Changing oceanographic conditions in East Greenland

East Greenland is a region of significant climatic and ecological importance. The region provides three main ecosystem functions: climate regulation, carbon sequestration and fisheries production. The physical oceanographic environment underpins these vital ecosystem functions. Three features of the East Greenland system warrant particular attention in this regard. Firstly, stratification as it regulates primary production, the formation of dense waters that feed into the Atlantic overturning circulation, and the ventilation of the interior ocean. Secondly, oceanographic fronts as they facilitate enhanced primary production and promote carbon sequestration. And finally, the low-salinity water carried by the East Greenland Current in light of its role in returning freshwater from high to low latitudes and its potential to impinge on vertical exchanges. Ongoing and future changes in oceanographic conditions in the region are expected to impact the functioning of the region with potentially far-reaching implications.

Based on a multitude of data types covering the northeastern Atlantic, Nordic Seas and East Greenland shelf, this thesis sheds light on how and why the key physical oceanographic features that support these regions’ ecosystem functions have changed over the past three to four decades. Specifically the thesis addresses changes in stratification over the open Greenland Sea (Paper I), hydrographic changes on the Northeast Greenland shelf (Paper II), variability in frontal positioning and intensity along the Southeast Greenland shelf (Paper III), and carbon and freshwater export with the dense waters of the Atlantic overturning circulation (Paper IV).

In Paper I, we analyse an ocean reanalysis dataset to investigate changes in the strength of stratification and the locally dominant stratifying property in the East Greenland region and adjacent seas. The Greenland Sea emerges as a hotspot of change. We find a transition in the controls of winter stratification in the upper 1000 m. This shift involved a westward movement of the boundary between salinity and temperature-stratified waters, leading to a switch from salinity to temperature-dominated winter stratification over the entire Greenland Sea between 1990 and 2010. A similar switch occurred in the upper 100 m during summer, where surface waters transitioned from variable stratification, intermittently set by both salinity and temperature, to a stable temperature-stratified regime. This transition was linked to reduced sea-ice concentrations following the disappearance sea-ice after 1997. The previously common high sea-ice conditions in the Greenland Sea are now rare, suggesting that this transition will persist, with potential impacts on local sea-ice formation and marine ecology.

In Paper II, we provide a further in-depth analysis of hydrography of the Northeast Greenland shelf. We consolidate previously desperate hydrographic observations from the shelf and fjords extending back to the 1930’s through to 2020. This collection of data was analysed to quantify long-term changes in fjord and coastal waters. We find significant freshening of the surface and Polar Water layers of 1.8 and 0.68 units of salinity from the early 2000’s to 2020, respectively. Concurrently, the Atlantic Water layer warmed by up to 1 °C. In addition, we observed changes in the vertical extent of the water masses, with the Polar Water layer thinning by approximately 50 m and the Atlantic Water layer expanding by around 60 m. These changes have substantially weakened stratification south of ∼74°N, indicating increased presence of the comparatively warm and nutrient-rich Atlantic Water, with potential knock-on effects for ecosystem productivity and structure.

In Paper III, we investigate the role of the East Greenland Polar Front (EGPF) in mediating climate-ocean-ecosystem variability in Southeast Greenland where the majority of fisheries are located. We find periodic switching between periods of a sustained strong and weak EGPF and relate these switches to regional Subpolar gyre (SPG) dynamics. By adopting a composite analysis of sea surface height, sea surface temperature, seasurface chlorophyll concentrations, and logbook data from the commercial Greenland halibut fisheries, we clarify how this SPGlike variability is manifested in the Southeast Greenland ecosystem. We show elevated chlorophyll concentrations along the continental slope when the Irminger gyre is spun-up and the EGPF is strong and vice versa. In addition, we find that the main sea surface temperature front over the Ammassalik shelf between 64-65 °N and 40-35 °W, has migrated 90 km shoreward in tandem with inter annual sea-ice retreat. This shift is also reflected in the chlorophyll distribution entailing reduced production over the slope and increased production over the shallow shelf. Our findings underscore the role on the EGPF in mediating interactions between the physical and biological components of the Southeast Greenland ecosystem.

In Paper IV, we turn our focus to the lowsalinity outflow in the East Greenland Current. Using dissolved organic matter as a tracer, we track the freshwater embedded in this current into the deep ocean and estimate that it contributes ∼ 1 % to the densest component of the Atlantic overturning circulation. The freshwater carries with it a distinct terrestrial signal. We trace this signal upstream to its source on the Siberian Shelves in the central Arctic Ocean. In concert with a numerical ocean model, the observations presented in this paper reveal a direct physical pathway for terrestrial material to enter the deep ocean that represents a pathway of carbon sequestration in the East Greenland region that is distinct from the biological carbon pump. We argue that our approach can be adapted to a variety of autonomous observation platforms to elucidate freshwater and organic carbon exchanges across the Arctic-subarctic boundary.

In combination, the four papers advance our understanding of the oceanographic features that support ecosystem functions in the East Greenland region. In particular, the results highlight the region’s sensitivity to changing sea-ice cover, surface warming, and variable Atlantic Water inflow across the Greenland-Scotland Ridge. The results emphasize East Greenland as a transition zone between Arctic and Atlantic type conditions and collectively suggest that East Greenland is subject to the broader current trend of Atlantification, referring to the expansion of Atlantic type conditions at the expense of typical Arctic type conditions. Furthermore, the results demonstrate large-scale connectivity that runs through East Greenland, highlighting that the region is not only a recipient, but also a source of globally relevant change. Finally, the thesis underscores the need for increased monitoring efforts to accurately predict how ecosystem functions and downstream implications of oceanographic changes will manifest under future climate change.