UC San Diego

Ocean dynamics in the subpolar North Atlantic play an important role in the global climate system: Deep convection in the open ocean drives a large-scale overturning circulation that redistributes heat around the globe and cold currents circulating around Greenland protect marine-terminating glaciers from warmer water offshore that drives ice-melt and sea level rise. However, the drivers of natural variability within this complex system, and how the region will respond to anthropogenic forcing, are not fully understood. This thesis sheds light on two regions of the subpolar North Atlantic: the Irminger Sea and the Greenland continental shelf, where the mesoscale dynamics that deliver heat to these otherwise cold regions are studied.The Irminger Sea is a region of deep convection, contributing to the water mass transformation and large-scale circulation. In Chapter 1, observations from profiling Argo floats and repeat hydrography are used to track buoyancy changes in the Irminger interior and study the region’s recovery from wintertime convection. This work shows that the slow recovery of the Irminger interior after a year of particularly strong deep convection can be attributed to reduced transport of warm, buoyant eddies from the Irminger Current.On Greenland’s continental shelf, cold, fresh waters insulate the Greenland Ice Sheet. Here, cross-shelf exchange that delivers heat on-shelf is studied. In Chapter 2, observations from a two-week ship-based survey of Narsaq Trough are used to investigate how troughs (glacially carved depressions in the continental shelf) modify the local continental shelf environment. This snapshot suggests that the trough drives a subsurface exchange flow that produces mid-depth waters in the trough that are a mixture of on-shelf and off-shelf waters. Chapter 3 contextualises the findings from Chapter 2 with two decades of repeat observations over Narsaq Trough by voluntary observing ships. This extensive time-series shows a bathymetrically-steered exchange flow is a persistent feature at the trough, both above and below the shelf, driving cross-shelf exchange of heat and salt.Cumulatively, the studies described here show how eddies and bathymetrically-steered flows at the boundaries of the sub-polar North Atlantic basins supply heat to regions which are unreachable by large-scale ocean processes.