Do surface temperature indices reflect trends in Atlantic Meridional
Overturning Circulation strength?
The difference between North Atlantic subpolar gyre sea surface
temperatures (SPG SSTs) and hemispheric- or global-scale surface
temperatures has been utilized as an index of centennial-timescale
changes in Atlantic Meridional Overturning Circulation (AMOC) strength.
Here, using Community Earth System Model ensembles, we show that surface
temperature-based indices (STIs) proposed to date largely reflect
global-scale temperature trends and thus do not reflect dynamical
relationships with AMOC. More broadly, we find that relationships
between STIs, SPG SSTs, and AMOC strength differ greatly in significance
and magnitude over different time periods because they are dependent
upon the nature of external forcing. In the 20th century, characterized
by offsetting greenhouse gas and aerosol forcing, the relationship
between SSTs and AMOC strength varies widely and changes sign across a
20-member ensemble. We conclude that STIs and SPG SSTs are poor
predictors of centennial-timescale AMOC strength variations.