Xavier Levine, Ph.D. 2013 Caltech, B.A. 2006 University of Chicago
Xavier Levine (2013-15) studies the dynamics of tropical circulations and their response to climate changes. Xavier investigates how monsoon circulations can remotely affect aridity over deserts in the subtropical regions, both in present-day climate and with global warming. A particular application of his work is quantifying the impact of the Indian monsoon in maintaining arid conditions over the parched Persian Gulf and Eastern Sahara regions.
Deserts cover about 40% of Earth’s surface and are primarily found in the tropics. The location of these deserts are controlled to a large extent by the dynamics of the tropical atmosphere; Although dryness is easily seen on land through the existence of deserts, dry zones also extend well over the oceans. In the Northern Hemisphere, the varying properties of the surface leads to large contrasts in hydrology on regional scales: for instance, while Dhaka (24N-90E) in Bengal receives about 2 meters of rain a year primarily during summer, Karachi (24N-67E) in Pakistan located 2500 km to the west receives less than 0.2 meter primarily during winter. While local feedbacks play a role in maintaining arid conditions, the distributions of deserts on Earth is to a large extent controlled by global circulation patterns, which may be more sensitive to remote than local surface conditions. Monsoon circulations, in particular, may create arid conditions thousands of kilometers away from their well-known regions of heavy rain.
In collaboration with Prof. William Boos (Yale), I am investigating the influence of monsoon circulations on Earth’s dry regions, which include not only land deserts but also neighboring ocean regions where evaporation exceeds precipitation. An idealized general circulation model (GCM) allows us to describe in relative simple terms how subtropical dryness changes in response to a monsoon system developing in a spatially-confined subtropical region. A primary focus of our work is to better understand how subtropical dryness changes in global warming or cooling scenarios when monsoons and the entire dynamics of the atmosphere are affected by a radiative change, for instance due to an increase in greenhouse gases concentrations. Our goal is to establish the contributions of monsoons systems in setting dryness in the subtropics, and how this contribution may change with climate change.
Most low latitude deserts on Earth are located in the subtropics in wide latitudinal bands ranging from about 10 to 40 degrees latitude in each hemisphere. This dryness is not confined to continental regions but extends over ocean basins as well. The large extent of these dry regions relates in part to the existence of a meridional overturning circulation in the atmosphere in the low latitudes, namely the Hadley circulation. The Hadley circulation is organized in a cell-like structure in each hemisphere, with a narrow latitudinal band of net airflow ascent no more than a few degrees wide between the equator and 10 degrees latitude and wide latitudinal bands of weak airflow descent extending throughout the subtropics. This circulation is not specific to Earth, but exists on any rotating and differentially heated planet or satellite. Poleward of the Hadley circulation waves and instabilities create a macroturbulent flow, which for instance is responsible for the fickle weather we experience in the Northeast.