Density changes in the ocean due to mixing - and its effects on circulation and sea level change -

Abstract

Changes in ocean density influence ocean sea level and global ocean overturning circulation. Therefore, understanding ocean density changes is crucial to understand ocean circulation and its role in the climate system, by influencing heat and carbon uptake, storage and distribution. In this thesis, we use observational data of temperature and salinity to calculate global density changes and investigate its effects on ocean circulation and sea level change. We focus on the contributions related to mesoscale and small-scale mixing processes, where the former are separated in a horizontal component near the surface and an isoneutral component in the interior. We further distinguish between processes that only redistribute density and processes that have a net effect on global mean density in the ocean (Cabbeling and Thermobaricity). To relate density changes to the ocean circulation, we use the water mass transformation framework, which quantifies mass transport due to density changes. Analysing the different mixing processes, we can show that Cabbeling and Thermobaricity are important for the production of intermediate water along the Antarctic Circumpolar Current, the Gulf Stream and the Kuroshio, and for the production of Nord Atlantic Deep Water in the Atlantic and Antarctic Bottom Water in the Southern Ocean; where the effect of Thermobaricity is one order of magnitude smaller than the effect of Cabbeling. Further, we identify that horizontal mixing processes, redistributing density in the boundary layer, are strongly related to ocean surface currents. Redistribution by vertical mixing depends predominantly on vertical buoyancy gradients and bathymetry. When combining all terms, we find a resulting global mean sea level change of -2.4 mm/yr, which is of the same order of magnitude as the +3.7 mm/yr observed global mean sea level rise. This emphasises the importance of accurately representing ocean mixing, to understand ocean density change and its impact on circulation and sea level rise.