Climate controlled mechanisms of subpolar North Atlantic carbon uptake

Abstract

The North Atlantic is a key region for global climate variability. Earlier studies have shown changes in the large-scale circulation, air-sea heat and freshwater fluxes and the carbon cycle in the North Atlantic in relation to anthropogenic forcing, but changes attributed to natural variability are less certain. However, in order to fully comprehend the ocean carbon sink and its variability, both, anthropogenic and non-anthropogenic driving mechanisms need to be evaluated. The results of this thesis are presented in four papers. In the first paper, the impact of atmospheric forcing on convection in the subpolar North Atlantic, in particular with regard to ocean ventilation and carbon sequestration is presented based on a unique dataset collected during a winter cruise to the Irminger Sea. In the second paper, a long term perspective on inorganic carbon inventory changes and its natural and an- thropogenic components is given. Inventory changes were decomposed into their main driving mechanisms and related to the evolution and distribution of the main water masses in the Irminger Sea. The third paper discusses surface f CO2 and pH trends across the Irminger Sea and Iceland Basin and their main driving mechanisms. The North Atlantic shows large decadal freshening and cooling signal, which produce a large non-anthropogenic driving mechanism on surface ocean f CO2 trends. The fourth paper discusses pH changes in the interior Irminger Sea and the Iceland Basin as well as the driving mechanisms for the observed variability. Combined, the four papers in this thesis increase our understanding of fundamen- tal interactions between atmospheric, hydrographic and biogeochemical processes in the subpolar North Atlantic. In particular, trends associated to natural variability and an- thropogenic climate change are distinguished, which contribute to a better understanding of the variability of carbon cycle processes in the modern ocean.