Heat released from the depths of the Arctic Ocean: amplified warming and tipping points

Surface temperature observations indicate considerably faster warming in the Arctic than in the rest of the globe. This raises questions regarding why the warming has been so concentrated in the Arctic and how the rapidly changing Arctic climate will evolve in the future. The Arctic Ocean, which covers much of the Arctic region, is characterized by an ice-covered layer of cold and relatively fresh water above warmer and saltier waters below. It is estimated that enough heat is stored at depth in the Arctic Ocean to melt all the Arctic sea ice many times over. But this heat has historically remained trapped at depth: the seawater density differences are dominated by salinity, making the vertical stratification stable, and the sea ice cover damps wind-generated internal waves that could otherwise mix the warm waters up to the surface. In this talk, I will discuss the implications of this potentially precarious state of the Arctic Ocean as the climate warms. The talk will draw on idealized physical models with varying levels of complexity, as well as more comprehensive global climate models and findings from in situ observations. In the first part of the talk, I will discuss a proposed physical mechanism by which changing vertical heat fluxes in the Arctic Ocean contribute to the observed Arctic amplification of global warming. In the second half of the talk, I will present a novel positive feedback process involving the release of subsurface heat in the Arctic Ocean. Idealized model results show that this feedback process can give rise to a hysteresis window bounded by saddle-node bifurcations, featuring an abrupt "tipping point" under global warming when the bifurcation point is crossed. The hysteresis occurs for only a limited range of plausibly realistic parameters, however, and questions remain regarding the likelihood that this potential tipping point could occur under global warming during the coming century. Nonetheless, even in the absence of a tipping point, this positive feedback could substantially accelerate the melt of Arctic sea ice.