The dynamics of quasi-stationary atmospheric rivers and their implications for monsoon onset

Abstract

A global Hovmöller diagram of column water vapor (CWV) at 308N from daily ERA-Interim data shows seasonally migrating North Pacific/Atlantic quasi-stationary atmospheric rivers (QSARs) located in the eastern Pacific/Atlantic in winter and propagate to the western Pacific/Atlantic in summer. Simplified general circulation model (GCM) experiments produce QSAR-like features if the boundary conditions include 1) the sea surface temperature contrast from the tropical warm pool-cold tongue and 2) topographic contrast similar to the Tibetan Plateau. SimulatedQSARs form downstream of topographic contrast during winter and coincide with it in summer. Two models of baroclinic instability demonstrate that QSARs coincide with the location where the most unstable mode phase speed equals that of the upper-level zonal winds. A consistent interpretation is that the waves become quasi stationary at this location and break. The location of quasi stationarity migrates from the eastern Pacific/Atlantic in the winter, when upper-level winds are strong and extended over the basin, to the western Pacific/Atlantic when winds are weak and contracted. Low-level wind convergence and moist static energy coincide withQSARs, and since the former two are essential ingredients tomonsoon formation, this implies an important role for QSARs in monsoon onset. This connection opens a new window into the dynamics of subtropical monsoon extensions.