A CASE STUDY OF THE UNCORRELATED RELATIONSHIP BETWEEN TROPICAL TROPOPAUSE TEMPERATURE ANOMALIES AND STRATOSPHERIC WATER VAPOR ANOMALIES

Using the measurements from the Halogen Occultation Experiment(HALOE)and the European Centre for Medium-Range Weather Forecasts(ECMWF)Interim reanalysis data for the period 1994-2005,we analyzed the relationship between tropical tropopause temperature anomalies and stratospheric water vapor anomalies.It is found that tropical tropopause temperature is correlated with stratospheric water vapor,i.e.,an anomalously high(low)tropical tropopause temperature corresponds to anomalously high(low)stratospheric water vapor during the period 1994-2005,except for 1996.The occurrence frequency and strength of deep convective activity during the‘mismatched'months is less and weaker than that during the‘matched'months in 1996.However,the instantaneous intensity of four short periods of deep convective activity,caused by strong surface cyclones and high sea surface temperatures,are greater during the‘mismatched'months than during the‘matched'months.Water vapor is transported from the lower troposphere to the lower stratosphere through a strong tropical upwelling,leading to an increase in stratospheric water vapor.On the other hand,deep convective activity can lift the tropopause and cool its temperature.In short,the key factor responsible for the poor correlation between tropical tropopause temperature and stratospheric water vapor in1996 is the instantaneous strong deep convective activity.In addition,an anomalously strong Brewer-Dobson circulation brings more water vapor into the stratosphere during the‘mismatched'months in 1996,and this exacerbates the poor correlation between tropical tropopause temperature and stratospheric water vapor.

Trends of Lower- to Mid-Stratospheric Water Vapor Simulated in Chemistry-Climate Models

Using the outputs from 16 chemistry-climate models(CCMs), the trends of lower- to mid-stratospheric water vapor(WV) during the period 1980–2005 were studied. Comparisons were made between the CCM results and European Centre for Medium-Range Weather Forecasts(ECMWF) Interim Reanalysis(ERA-Interim).The results of most of the CCMs, and those based on ERA-Interim, showed the trends of lower- to mid-stratospheric WV during the period 1980–2005 to be positive, with the extent of the trend increasing with altitude. The trend of lower- to mid-stratospheric WV in the ensemble mean of the CCMs was 0.03 ppmv per decade,which was about twice as large as that based on ERA-Interim. The authors also used a state-of-the-art general circulation model to evaluate the impacts of greenhouse gas(GHG) concentration increases and ozone depletion on stratospheric WV. The simulation results showed that the increases of lower- to mid-stratospheric WV affected by the combined effects of GHG and ozone changes happened mainly via warming of the tropopause and enhancement of the Brewer-Dobson circulation(BDC), with the former being the greater contributor.GHG increase led to a higher and warmer tropopause with stronger BDC, which in turn led to more WV entering the stratosphere; while ozone depletion led to a higher and cooler tropopause, which caused the decreases of lowerto mid-stratospheric WV, despite also causing stronger BDC.