On the Ability of Sea Surface Temperature Indices to Hindcast AMOC Variability

Paleoclimatic records of ocean circulation are sparse and often indirect. A robust indirect index of the Atlantic Meridional Overturning Circulation (AMOC) would be a highly valuable tool, both for reconstructing ocean circulation, and for interpreting the broader paleoclimate record. To this end, previous studies have proposed that North Atlantic subpolar gyre sea surface temperatures (SSTs), either independently, or relative to global or Northern Hemisphere mean temperatures, reflect variations in AMOC strength. However, other modelling studies have shown strong dependence on timescale and/or climate forcing. Here, using the Community Earth System Model Last Millennium Ensemble (an ensemble of 10 climate model simulations of the 850-2005 period, and 4 from 2005-2100), we find that the strength, nature, and evolution of external forcing fundamentally alters AMOC-SST relationships. Temporal variations in the sign and amplitude of correlations suggest that different physical mechanisms mediate paleo- and modern, and internally- and externally-forced, AMOC-SST relationships. For example, lead/lag relationships indicate that external forcing inverts the causal linkage between AMOC and Northern Hemisphere SSTs. Regression (scaling) coefficients between an SST index and AMOC vary from 0.68 to 3.9 Sv/K, depending on the time period considered. To quantify the limitations of a stationary SST-based proxy, we hindcast AMOC strength using a scaling relationship derived from modern and future periods.