Hadley Cell Expansion and Shifting Subtropical Highs

The Hadley cells—large-scale overturning circulations with rising air near the equator and sinking air in the subtropics—dominate the Earth’s climate at low latitudes. Observations have shown that the Hadley cells are expanding poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to continue throughout this century as a result of global warming. This has led to concerns about potential surface impacts, including a poleward shift of the subtropical dry zones.

However, the impacts of Hadley cell expansion are zonally asymmetric—especially in the Northern Hemisphere—raising the question of whether a more regional focus is necessary. In particular, the North Pacific and North Atlantic subtropical high-pressure systems are related to the Hadley cell, but also have a considerable amount of independent variability of their own. In this study, I contrast the impacts of the Northern Hemisphere subtropical highs and the Northern Hemisphere Hadley cell on surface-level processes including precipitation, surface-level wind, wind-driven ocean currents, and marine chlorophyll. After considering the impacts of shifts in these circulation features, it is also worthwhile to ask what dynamical drivers are responsible for such shifts. I use a timescale analysis to explore these causes—which appear to differ strongly between ocean basins—before describing several hypotheses for the drivers and outlining plans for further tests of these hypotheses.