Transient ocean overturning response to changes in boundary conditions.

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The rate at which heat and carbon are exchanged between the atmosphere and ocean strongly influences the Earth's climate. In this context, the ocean can be seen as a huge reservoir that stores atmospheric tracers and, as a result, regulates the climate. The ocean's role in this process is not solely passive. At the center of this picture is the ocean meridional overturning circulation (MOC). The MOC sets the amplitude and spatial structure of oceanic heat and carbon absorption and redistributes these tracers into the abyss. Recent advances the MOC's interaction with surface atmospheric fluxes has focused primarily on long-term equilibrium solutions under steady forcing. However, the ocean has most likely never been in a state of equilibrium and many aspects of the more realistic time-dependent response to transient climate change remain poorly understood. The long-term goal of this research proposal is to clarify the ocean's role in the transient climate change problem through physical and biogeochemical processes. The short-term goals are to focus on three key aspects of the time-dependent response to transient climate change in isolation. The first aspect relates to the surface response in temperature anomaly under global warming. While most of the surface of the globe shows positive temperature anomalies, a surprising region of cooling, called "warming hole", is nonetheless observed in the northwestern Atlantic. In order to gain understanding of how this structure develops, we will investigate the coupled interaction between the surface wind-driven gyres and the deeper pathways of the MOC. The second aspect relates to the polar amplification of climate change. The flux of warm Atlantic waters into the Arctic is predicted to increase under global warming despite the slowdown of the MOC. In order to understand what mechanisms control the oceanic heat transport into the Arctic, we will diagnose the Arctic overturning circulation and investigate the coupled interaction between an estuary-type circulation and a wind-driven-type circulation. The third aspect relates to climate variability during the last glacial period when abrupt climate variations, called Dansgaard-Oeschger cycles, took place. We intent to develop a simple predictive framework that captures the main characteristics of the coupled ocean-sea-ice interaction and investigate its effect on the global carbon cycle. Overall, the completion of this work will help reduce uncertainty related to the overturning circulation in climate models, improve predictions in ocean heat transport into the Arctic, and clarify our understanding of key mechanisms controlling the variability of the overturning. By contributing to improvements in climate projections, this research will help Canadian climate adaptation policy makers take more informed decisions for local populations and industry.