Nonlinear Response of Tropical Lower Stratospheric Temperature and Water Vapor to ENSO: Implications for the Millenium Drop

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 Nino-Southern Oscillation in this region is nonlinear. Both strong La Nina and strong El Nino events lead to enhanced entry water vapor and stratospheric moistening. While moderate El Nino events lead to cooling in this region, strong El Nino events appear to lead to warming, even as the response of the large scale Brewer Dobson Circulation appears to scale linearly with El Nino. The tropospheric warming associated with strong El Nino events extends into the tropical tropopause layer and up to the cold point, where it enables more water vapor entering the stratosphere. These results lead to the following interpretation of the millenium drop in water vapor in 2001: the very strong El Nino event in 1997/1998 followed by more than two consecutive years of La Nina led to enhanced lower stratospheric water vapor. As this period ended in early 2001, entry water vapor concentrations declined. The magnitude of this effect is 0.14ppmv, and thus accounts for approximately 20% of the observed water vapor drop.