Aquaplanet GCM simulations of tropical intraseasonal variability

Aquaplanet experiments are conducted with an atmospheric general circulation model to determine how the tropical basic state influences the simulation of tropical intraseasonal oscillations. Perpetual equinoctial insolation and other forcings are used. Three 15-year simulations are conducted: one with tropical SST distribution resembling that in March, a distribution with reduced meridional SST gradient, and a zonally-symmetric distribution. The zonally-symmetric distribution produces very little intraseasonal variability at 30-90 day timescales, suggesting that an easterly low-level basic state on the equator does not produce realistic tropical intraseasonal variability. The strongest and most realistic eastward-propagating wind and precipitation variability, with a narrow spectral peak centered near 50 days and slow eastward propagation at 5 m s^-1, is produced in the experiment with realistic zonal asymmetries, but reduced meridional SST gradient.

The reduced meridional SST gradient simulation is analyzed during the rest of the talk. Precipitation exhibits a very tight coupling with column saturation fraction in this model, one of the criteria for existence of moisture modes that occur in moist atmospheres of weak horizontal temperature gradients. Moist static energy and moisture budget analysis of the model intraseasonal oscillation indicates that horizontal advection and surface latent heat flux anomalies appear most important for determining the character of intraseasonal variability in the model. Wind-evaporation feedback appears to destabilize the intraseasonal moisture mode, and zonal moisture advection by the basic state flow appears to propagate it slowly eastward. Intraseasonal variability collapses in a mechanism denial experiment in which wind-evaporation feedback is withheld. However, this No-WISHE experiment still includes high space and time frequency convective structures that propagate slowly eastward at 5 m s^-1, supporting the role of horizontal advection in eastward propagation. The broader implication of these experiments for the dynamics of tropical intraseasonal variability is discussed, including how gross moist stability determines the nature of intraseasonal variability in the model.