Quantification of isentropic water-vapour transport into the lower stratosphere

Abstract

The contour advection technique is used to quantify the quasi-horizontal isentropic transport of water vapour across the tropopause between the tropical upper troposphere and the extratropical lower stratosphere. The calculations are based on European Centre for Medium-Range Weather Forecasts analyses for 1997 and 1998. It is found that the isentropic transport of water vapour into the extratropical lower stratosphere has a similar magnitude to the upward (cross-isentropic) transport of water vapour into the stratosphere in the tropics. The isentropic flux is largest during summer, and is more than enough to account for the observed summertime increase in specific humidity in the lowermost stratosphere. The annual isentropic moisture transport into the extratropical lower stratosphere in the northern hemisphere is about one order of magnitude larger than that in the southern hemisphere. This difference in transport explains why the extratropical lower stratosphere is moister in the northern hemisphere during northern summer than it is in the southern hemisphere during southern summer. The results further show that there was more moisture transport into the lower stratosphere of both hemispheres during 1998, under El Nino conditions, than during 1997.