The Gathering Storm: Clouds Determine Drought or Drowning
Earth’s trade winds meet in a trough of low pressure near the equator in an area that scientists refer to as the Intertropical Convergence Zone (ITCZ). Heat and moisture generated by deep, or convective, cloud systems in the ITCZ drive atmospheric circulation around the globe. Scientists from the University of Maryland, NASA Goddard Space Flight Center, and the U.S. Department of Energy’s Pacific Northwest National Laboratory conducted a detailed modeling study to investigate how interactions between large scale atmospheric circulation, clouds, and Earth’s incoming and outgoing energy modulate the ITCZ and circulation on global scales. Results showed that radiation-cloud-convection-circulation interactions, or RC3I, increased the heating contrasts between wet and dry regions, leading to larger and drier dry regions and narrower and wetter wet regions and that subsequent adjustments by the atmosphere influence general circulation patterns.
RC3I plays fundamental roles in the Earth’s response to changes in the global energy balance, but the mechanisms for the response are not well understood. This modeling study isolated the impacts of RC3I on global circulation patterns and showed that RC3I increases the heating contrasts between wet and dry regions, and that atmospheric adjustments to this change influence general circulation patterns. The results lay a foundation for evaluating how warming temperatures may influence global circulation through RC3I, with implications for projecting future changes in the ITCZ and precipitation worldwide.
Researchers explored how radiation-cloud-convection-circulation interactions (RC3I) affect the ITCZ and circulation at the global scale. The team used a global climate model coupled to embedded cloud resolving sub-models (i.e., super-parameterization), to conduct 10-year simulation experiments with and without cloud-radiation feedback and using observed sea surface temperature. Cloud-radiation feedback induced anomalies of the atmospheric energy balances due to changes in atmospheric heating (shortwave, longwave radiation, and latent heating), adiabatic processes, and heat transport. Several key results surfaced.
First, RC3I leads to warmer and moister tropics, with deeper convection, intensified precipitation in the ITCZ core, and a narrowing of the ITCZ ascent region. These changes were amplified by increased heating of the tropical troposphere due to increased trapping of longwave radiation by enhanced deep clouds and water vapor, as well as increased shortwave absorption by high clouds. Second, models showed that with RC3I, the subtropical dry zone becomes drier and expanded, leading to increased longwave cooling above clouds and increased warming below clouds. Third, researchers found that RC3I leads to an increased tropics-to-pole tropospheric thermal gradient along with a poleward shift of storm tracks, increased poleward and upward heat transport, and enhanced longwave cooling to space in the extratropics. Finally, enhanced convective aggregation in the ITCZ, i.e., clustering of deep convective into smaller areas, coupled to expanding drier and less cloudy areas in marginal convective zones under RC3I led to a new balance between increase latent heating and more efficient cooling by longwave radiation to space.
The RC3I induced changes are consistent with, and provide a deeper understanding of, the roles of cloud radiation feedback in the “Deep Tropical Squeeze” (DTS) reported in numerous observational and climate model projection studies. This phenomenon refers to a sharpening of the ITCZ, with an expansion of the subsiding branch of the Hadley circulation and a poleward shift of storm tracks under greenhouse warming.