Effects of Low-Frequency Wave Interactions on Intraseasonal Oscillations

Abstract

Intraseasonal oscillations (ISOs) control much of the large-scale variability of convection in the Tropics on time scales of about 15?100 days. These disturbances are often thought to be dominated by eastward-propagating modes, especially during austral summer, but disturbances that propagate westward are also important. This work demonstrates by means of a multiple linear regression model and a brief case study that eastward- and westward-moving intraseasonal modes often cooperatively interact with one another to produce many of the characteristics of the observed Southern Hemisphere summer ISO. These interactions appear to be facilitated by topography and/or by the convective anomalies that are cooperatively induced by the eastward- and the westward-moving components of the oscillations. These interactions do not occur during every period of intraseasonal convective activity, but they do commonly occur during periods of high-amplitude convective anomalies. This analysis shows that eastward- and westward-moving intraseasonal modes should not be generally assumed to be linearly independent entities.