Nonlinear response of tropical lower stratospheric temperature and water vapor to ENSO

Abstract

Abstract. A series of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model are analyzed in order to assess interannual and sub-decadal variability in tropical lower stratospheric temperature and water vapor over the past 35 years. The impact of El Niño-Southern Oscillation in this region is nonlinear. While moderate El Niño events lead to cooling in this region, strong El Niño events appear to lead to warming, even as the response of the large scale Brewer Dobson Circulation appears to scale nearly linearly with El Niño. The tropospheric warming associated with strong El Niño events extends into the tropical tropopause layer and up to the cold point, where it allows for more water vapor to enter the stratosphere. The net effect is that both strong La Niña and strong El Niño events lead to enhanced entry water vapor and stratospheric moistening. These results lead to the following interpretation of the millennial drop in water vapor in 2001: the very strong El Niño event in 1997/1998 followed by more than two consecutive years of La Niña led to enhanced lower stratospheric water vapor. As this period ended in early 2001, entry water vapor concentrations declined. The net effect is that sea surface temperature variability led to a decrease in water vapor of 0.14 ppmv after 2001, which accounts for approximately 23&thinsp.% of the observed drop.