Speaker: Dr. Rei Chemke from Columbia University

Title: "The response of the tropical circulation to increasing greenhouse gases: recent trends and the role of ocean heat transport"

Abstract: The Hadley cell plays an important role in setting the strength and position of the hydrological cycle. Climate projections show a weakening of the Hadley cell, together with widening of its vertical and meridional extents. These changes are projected to have profound global climatic impacts. Current theories for the Hadley cell response to increased greenhouse gases account only for atmospheric and oceanic thermodynamic changes, but not for oceanic heat transport changes. Here, the effects of ocean heat transport changes on the Hadley cell response to increased greenhouse gases are examined. First, using a hierarchy of ocean-model configurations under increased greenhouse gases or arctic sea-ice loss, we show that, by cooling the surface and atmosphere, ocean heat transport contracts and strengthens the Hadley cell, and thus reduces its projected response. Finally, we examine whether the projected weakening of the Hadley cell has already been emerged. While climate models simulate a weakening over the last 40 years, atmospheric reanalyses indicate a strengthening of the Hadley circulation. Using observations and large-ensemble of model simulations we elucidate this discrepancy between climate models and reanalyses, showing that it does not stem from internal climate variability or biases in climate models, but appears related to how latent heating is represented in the reanalyses. Our results highlight the role of anthropogenic emissions in the recent slowdown of the atmospheric circulation, and question the reliability of reanalyses for estimating trends in the Hadley circulation. [Paper]