A preferred pattern of variability in the tropical upper troposphere and lower stratosphere and its relationship to recent trends

Equatorial planetary waves are a fundamental component of the tropical climate system. Recent studies have examined the role of these waves in the climatological zonal-mean momentum balance of the tropics and in the long-term mean upwelling across the tropical tropopause. In this study, the authors show that equatorial planetary waves also strongly modulate interannual and intraseasonal fluctuations in the momentum balance of the tropical upper troposphere and lower stratosphere through a preferred pattern of variability. The observed pattern of variability contains equatorial planetary wave structures that are reminiscent of those derived from idealized modeling experiments: anomalous eddy momentum flux convergence and westerly wind anomalies in the tropical upper troposphere, anomalous eddy-driven circulations in the subtropics of each hemisphere, cold temperature anomalies at the tropical tropopause, and a couplet of anomalous upper tropospheric anticyclones positioned just to the west of a region of enhanced convection. The pattern of variability is strongly related to both the El Niño-Southern Oscillation and the Madden-Julian Oscillation, and because of a sharp increase in amplitude around 2001, it may also be relevant in interpreting recent changes in stratospheric water vapor and the width of the tropics.