The energy budget of a tropical intraseasonal oscillation in a climate model

The moist static energy (MSE) budget is analyzed in a general circulation model that produces a realistic tropical intraseasonal oscillation (ISO). A sufficiently high convective rainfall re-evaporation fraction generates realistic intraseasonal variability in the model. An analysis of MSE anomalies during a model ISO lifecycle appear to support the recharge-discharge paradigm for the ISO, in which column MSE builds before the onset of ISO convection, and then is discharged during and after the convective event.

Horizontal advection dominates the ISO MSE budget in this model. MSE recharge near 850 hPa during the low-level easterly phase of the ISO appears to be caused by suppression of tropical synoptic-scale disturbances, which suppresses advection of dry air from the subtropics into the equatorial region. Eddy kinetic energy is enhanced during the convectively active/westerly phase of the ISO, and is associated with an anomalous drying of the near-equatorial region through meridional advection. Latent heat flux is the second most important term in the intraseasonal moist static energy budget, and appears to slow the discharge of MSE during the convectively active phase.

This study suggests that model intraseasonal variability should be sensitive to the mean humidity gradient from the tropics to subtropics. In particular, a more realistic humidity distribution and stronger meridional humidity gradient is produced by increasing the model convective rain re-evaporation fraction. The increased gradient appears to allow a recharge-discharge cycle regulated by meridional humidity advection to operate.