Widening of the Regional Meridional Overturning Circulation

The observed and projected widening of the tropics and subtropics are often described as a changing global overturning Hadley circulation. This provides a theoretical basis with which to understand climate change, but mostly in the zonal mean. To study regional change, sea level pressure (SLP) diagnostics are often used, but they lack some of the theoretical elegance of Hadley theory. Here we calculate longitudinal contributions to the Hadley circulation using the divergent component of winds in CMIP5 models, with an aim to relate zonal mean Hadley cell changes to regional shifts, and to study the impacts of widening on regional circulation. This field reveals more of the zonal structure of the tropics than does SLP.

Many previous studies analyze changes in subtropical SLP maxima to quantify regional changes in the midlatitude jet and in the width of the tropics. We analyze both the SLP and the divergent overturning circulation together to produce a more detailed depiction of tropical and subtropical circulation variability and change. We show that the SLP changes highlight the circulation response in the middle and high latitudes, where geostrophy is stronger, while the overturning circulation highlights changes in the tropics, where divergence is stronger. Furthermore, SLP emphasizes the circulation over the ocean, while the divergent circulation emphasizes circulation change over land as well. The divergent circulation also isolates circulation patterns that contribute to the zonal mean Hadley circulation, ignoring regional rotational (i.e. geostrophic) circulations. During solstitial seasons, for example, the winter Hadley cell, its variability, and its changes, are all dominated by contributions from the African and Asian monsoon sectors. During equinoctial seasons, contributions from other longitudes (i.e. the Eastern Pacific, the Americas and the Atlantic) generally play a larger role.