ARI: Where the Arctic Meets the North Atlantic: Where does the freshwater go?

Robert S. Pickart, Physical Oceanography
Carin J. Ashjian, Biology

Abstract

The Arctic Ocean has been undergoing unprecedented change, seemingly the result of global warming.Among the most notable changes has been an acceleration of the hydrological cycle, including more melting of sea-ice, more storms (hence more precipitation), fresher input from the Pacific, and an increase in river run-off. These changes are consistently of one sign: to increase the freshwater discharge to lower latitudes. Consequently, there is widespread concern about the sensitivity of the North Atlantic meridional overturning circulation (MOC) to these northern freshwater sources.In particular, it is thought that the increased freshwater—roughly half of which passes to the east of Greenland—will reduce convective overturning in the North Atlantic and consequently slow down, or halt, the MOC. However, in order to accomplish this, the outflowing freshwater needs to be transferred from the East Greenland boundary current system into the basin interior, and it is presently unknown how this will happen, how quickly it will occur, or what the biological ramifications will be.

We propose to deploy a high-resolution moored array across the continental shelfbreak just south of Denmark Strait, where the Arctic-origin water first meets the open North Atlantic. The sharp front that forms here between the cold, fresh out-flowing Arctic water and the warm, salty recirculating Atlantic water is known to be leaky—likely allowing Arctic water to escape offshore—but historical measurements from ships have been too coarse to determine why this might be or how much water and biogenic material is exchanged. The proposed array will offer the first-ever view of this front, and its associated current system, at the dynamically relevant lateral and vertical scales, by using profiling instrumentation spaced closely together across the shelfbreak.This will provide multiple vertical sections per day of hydrographic properties, velocity, and acoustic backscatter (zooplankton proxy) over the span of one complete annual cycle. Our aim is to determine the mechanisms by which Arctic-origin waters and their intrinsic zooplankton are exchanged across the frontal boundary, and determine the magnitude of the fluxes. The hypothesis is that this happens by two processes: hydrodynamic instability of the current that leads to eddy formation during periods of light winds, and downwelling during the frequent passage of storms along the North Atlantic storm track. The information obtained from this study will put us in a better position to understand how the predicted increase in freshwater discharge from the Arctic might impact the thermohaline structure, convection, and ecosystem of the sub-polar North Atlantic.