Trends in the temperature and water vapor content of the tropical lower stratosphere: Sea surface connection

The tropical lower stratosphere is an important region of the atmosphere, where strong convective activity in the underlying troposphere affects both its chemical and dynamical properties. Temperatures near the tropopause influence the input of water vapor from the troposphere and act as an indicator of the dynamical properties of the region. This paper addresses long-term trends in the temperature of the tropical lower stratosphere. Correlations with recent changes in tropical stratospheric water vapor are also noted. Special attention is given to the convectively active tropical western Pacific Ocean, where sea surface temperatures (SSTs) are among the highest in the world. The region contains several island radiosonde stations with records reliably extending over several decades. Results show only weak cooling trends occurred before the 1990s, but a strong and rapid cooling of 4 degrees to 6 degrees C took place in the mid-1990s and has persisted since that time. The properties of the temperature records during and following this cooling event are discussed, and a significant anticorrelation with SST anomalies in the underlying ocean is noted. The rate of ocean warming increased in the early 1990s, coinciding approximately with the mid-decade cooling event, while individual monthly anomalies in both time series are also anticorrelated. Past work has shown that cooling of the tropical lower stratosphere is a dynamical result of tropospheric convection, which in turn partially depends upon sea surface temperatures. Convection may therefore be the link between the ocean and the stratosphere, and the increased cooling may be an indication of strengthening tropical convection.