Equatorial planetary waves and their signature in atmospheric variability

Equatorial planetary waves are a fundamental component of the tropical climate system. Previous studies have examined their climatological-mean structure, their role in the climatological-mean momentum balance of the tropics, and their contribution to the climatological-mean upwelling across the tropical tropopause. In this study, the authors focus on the contribution of equatorial planetary waves to variability about the climatological mean.

The leading pattern of wavelike variability in the tropical atmosphere is identified via empirical orthogonal function analysis of the eddy kinetic energy field. The dominant pattern in the tropical eddy kinetic energy field bears strong resemblance to the long-term mean structure of equatorial planetary waves. Thus, equatorial planetary waves not only dominate the climatological-mean tropical general circulation but also play a prominent role in its intraseasonal and interannual variability. Regressions on an index of the equatorial planetary waves reveal that they are associated with a distinct pattern of equatorially symmetric climate variability. The pattern is characterized by variations in: 1) the eddy momentum flux convergence and zonal-mean zonal wind in the tropical upper troposphere, 2) the mean meridional circulation in the tropics and subtropics of both hemispheres, 3) temperatures in the tropical upper troposphere and lower stratosphere, and 4) the amplitude of the upper tropospheric anticyclones that straddle the Equator over the western tropical Pacific Ocean.

Variability in the equatorial planetary waves is linked to both the El Niño-Southern Oscillation and the Madden-Julian Oscillation, but the time series of the equatorial planetary waves also exhibits variability that is independent of these two phenomena. Consequently, equatorial planetary waves may reflect a preferred pattern of variability in the tropical atmospheric circulation that emerges in response to any forcing that projects onto the climatological distribution of tropical convection. It is argued that equatorial planetary waves may play a key role in a range of time-varying tropical phenomena and may contribute to both interannual variability and trends in troposphere/stratosphere exchange and the width of the tropical belt.