Graduate Lecture Series: David McGee (EAPS)

Speaker: 
David McGee, Associate Professor
Date: 
Friday, April 6, 2018 - 4:30pm to 5:30pm
Location: 
54-915

Toward a Global Picture of Atmospheric Circulation in the Past

I will report on efforts to understand the global atmospheric circulation during millennium-long periods of the last ice age known as Heinrich stadials, during which the Northern Hemisphere cooled dramatically relative to the Southern Hemisphere. These events represent the largest perturbations of the balance of surface temperatures between the hemispheres in the recent (last 500,000 years) paleoclimate record, and as such they provide a natural laboratory for understanding the atmospheric response to asymmetric heating/cooling. I will begin by reviewing work on the response of the tropical rainbelt associated with the Intertropical Convergence Zone (ITCZ), demonstrating that the ITCZ consistently moves toward the warmer hemisphere, but that energetic constraints limit zonally averaged north-south ITCZ movements to around 1 degree. I will then investigate the response of the broader Hadley circulation, showing that theory, model simulations and data support intensification of the Hadley circulation in the cooling hemisphere, and weakening in the warming hemisphere. The accompanying trade wind changes induce responses in the ocean’s subtropical cells that increase heat transport into the cooling hemisphere, further damping the ITCZ’s response to asymmetric temperature anomalies. Finally, I will examine subtropical/mid-latitude responses during Heinrich stadials, with a focus on the western U.S. Subtropical jet changes that accompany Northern Hemisphere Hadley cell intensification during Heinrich stadials drive increases in precipitation in the southwestern U.S., similar to dynamics observed in modern interannual variability. This finding provides a dynamical framework for understanding lake expansions in the U.S. Great Basin during Heinrich stadials. I will close with a brief summary of remaining questions and directions for future work.

About the Speaker

David McGee’s research focuses on understanding the atmosphere’s response to past climate changes. By documenting past changes in precipitation and winds using geochemical measurements of stalagmites, lake deposits and marine sediments and interpreting these records in the light of models and theory, he aims to offer data-based insights into the patterns, pace and magnitude of past hydroclimate changes. His primary tool is measurements of uranium-series isotopes, which provide precise uranium-thorium dates for stalagmites and lake deposits and allow reconstructions of windblown dust emission and transport using marine sediments.

About the Series

The Graduate Lecture Series [GLS] is a weekly lecture featuring EAPS Professors geared towards EAPS Graduate Students, Researchers and Postdocs. Lectures usually take place on Fridays from 4:30-5:30 pm in 54-915 unless otherwise noted (term-time only). For more information please contact: Allison Provaire, provaire@mit.edu.