The warming of tropical central Pacific drying the lower stratosphere

The amount of water vapor in the tropical lower stratosphere (TLS), which has an important effect in determining the radiative energy budget of the climate system, is highly modulated by the temperature variability in the tropical tropopause layer (TTL). The TTL temperature variability is caused by a complex combination of stratospheric quasi-biennial oscillation (QBO), tropospheric convective processes in the tropics and the Brewer-Donson circulation (BDC) driven by the mid-latitude and subtropical atmospheric waves. In the year 2000, the water vapor amount in the TLS exhibited a stepwise transition to a dry phase, which was believed to be caused by a change in the BDC. Here we present observational and modeling evidences that the water vapor “drop” event during the year 2000 was at least partly caused by the concurrent Sea Surface Temperature (SST) warming in the tropical central Pacific. This SST warming cools the TTL temperature above by inducing a tropical wave in the troposphere, which consequently reduces the amount of water vapor entering the stratosphere. The QBO was identified to primarily affect the TLS water vapor on the inter-annual time scale while a ‘classical' El Nino Southern Oscillation (ENSO) event has a weak impact on the tropical-mean TLS water vapor because of its spatially out of phase responses. This study suggests that on inter-decadal time scales, the tropical central Pacific SST is the leading factor in the tropics in controlling the TLS water vapor variability. Thus the ability to accurately simulate the changes in the tropical SST patterns is required for reliable modeling the future change in the TLS water vapor.