The Southward Returning Pathways of the AMOC and Their Impacts on Global Sea Surface Temperature

Our recent study showed that there exists a coherent spatial pattern of inter-hemispheric global model sea surface temperature (SST) biases in CMIP5 (Coupled Model Intercomparison Project phase 5) climate models and this global pattern of model SST biases is closely linked to the strength of simulated Atlantic Meridional Overturning Circulation (AMOC). The models with a weaker AMOC are associated with cold SST biases in the entire Northern Hemisphere, and with an anomalous atmospheric pattern that resembles the Northern Hemisphere annular mode. These models are also associated with a strengthening of Antarctic Bottom Water (AABW) formation and warm SST biases in the Southern Ocean. However, in many of these models, the amplitudes of the AMOC agree very well with or are even larger than the observed value of about 18 Sv at 26.5°N, but they still show cold SST biases in the Northern Hemisphere. This suggests that the AMOC strength may not be the only factor that causes the cold SST bias. A common symptom in these models is that the returning flow of the AMOC at depth is too shallow. A shallow returning flow would carry excessive heat southward; thus the net northward heat transport by the AMOC would be weaker than the observed. The shallow returning flow in CMIP5 models should be linked to the bias in the southward pathways of the AMOC at depth. We propose to continue our investigations to (1) diagnose the meridional heat transport and its link to model SST biases in CMIP5 models, (2) perform and analyze “robust diagnostic” simulations of the AMOC to reconstruct realistic southward returning flow pathways of the AMOC, (3) explore AMOC southward returning flow pathways and sources of the shallow returning flow of the AMOC in CMIP5 models, (4) investigate the relationship of North and South Atlantic water masses associated with the AMOC, and (5) examine the impacts of improved AMOC on global SST. We will use available hydrographic observations interpolated into isopycnal surfaces, CMIP5 outputs, and model experiments of NCAR Community Earth System Model and an intermediate complexity model.
The proposed work directly contributes to the priority for NOAA FY2016 CPO/CVP funding: “Solicits projects that will refine the current scientific understanding of the AMOC state, variability, and change. Specifically, projects are sought that use newly deployed and existing observations in combination with modeling experiments to refine our understanding of the present and historical circulation (and related transports of heat and freshwater) in the North and/or South Atlantic. An emerging priority is to provide a more detailed characterization of AMOC flow pathways and their impact on variability.” The main outcome of this study will greatly improve our understanding of the decadal predictability of the AMOC and associated climate impacts, and help improve CMIP5 models.