Convection, Waves, and Double ITCZs—An Angular Momentum Perspective

Abstract:

Here I propose that enhanced equatorial wave activities lead to the double ITCZ rainfall pattern by modulating the atmospheric angular momentum budget. Convectively coupled equatorial waves can transport angular momentum to the equator, and this momentum convergence accelerates the zonal flow. As a result, anomalous equatorward flows in the upper troposphere are induced to balance this acceleration. The air mass converges and descends at the equator. Deep convection is therefore suppressed. Near the surface, the air mass moves away from the equator, and then converges and ascends at higher latitudes. If the convectively coupled waves are strong enough, this anomalous circulation can give rise to double ITCZs.

In this presentation, I will first focus on a case study during March 2015. During this period, the strongest MJO event on record occurred, and equatorial wave activities were enhanced. The double ITCZs also emerged during the same period over the western and central Pacific. I calculate the momentum budget in the tropical atmosphere by decomposing the zonal wind into the zonal mean, stationary eddies, and transient eddies. Our results suggest that transient eddies play an important role in forming double ITCZs. Long-term statistics by using the reanalysis data and results from GCM simulations will also be presented.