Global wet and dry contrasts exacerbated by radiation-cloud-convection-circulation interactions during 1980-2014: An observational portrayal

Abstract:

In this study, long-term structural changes in the Inter-Tropical Convergence Zone (ITCZ) and associated radiation-cloud-convection-circulation interactions (RC3I) are examined using multiple sources of reanalysis data for temperature, winds, moisture, and observed precipitation and outgoing longwave radiation (OLR) during 1980-2014. Consistent with CMIP5 climate model projections of the “Deep Tropical Squeeze” under greenhouse warming, this period witnessed a warming and wetting (increased specific humidity) global trend, characterized by a narrowing of the ITCZ core with increased precipitation, coupled to widespread tropospheric drying (deficient relative humidity), increased OLR in the subtropics and mid-latitudes, in association of a widening of the descending branches of the Hadley Circulation, and a poleward shift of the jetstreams in both hemispheres. The widespread tropospheric drying stems from a) faster rate of increased saturated water vapor with warming, relative to the increase in ambient moisture due to convective and large-scale transport, and b) enhanced anomalous subsidence, and low-level moisture divergence in the subtropics and midlatitudes.

The long-term trend in enhanced precipitation (latent heating) in the ITCZ core region is strongly coupled to increasing OLR (radiative cooling to space) in the expanding dry zones, particularly over land regions in the subtropics and midlatitudes, arguably as a necessary condition for global thermodynamic energy balance, consistent with the “furnace-radiator fin” paradigm (Pierrehumbert, 1995). Analyses of the trend patterns in vertical profiles of p-velocity, temperature, and relative humidity with respect to ITCZ precipitation and OLR reveal that the contrast between the wet and dry regions in the troposphere has been increasing globally, with the ITCZ core getting wetter and contracting, while the marginal convective and dry zones, getting drier and expanding.