The Tropical Response to Ocean Circulation Collapse

Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, USA

Global warming could bring about a reduction, or even a collapse of the Atlantic
Meridional Overturning Circulation (AMOC) – a system of ocean currents that plays a
crucial role in global heat transport. In addition to producing localized impacts over the
North Atlantic, a reduction in the strength of the AMOC would have global impacts,
particularly over the tropics, where models and theory predict large shifts in rainfall
patterns. The physical mechanisms whereby an AMOC collapse influences the global
tropics are poorly understood limiting our ability to predict future changes. We addressed
this question using a large ensemble of numerical simulations validated against
hydroclimate changes during Heinrich Stadial 1 (HS1) – the most recent, best-documented
AMOC collapse. A subset of simulations in the ensemble explains the proxy-inferred
changes in rainfall across the global tropics with unprecedented detail. This response is
driven by cooling over the tropical North Atlantic and communicated by atmospheric
responses across tropical landmasses triggering air-sea interactions spanning the Pacific
and Indian oceans. These responses uniquely explain proxy-inferred patterns of rainfall
change across the globe. A similar global response is active in simulations of future
greenhouse warming, but model disagreement regarding the pattern of tropical cooling in
the North Atlantic produces divergent rainfall predictions across the tropics. Models with
responses consistent with the paleodata predict more pronounced rainfall reductions
across the tropics, revealing a heightened risk of drought over vulnerable societies and
ecosystems worldwide.