Predictability of Tropical Precipitation and Gravity Waves, and Relation to Circulation in the Upper Troposphere and Lower Stratosphere

Atmospheric gravity waves generated by tropical convection influence large-scale winds in the upper troposphere and stratosphere.  Winds at these levels guide Rossby wave propagation and teleconnection patterns that strongly influence the simulation of regional-scale climate and skill of long-range weather forecasts.  Most climate and weather forecasting centers have raised their model lids in recognition of the importance of these upper level winds and the importance of simulating the processes that control them.  At seasonal forecast model resolutions, small-scale waves remain severely under-resolved, yet the influence of their drag forces on the circulation in the upper troposphere and stratosphere make them key players in predictability.   Gravity wave drag parameterizations are used to tune both climate and forecast models with demonstrated effects on bias reduction and forecast skill. The tropical lower stratosphere quasibiennial oscillation, in particular, has demonstrated influence on seasonal predictability, and this circulation is forced in large part by non-orographic gravity waves emanating from tropical convection.

The El Nino Southern Oscillation (ENSO) and Madden-Julian Oscillation (MJO) are two major modes of tropical precipitation variability, and the small-scale rain events within these oscillations are major sources of tropical gravity waves.  We present results of idealized models with realistic gravity waves generated by observed precipitation variability to examine regional changes in gravity wave generation and gravity wave drag and relationships to ENSO and MJO precipitation and circulation patterns.  We find that ENSO changes not only the regional pattern of tropical gravity wave sources, but also their propagation properties and their effects on the stratospheric circulation.  We also investigate the mechanism behind a previously identified relationship between stratospheric gravity waves and MJO precipitation and circulation patterns.  Specifically, stratospheric gravity wave activity is enhanced where tropopause winds are stronger easterly.  We find that the observed relationship is further associated with shifts in the stratospheric circulation, and we investigate potential causes of these regional relationships among precipitation, wave activity, and circulation.