resulting in an increase of the TTL thickness; while the tropical regions of beyond the tropical central and eastern Pacific, the CPT is cooling and lifting for both the top and the bottom of TTL, which results in a decrease of the TTL thickness. Sensitive simulations with the WACCM4 proved that these zonal asymmetry of the trends in TTL during 1979-2005 is mainly driven by the SST changes via resulting in an intensified Walker circulation, with more convection activities in the Indian Ocean and western Pacific warm pool regions, which results in a cooler and higher TTL. While the downward branch of Walker circulation over the central and eastern Pacific leads to the increasing of dynamical warming, which results in the warmer TTL. The warming of TTL’s top over the CEP regions leads to more water vapor entering the stratosphere.(6) Using the outputs from 16 CCMs, the trends of lower-to mid-stratospheric water vapor during the period 1980-2005 were studied. Comparisons were made between the CCM results and ERA-Interim. Most of the CCMs showed the trends of lower-to mid-stratospheric water vapor during the period 1980-2005 are positive, with the extent of the trend increasing with altitude, which is consistent with the results in ERA-Interim. The trend of lower-to mid-stratospheric water vapor in the ensemble mean of the CCMs was about 0.003 ppmv/decade, which was about twice as large as that based on ERA-Interim. Using the general circulation model without chemistry processes, we also analyzed the impacts of GHG concentration increases and ozone depletion on stratospheric water vapor. The simulation results showed that the increases of lower-to mid-stratospheric water vapor affected by the combined effects of GHG and ozone changes are mainly caused by the warming of tropopause and the enhancement of BDC, with the former being the greater contributor. GHGs increase led to a higher and warmer tropopause with stronger BDC, which in turn led to more water vapor entering the stratosphere; while ozone depletion led to a higher and cooler tropopause, which caused the decreases of lower-to mid-stratospheric water vapor, despite also causing stronger BDC.