Relating Patterns of Added and Redistributed Ocean Warming

Ocean warming patterns are a primary control on regional sea level rise and transient climate sensitivity. However, controls on these patterns in both observations and models are not fully understood, complicated as they are by their dependence on the "addition" of heat to the ocean's interior along background ventilation pathways and on the "redistribution" of heat between regions by changing ocean dynamics. While many previous studies attribute heat redistribution to changes in high-latitude processes, here we propose that substantial heat redistribution is explained by the large-scale adjustment of the geostrophic flow to warming within the pycnocline. We explore this hypothesis in the University of Victoria Earth System Model, estimating added heat using the transport matrix method. We find that throughout the midlatitudes, subtropics, and tropics, patterns of added and redistributed heat in the model are strongly anticorrelated (R approximate to -0.75). We argue that this occurs because changes in ocean currents, acting across pre-existing temperature gradients, redistribute heat away from regions of strong passive heat convergence. Over broad scales, this advective response can be estimated from changes in upper-ocean density alone using the thermal wind relation and is linked to an adjustment of the subtropical pycnocline. These results highlight a previously unappreciated relationship between added and redistributed heat and emphasize the role that subtropical and midlatitude dynamics play in setting patterns of ocean heat storage. Significance StatementThe point of our study was to better understand the geographic pattern of ocean warming caused by human-driven climate change. Warming patterns are challenging to predict because they are sensitive both to how the ocean absorbs heat from the atmosphere and to how ocean currents change in response to increased emissions. We showed that these processes are not independent of one another: in many regions, changes in ocean currents reduce regional variations in the build-up of new heat absorbed from the atmosphere. This finding may help to constrain future projections of regional ocean warming, which matters because ocean warming patterns have a major influence on regional sea level rise, marine ecosystem degradation, and the rate of atmospheric warming.