Regional Precipitation Sensitivity is Sensitive to Changes in
Cross-Equatorial Ocean Heat Transport
Abstract
Weakening of the Atlantic Meridional Overturning Circulation (AMOC) with
greenhouse gas forcing is a common feature in most Earth System Models
(ESMs). Here, we show that uncertainty in regional precipitation
projections in CMIP6-class ESMs, particularly over the deep tropics, is
linked to uncertainty in how much the AMOC weakens. Specifically, ESMs
with greater AMOC weakening exhibit less strengthening (or greater
weakening) of monsoonal circulations north of the equator, consistent
with a pronounced southward shift in the tropical precipitation maximum.
Using an idealized ESM, we show that decreasing the (northward)
cross-equatorial ocean heat transport (X-OHT) while doubling CO2 is
sufficient to shift the tropical precipitation maximum southward and
thereby weaken northern hemispheric monsoon circulations, relative to an
experiment where X-OHT remains unchanged. These findings imply that
uncertainty in precipitation projections over key regions, particularly
in the tropics, may be reduced by better constraining changes in
large-scale ocean dynamics with warming.