OCEAN DYNAMICS IN THE SUBPOLAR NORTH ATLANTIC IN A CLIMATE SIMULATION

Abstract

The Subpolar Gyre (SPG) is a dominant oceanic feature of the upper circulation in the Northwest Atlantic Ocean, characterized by a cyclonic circulation pattern. It is a critical component of the Earth’s climatic system, as it modulates the exchange of heat, salt, fresh water and sea ice between the North and Tropical Atlantic and Arctic Ocean, and it interacts with large-scale oceanic features like the Atlantic Meridional Overturning Circulation and the Global Thermohaline Circulation. Currently, the SPG variability is thought to be largely controlled by an east-west seawater density gradient in the SPG region that is determined by the contrast between hot and salty water masses from mid-latitude Atlantic Ocean, mainly transported by the Gulf Current, and fresh and cold water masses from the Arctic Ocean. In this thesis, I investigate ocean dynamics in the SPG region, using a state-of-the-art global ocean – regional atmosphere coupled model. The goal of this research is to determine how ocean-atmosphere coupling affects the general mechanism of SPG variability in this state-of-the-art model. To this purpose, I focus on interannual and decadal changes in the strength of the SPG, identified by variations in the mass barotropic streamfunction. The main analysis uses an ensemble of three simulations with ocean-atmosphere coupling activated in the SPG basin and Arctic region and differing for the resolution of the atmospheric model. Secondly, I compared these simulations to an additional simulation where the ocean-atmosphere coupling is activated in the tropical Atlantic instead of in the SPG/Arctic to disentangle different contributions to SPG dynamics, specifically those regarding heat and salt input from the Tropical Atlantic and fresh water fluxes and sea ice export from the Arctic. At the end I also evaluate the linkages between SPG and the Atlantic Meridional Overturning Circulation (AMOC), especially their relationship with meridional ocean transport of heat and mass. The investigation makes use of statistical methods to identify lead-lag relationships between the SPG strength and selected driving variables as triggers of the described variability (sea surface temperature, salinity and density, anomalies of sea ice thickness and fraction, fresh water fluxes) and heat and mass transport in the AMOC. Methods include also time series analysis, cross-correlation and linear regression analysis, composite analysis and wavelet analysis. A first key aspect of the research is the identification recurrent features of SPG variability in the three simulations to determine whether the SPG variability is due to internal variability (differences between simulations) or to external forces (consistent behavior). The study of consistent cross-correlation spatial patterns describing the relationship between SPG strength and the driving variables allows to unveil sources and pathways of SPG variability. Comparison between the simulations will reveal potential effects of model resolution. Finally, the analysis of time series will describe the connection between the AMOC, identified by changes in heat and mass transport, and SPG strength.