Wodzicki, Kyle R (2021-03). Changes in Tropical Convection and Cloud Populations With ITCZ Width and Implications for the Hadley Circulation. Doctoral Dissertation. Thesis uri icon

abstract

  • The Intertropical Convergence Zone (ITCZ) and Hadley and Walker circulations are primary drivers of the hydrologic cycle in the tropics. The ITCZ controls some of the wettest regions of the globe, while the Hadley circulation controls subtropical deserts and the Walker circulation acts to regulate convection within the ITCZ. As convection within the ITCZ drives the Hadley circulation, understanding the connections between ITCZ and circulation variability is vital to improving climate forecasts. Although long-term trends from observations and climate models indicate a narrowing of the ITCZ and an intensificaiton of associated precipitation in a warming climate, few studies have examined the relationship between ITCZ width and the characteristics of convection within the ITCZ. Using the Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database, Moderate Resolution Imaging Spectroradiometer (MODIS) level 3 data, an ITCZ identification database, and European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis Interim (ERA-Interim) data the variability of cloud and convective populations are studied as a function of ITCZ width. In the Pacific basin, convection is more (less) intense, with large (small) stratiform rain fractions and high (low) maximum echo-top heights, when the Pacific ITCZ is wide (narrow). This apparent discrepancy with long-term changes is linked to Walker circulation and El Nino-Southern Oscillation (ENSO) variability, with wide (narrow) ITCZs tending to occur during El Nino (La Nina). Further analysis of variability within the Pacific, controlling for signals such as ENSO and the seasonal cycle, further indicates a Walker circulation influence over variations of convective intensity with ITCZ width. The relationships between the Hadley circulation and ENSO both within and outside of the Pacific Ocean show clear zonal variability, with a strengthening and widening (narrowing and weakening) of the circulation within (outside) the Pacific during El Nino. To control for zonal differences, tropical ascent area fraction is used to study global variability in convection with changes in the area of ascent. Convection in the ascent region becomes more intense (i.e., higher cloud-top heights and large stratiform area fractions) when tropical ascent area fraction is low; a relationship similar to that of long-term trends. In descent regions, variability in clouds with respect to tropical ascent area fraction differs between cloud regions (i.e., shallow cumulus and stratocumulus regions), but both indicate an intensification of subsidence.

publication date

  • March 2021