Do Surface Temperature Indices Reflect Centennial‐Timescale Trends in Atlantic Meridional Overturning Circulation Strength?. Issue 22 (14th November 2020) - Little, Christopher M.
Abstract: 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 twentieth 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. Plain Language Summary: The short observational record of the Atlantic Meridional Overturning Circulation (AMOC) limits our ability to assess changes in its strength over the instrumental and preinstrumental periods. Indirect proxies of ocean circulation are thus required to make inferences about past trends, for example, those over the past century. Several previous analyses have used surface temperature indices to interpret twentieth century AMOC trends. However, the robustness of this indirect AMOC proxy, including its sensitivity to time period, timescale, and/or climate state, has not been assessed. We use two state‐of‐the art climate model ensembles to assess AMOC/surface temperature relationships over century timescales, finding a strong dependence upon time period and climate forcing. Our results clarify the origins of discrepancies in AMOC/surface temperature relationships and suggest that interpretations of twentieth century climate and ocean circulation change based on surface temperature indices are limited. Key Points: Previously proposed surface temperature indices (STIs) of AMOC strength are dominated by global temperature trends STIs are poor predictors of AMOC strength outside their calibration period, calling into question previous interpretations of twentieth century trends Over centennial timescales, AMOC/STI relationships are sensitive to the nature of external forcing and unforced variability