Development of a Freshwater Flux Array off Southeast Greenland
OCCI Project Funded: 2004
There is growing observational and modeling evidence that the role of the Arctic in global climate change may act through exchange with seas further south. Thus, decadal variability may be imposed on the Arctic Ocean through changes in the inputs of heat and salt caused by climatic forcing over Nordic seas. In turn, changes in the heat and freshwater flux from the Arctic to subjacent seas can alter the rate and characteristics of the Atlantic thermohaline circulation (THC), which helps maintain the relative warmth of the climate of NW Europe. To monitor the exchange of water between the Arctic and the subarctic seas, two collaborative arrays have been deployed off Southeastern Greenland (Figure 1).First, a current meter array which, discontinuously since 1986, has been set to measure the transport of the cold dense Denmark Strait overflow where it descends across the Continental Slope off SE Greenland. Originally deployed by CEFAS, this array has been jointly maintained since 1997 by the Center for Environment, Fisheries and Aquaculture Science (CEFAS), the University of Hamburg and the Finnish Institute of Marine Research Helsinki under the European Climate Variability of Exchange in the Northern Seas (EC VEINS) and Arctic/Subarctic Ocean Fluxes (ASOF) programs. It is intended that this array will be maintained to a point where decadal measurements of overflow transport and its hydrographic characteristics have been recovered in order to assess the causes of long term variability in overflow and to identify the links between that variability and climatic forcing. The justification lies in the fact that this overflow both ventilates the deep North Atlantic and ‘drives’ the abyssal limb of the THC.
The second deployment off SE Greenland is at a less advanced state of development. Set across the continental shelf (Figure 1 again, in collaboration with UHH Hamburg and IMR Helsinki), its specific objectives are to use an array of fixed or profiling moored salinity sensors and current measurements to measure the major (and, at present, totally unmeasured) component of freshwater flux passing south from the Arctic Ocean to the North Atlantic under the ice of the East Greenland shelf south of Denmark Strait. Its justification is again climatic, namely that an increasing outflux of freshwater from the Arctic has been implicated in advanced coupled climate models with slowing down the Meridional Overturning Circulation of the North Atlantic.
Purpose of the Project
Though the instruments moored across the Denmark Strait Overflow are mainly conventional current meters, the freshwater flux array has been largely experimental and the rate of its expansion across the shelf and hence the breadth of the freshwater stream covered has depended on a series of trials. Hitherto, we have depended on 35-45 meter long ‘tube’ moorings to carry the salinity sensors up to the ice-base (Figure 2); but, though these have been successful, resulting in a growing recovery of long salinity records, two additional features are urgently needed to upgrade the array to the point where the first freshwater flux estimates can be made. 1) We need to achieve a better vertical resolution of salinity distribution under the ice by introducing an increased number of salinity sensors (salinity-temperature recorders). 2) Since our past attempts to measure the current profile on the shelf by conventional means have provided only short records (typically 6-month records from one-year deployments), we need to rapidly achieve a longer-term measure of the current speeds under the ice and their variability in space (including depth) and time. We intend to do this by adapting two existing CEFAS acoustic Doppler current profilers (ADCPs) for use at Greenland.
This project is using WHOI funding support from the OCCI for these two measures: i.e., for the purchase of 6 salinity-temperature recorders and for the adaptation for Greenland waters of the 2 existing ADCP units (i.e., increasing their depth capability from 300 to 500 meters, building bottom frames, and adding a groundwire recovery system). The third element of this project is to analyze the resulting salinity-temperature records that potentially total 20 instrument years on recovery in summer 2004.
The planned accelerated development of the number and types of moored
gear together with additional effort in analyzing and interpreting
their records will rapidly overcome the present deficiencies of the
array and will equally rapidly allow the calculation of the first
estimate of the climatically-important freshwater flux passing south
along the East Greenland Shelf.
The scientific return from the two arrays to date has been high. Results from both arrays and from the larger-scale analysis of hydrographic data are of two types: a) those which continue to confirm the pre-existing working hypothesis; and b) those which provide new insights into local, regional or larger scale variability. They are itemized as follows:
- There continues to be little evidence of any systematic seasonal or interannual change in the speed or transport of the Denmark Strait overflow, in contrast to that through the Faroe-Bank Channel, which appears (from proxy evidence) to be slowing.
- The temperature of the Denmark Strait overflow off SE Greenland appears to be determined by North Atlantic Oscillation (NAO) forcing of the upper ocean in the Fram Strait, some 2,500 kilometers upstream and 3 years earlier.
- The hydrographic character of the deepening overflow off SE Greenland determines the density at abyssal depths in the Labrador Sea one year later.
- The progressive 4-decade freshening of the entire system of overflow and entrainment that ventilates the deep Atlantic has continued.
- The freshening of the water column of the NW Atlantic is among the largest changes ever observed in oceanography.
- The freshening observed in the northern North Atlantic over the past four decades appears to be part of a circumpolar freshening of the World Ocean in both hemispheres. Coupled with an increase in upper ocean salinities throughout the subtropics, this large-scale and long-term redistribution of ocean salinity is suggested to be the ‘telltale’ of an accelerating water cycle, long anticipated as a result of global warming.
- Despite the harsh and remote fieldwork site off SE Greenland, the above results have been underpinned by a high recovery-rate of gear during the contract period, including a 95% good data return from current meters.
The flows that form the subject of this project are of central importance to Earth's climate system. The cold, dense overflow from the Arctic, whose characteristics and variability are being measured by the Slope array, drives the abyssal limb of the Atlantic meridional overturning circulation. The freshwater flows from the Arctic on either side of Greenland are implicated in model experiments by slowing that circulation down. Developing an understanding of the longer-term variability of both flows will be critical to the continued development of climate models. The overflow array has already been proven and is fully-developed. This WHOI OCCI-supported project will permit the rapid development of the freshwater flux array on the neighbouring shelf to the point where the first measurement can be made of the largest of the freshwater fluxes that reach the North Atlantic from Arctic and subarctic seas. The developed freshwater flux array should be kept in place for at least two years, with data analysis and interpretation continuing for a third year. Thereafter, the continuation of these arrays will depend on the long-term research policy of CEFAS and its collaborators and the availability of an adequate scale of funding to permit their continuation.
Originally published: January 1, 2004