The Arctic Ocean in a Fresh and Warm Future

The Arctic Ocean remains one of the least known ocean regions due to its remote location, year-round sea ice cover, and harsh weather conditions. Today, knowledge of the Arctic atmosphere-sea ice-ocean system increases in parallel with the need to understand this changing environment under anthropogenic greenhouse warming. Indeed, the Arctic is an area where the effects of anthropogenic greenhouse warming are already visible and among the strongest on Earth; the atmosphere and the ocean are warming, the sea ice cover is diminishing, and the freshwater input to the ocean through river inflow, precipitation, and ice melt is increasing. It is this rapidly changing, but poorly understood, Arctic climate system that motivates us to study its fate in a fresh and warm future. Our objective is to assess how the ocean circulation, the ocean heat content, and the ice cover respond to increasing freshwater input and overall greenhouse warming. We also ask whether changes in the ocean affect the atmosphere, i.e., is the atmospheric surface warming modified by the changing ocean? We choose to seek answers to these questions with a hierarchy of model simulations. We focus on the North Atlantic-Arctic sector and examine changes in the ocean circulation and ocean heat content under greenhouse warming. We use idealized model simulations to assess changes in freshwater forcing, and global climate model simulations to examine the changing ocean heat budget. With this hierarchy of models we build a comprehensive understanding of the changing high latitude climate system and compile this dissertation around three main scientific findings. First, we increase the Arctic river runoff in an idealized column model which represents the large scale average conditions of the Arctic ocean-sea ice-atmosphere system. A larger Arctic river runoff leads to a new equilibrium with a fresher surface and a warmer subsurface. Interestingly, even though the fresher surface leads to larger vertical density differences and suppresses vertical mixing, the vertical heat flux towards the surface remains close to constant. This is because stronger density and temperature differences balance the heat flux: even a relatively small amount of warm water carries a relatively large amount of heat. As a result changes in the sea ice thickness remain small. Second, we extend our focus to larger scales and increase river runoff in a global ocean-sea ice model. Again, we find a fresher surface and a warmer subsurface Arctic Ocean as a response to increasing Arctic river runoff. The model also simulates a slightly weaker flow of water between the Arctic Ocean and its surrounding ocean basins. However, the heat exchanges between the central Arctic Ocean and the lower latitude oceans remain relatively constant. In a wider North Atlantic perspective, the subpolar North Atlantic shows an opposite response to the Arctic Ocean. The river runoff that enters the Arctic Ocean flows south along the coasts of Greenland and through the Canadian Arctic Archipelago and mixes into the subpolar North Atlantic. The additional freshwater weakens the large scale horizontal and vertical density differences and the ocean flow that depends on these density differences. The weaker ocean circulation brings less warm waters to the subpolar North Atlantic and the ocean cools as a result. Third, we find that, as the ocean heat content increases under greenhouse warming, the rate of the increase only weakly depends on the latitude in climate models. Only the Arctic Ocean, the northern part of the Southern Ocean, and the mid-latitude North Atlantic are warming slightly faster than the global average. We find that this stronger warming is associated with changes in the surface heat fluxes between the atmosphere and the ocean. In contrast, the subpolar North Atlantic is warming slightly slower than the global average because of the weaker ocean circulation that transports less warm waters towards the north. In summary, under greenhouse warming the high latitude ocean freshens and warms. Freshening at northern high latitudes acts to weaken the vertical heat exchanges between surface and subsurface waters which warms the Arctic Ocean. However, freshening in the north also acts to slow down the ocean circulation in the subpolar North Atlantic which reduces the northward ocean heat transport and cools the ocean there. Greenhouse warming leads to ocean warming and most of the small differences in the rate of ocean warming from latitude to latitude can be explained by changes in surface heat fluxes