Tropical Dynamics and Rainfall

The tropics are characterized by large variations in rainfall seasonally, interannually, and from one region to another, with these spatiotemporal variations governing the water availability for more than half of the world’s population. Tropical rainfall variations are tightly coupled to the atmospheric circulation, which transports water that is evaporated in one region and deposits it as precipitation in another, for example leading to rainfall bands such as the Intertropical Convergence Zone (ITCZ) in regions of lower-tropospheric convergence. As the climate warms, the atmosphere can hold more water vapor, increasing the contrast between wet regions and dry regions and between wet periods and dry periods. However, changes in the atmospheric circulation can modify which regions are wet and which are dry or modulate the strengthening of rainfall contrasts. Our past work has shown that an overall weakening of tropical circulations makes the spatial variability of tropical rainfall increase less quickly than would be expected from the increase in atmospheric moisture content (Download Wills et al. 2016) and used a simplified model of the global atmosphere to understand how this weakening of tropical circulations comes about from the thermal expansion of the troposphere (Download Wills et al. 2017).

The dynamics of tropical circulation changes and their influence on rainfall are core to several ongoing projects in the group, spanning multiple timescales and regions:

• The Ph.D. project of Nora Fahrenbach investigates how changes in forest cover influence tropical circulations and the tropical hydrological cycle. Trees influence the surface fluxes of water, energy, and momentum, meaning that deforestation or afforestation can potentially influence rainfall in remote regions.
• The Ph.D. project of Zhenghe Xuan investigates how tropical circulation and rainfall variability weakens or strengthens on different timescales in response to global warming, as discussed further in the Atmospheric Circulation Variability and Change page.
• Clarissa Kroll is investigating the influence of model tuning and increased resolution on climate models’ skill in representing the spatial pattern and seasonal cycle of tropical rainfall, with a focus on alleviating the double-ITCZ bias that has long been a problem in climate models. This is discussed further in the High-Resolution Climate Modeling page.

Overall, our focus on tropical dynamics and rainfall supports our group’s mission to contribute to improving regional climate projections, in particular concerning how water resources might change in the future.