Seasonal Sensitivity of the Cross-Equatorial Hadley Cell Response to Extratropical Thermal Forcings

Abstract:

This study explores the seasonal sensitivity of tropical circulation responses to an idealized extratropical thermal forcing using Community Atmosphere Model version 5 coupled to a slab ocean. The thermal heating over the Southern Ocean is held constant, and the tropical responses in each month of the year are investigated. An anomalous cross-equatorial cell and a southward tropical rain belt shift occur every month. The anomalous cross-equatorial cell has a stronger influence on the strengths of the Hadley Cell and the subtropical jet in the winter hemisphere, whereas it has nearly no influence on the Hadley Cell and the subtropical jet strengths in the summer hemisphere.

The seasonal standard deviation of the strength of the anomalous cross-equatorial cell is 22% of the annual mean anomaly. When the climatological Hadley Cell is more symmetric with respect to the equator, in May and November, a weak anomalous cross-equatorial cell introduces opposite surface wind speed changes in the two hemispheres. The resulting hemispheric asymmetry of anomalous evaporation reduces the anomalous northward cross-equatorial atmospheric energy transport in these months and has an influence on sea surface temperature two months later. The anomalous inter-hemispheric temperature gradient and cross-equatorial cell are therefore larger in July and January.