How the AMOC affects ocean temperatures on decadal to centennial timescales: the North Atlantic versus an interhemispheric seesaw

Variations of the Atlantic Meridional Overturning Circulation (AMOC) are believed to be an important driver of decadal to multi-decadal climate variability (e.g. Sutton and Hodson 2005; Knight et al. 2005; Zhang and Delworth 2006; Álvarez-Garcia et al. 2008; Seager et al. 2010; Semenov et al. 2010; Mahajan et al. 2011, for a recent review see Srokosz et al. 2012). In particular, it has been suggested that AMOC variations control or at least contribute to the Atlantic Multidecadal Variability (AMV), also referred to as the Atlantic Multidecadal Oscillation (AMO), see (Ting et al. 2011; Zanchettin et al. 2014). One important aspect of this connection is whether the AMOC variability affects sea surface temperatures (SST) mainly in the Northern Hemisphere or its impacts extend to the Southern Hemisphere as well. Accordingly, the goal of this study is to investigate this and other key aspects of the SST response to AMOC variations in climate models. A number of observational and modeling studies investigating the AMOC have linked an interhemispheric SST dipole, designed to reflect interhemispheric seesaw changes in SSTs, to fluctuations in the overturning circulation (Latif et al. 2006; Keenlyside et al. 2008). This temperature dipole is observed on multi-decadal and longer timescales, is separate from the interannual to decadal Northern Hemisphere tri-polar pattern (Visbeck et al. 1998), and is generally consistent with the observed interhemispheric signature of the AMV. While some studies use the dipole as a way of removing the global warming signal from the North Atlantic SST (Latif et al. 2004; Keenlyside et al. 2008), others employ the dipole as an index to investigate AMOC changes (Latif et al. 2006; Kamyokwsi 2010). In particular, in the absence of direct measurements of the AMOC extending beyond Abstract This study investigates how variations of the Atlantic meridional overturning circulation (AMOC) affect sea surface temperature (SST) within the simulations of the coupled model intercomparison project phase 5. In particular, we explore whether the SST response is interhemispheric in nature, specifically as reflected in the Atlantic SST Dipole index, or whether the response is localized more in the North Atlantic Ocean. In the absence of direct observational data, this Dipole index has been proposed to approximate AMOC variations over the duration of the instrumental temperature record. We find that typically, on timescales between decadal and centennial, the SST Dipole index correlates with the AMOC with coefficients ranging from 0.2 to 0.7, typically with a 0–6 year lag, and thus explains less than half of the AMOC variance. In just two models this value slightly exceeds 50 %. Even for the models with the highest correspondence between the AMOC and the Dipole index, the correlation between the two variables is controlled mainly by SST variations in the North Atlantic, not the South Atlantic, both for the model control and historical simulations. Consequently, in nearly all models, the North Atlantic SST provides a better indicator of AMOC variations than the Atlantic Dipole. Thus, on decadal to centennial timescales AMOC variability affects mainly the North Atlantic Ocean, with the sensitivity of the North Atlantic SST between 40 and 60°N, given by the multi-model average, of about 0.3 °C per 1 Sv of AMOC change, explaining roughly one third of the SST variance.