Sack Lunch Seminar (SLS)

SLS - Nathan Arnold (Harvard) - Organized Tropical Convection and Atmospheric Superrotation in Warm Climates
Date Time Location
March 13th, 2013 12:10pm-1:00pm 54-915
Superrotation, a circulation state with westerly mean equatorial winds, is observed in the atmospheres of Jupiter, Saturn, Venus and Titan, and has also appeared in numerical simulations of Earth's atmosphere. The phenomenon requires up-gradient transport of angular momentum by large-scale eddy motions, which must in turn be excited by organized convection in the tropics. In the present climate, major sources of equatorial momentum convergence include the stationary wave pattern around the west Pacific warm pool and the transient circulations associated with the Madden-Julian Oscillation (MJO). Simulations of warm climates which generate enhanced MJO-like variability seem particularly prone to superrotation, which has led to hypotheses that Earth's atmosphere could become superrotating in warmer climate regimes.

Here I consider a set of zonally symmetric aquaplanet simulations with a super-parameterized version of the NCAR Community Atmosphere Model, known to simulate a realistic MJO in a modern background climate. These simulations exhibit a tripling of MJO-like variability as equatorial surface temperature is increased from 26C to 35C. Both the magnitude of individual events and the frequency of events increases, with the MJO transitioning from an episodic phenomenon to one with a semi-regular period of 30 days. Eddy circulations associated with the model MJO lead to increased equatorward momentum transport and development of westerly equatorial winds. Moist static energy (MSE) budgets of composite MJO events are used to diagnose the physical processes responsible for the relationship with surface temperature. This analysis points to an increasingly positive contribution from vertical advection, associated in part with a steepening of the mean vertical MSE profile in the lower troposphere.