Zonally asymmetric circulations and Earth's dry regions

Monday, 15 December 2014
Xavier J Levine and William R Boos, Yale University, New Haven, CT, United States
Precipitation in Earth's subtropics is characterized by large zonal asymmetries, especially during summer. A number of mechanisms have been proposed to relate precipitation contrast between deserts and monsoon regions, for instance the planetary Rossby wave mechanism of Rodwell and Hoskins (1996) or the ventilation mechanism of Chou et al. (2001). Yet, little has been done to quantify seasonal or centennial changes in the precipitation contrast between deserts and monsoon regions. Moreover, the relevance of stationary circulations to seasonal or centennial changes in the hydrology over subtropical dry zones remains unclear, for instance when compared to the influence of storm tracks or Hadley cells.

Using reanalysis data for past decades, we quantify the effect of stationary circulations on precipitation and evaporation over dry regions throughout seasonal cycle. We then quantify the sensitivity of dry zones to changes in the strength and extent of stationary (zonally asymmetric) circulations with global warming, using a hierarchy of GCMs. We relate changes in precipitation over monsoon regions to that over dry regions using a simple mechanism for stationary circulation changes we devised and successfully tested in idealized GCM simulations over a wide range of climates. This mechanism is based on fundamental dynamical principles of the tropical atmosphere. It predicts stationary circulations to strengthen with a deepening of the tropical troposphere or with enhanced near-surface thermal zonal asymmetries. We test this mechanism in reanalysis data of present-day climate and in comprehensive GCM simulations of global warming scenarios. Seasonal or centennial changes in precipitation contrast between deserts and monsoon regions are then related to fundamental thermodynamic properties of the surface and atmosphere.