Lagrangian perspectives on the meridional overturning circulation in the North Atlantic Ocean

Thesis

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. However, the distribution of the strength and variability of overturning across the individual circulation pathways of the North Atlantic Subtropical Gyre (STG), Subpolar Gyre (SPG) and the Nordic Seas overflows remains poorly understood. In this thesis, we investigate the detailed nature of the North Atlantic overturning circulation by introducing a novel Lagrangian framework to analyse ocean general circulation models.

We show that dense water formation along the longest SPG pathways sourced from the central and southern branches of the North Atlantic Current (NAC) accounts for more than half of the time-mean strength of the eastern subpolar AMOC in an eddy-permitting ocean model. In contrast, the strong seasonality of subpolar overturning is explained by water parcels circulating rapidly in the upper Irminger and Central Iceland Basins, whose along-stream transformation depends on their month of arrival into the eastern SPG. Looking beyond seasonal timescales, we demonstrate that the amount of dense water formed along the path of the SPG in models can be skilfully predicted, based solely on the volume transport of light waters flowing northward in the branches of the NAC, which is closely related to the strength of the SPG circulation. Finally, we present the first Lagrangian decomposition of the time-mean meridional heat transport and overturning in the subtropical North Atlantic. We show that the substantial heat transport of the STG cannot be meaningfully distinguished from that of the AMOC, since water cooled within the gyre subsequently flows northward at intermediate depths to feed regions of dense water formation at subpolar latitudes.

Overall, our findings highlight the fundamental roles of geometry and connectivity in governing the strength and variability of the AMOC in the North Atlantic Ocean.

Authors: Tooth, OJ

• DOI: 10.5287/ora-zozabvawr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Atlantic Meridional Overturning Circulation; water mass transformation; circulation pathways; Lagrangian
• Subjects: Physical oceanography