Warm Season Drought Development over North America and Northern Eurasia: The Role of Stationary Rossby Waves

Abstract:

Quasi-stationary Rossby waves are known to play a key role in Northern Hemisphere warm season atmospheric circulation and surface meteorology variability on subseasonal time scales. In particular, such waves have been crucial to the development of a number of recent short-term warm season heat waves and droughts over North America (e.g. the 1988 and 2012 droughts) and northern Eurasia (e.g. the 2003 heat wave over Europe). Recently, new insights have been gained based on a case study of a stationary Rossby wave event that developed during 20May-15June 1988. Simulations with the NASA Goddard Earth Observing System model (GEOS-5) atmospheric general circulation model highlight the crucial importance of the mean jet streams in guiding and constraining the path and speed of wave energy propagation. In particular, in the presence of the strong north Pacific jet, convective anomalies that developed over the western Pacific in late May 1988 produce a predilection for persistent upper-level high anomalies to occur over central North America about one to two weeks later, leading to the rapid development of severe dry conditions there. There are indications of continued downstream wave energy propagation that reaches northern Eurasia about two weeks later, leading to the development of dry conditions over eastern Europe and western Russia, and cool and wet conditions over western Europe and central northern Eurasia. The local soil moisture feedback moderately amplifies surface warming anomalies, but exerts an overall small effect on the overlying atmospheric circulation anomalies and associated precipitation deficits. These results suggest that stationary Rossby waves can serve as a source of predictability for subseasonal development of droughts over North America and northern Eurasia. They also imply that a proper representation of stationary Rossby waves and their effects in a model could improve considerably the model skill in forecasting subseasonal drought development.