From Chief Scientist, Craig Lee – University of Washington, Applied Physics Lab
It’s Thursday midnight, and we’re making our last measurements of seawater chemistry before concluding this year’s field effort and returning to Greenland. We’ve been working to understand the oceanography of this critical region for over a decade. Why?
The Arctic Ocean connects to the lower latitude oceans (Pacific and Atlantic) though only a small handful of gateways. Bering Strait connects the Arctic to the Pacific Ocean, while Fram Strait, east of Greenland, and the Canadian Arctic Archipelago, west of Greenland, and the Barents Sea opening, connect it to the Atlantic. These gateways are important because they allow freshwater and heat to move between the Arctic and the rest of the world’s oceans. Such exchanges can have large impacts on the currents that carry water around the entire globe.
In our case, we are concerned with the waters between Baffin Island and Greenland, and in particular with the southward flow of relatively fresh waters from the Arctic Ocean into the Labrador Sea, and the northward flow of relatively warm Atlantic Ocean water along the Greenland coast.
Why fresh water? During wintertime, the cold, dry air and strong winds cool the upper layer of the Labrador Sea. As the upper layer cools it gets heavier (more dense). When it becomes heavier than the waters below, the situation is unstable, and the heavy upper layer sinks, similar to what happens when water is poured on top of oil. This is called “convection”, a process that is a critical part of the atmosphere-ocean engine that moves heat and freshwater around the global oceans. Salt content also contributes to the density of seawater. Waters exiting the Arctic (Baffin Island Current) are fresher, and thus less dense (lighter), than those in the Labrador Sea. Fresh Arctic waters that enter the Labrador Sea can make the surface layer lighter. Because lighter surface layers require more cooling to trigger convection, fresh Arctic waters that enter the central Labrador Sea can reduce convection and slow the ocean currents that it drives.
Why heat? The Greenland ice sheet melts from the top, due to warming by the sun, but may also melt from below, due to warming from the ocean. Waters from the Atlantic Ocean provide a source of heat for melting the ice sheet. These waters flow north in a surface current offshore of western Greenland (West Greenland Current), and in a current below the surface, located just beyond the shallow waters of the west Greenland shelf (West Greenland Slope Current). We are interested in the heat being carried northward in these currents due to their potential for accelerating melting of the Greenland ice sheet. Rates of ice sheet melting (in both Greenland and the Antarctic) have large impacts on forecasts of sea level rise, making this an important area of research.
Why Davis Strait? The Canadian Arctic Archipelago is a complex system of many interconnected narrow channels and small basins that lead from the Arctic to Baffin Bay. That complexity makes it difficult to measure. Fortunately for us, nearly all of these channels funnel into Baffin Bay. Davis Strait (the red line on the chart that marks where we’ve been working) bounds the southern end of Baffin Bay, and thus provides a convenient “choke point.” By measuring at this one location (rather than in the many channels to the north), we can capture all of the water going in and out of the Canadian Arctic Archipelago. This includes south-going freshwater from the Arctic, and north-going heat from the Atlantic. The Arctic outflow also has implications for ocean acidification and ecosystems, but that’s a story for another post.