Rapid freshening of the deep North Atlantic Ocean over the past four decades

Abstract

The overflow and descent of cold, dense water from the sills of the Denmark Strait and the Faroe Shetland channel into the North Atlantic Ocean is the principal means of ventilating the deep oceans, and is therefore a key element of the global thermohaline circulation. Most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse-gas concentrations predict a weakening thermohaline circulation in the North Atlantic as the subpolar seas become fresher and warmer, and it is assumed that this signal will be transferred to the deep ocean by the two overflows. From observations it has not been possible to detect whether the ocean's overturning circulation is changing, but recent evidence suggests that the transport over the sills may be slackening. Here we show, through the analysis of long hydrographic records, that the system of overflow and entrainment that ventilates the deep Atlantic has steadily changed over the past four decades. We find that these changes have already led to sustained and widespread freshening of the deep ocean.

The Medieval Climate Anomaly and Little Ice Age in Chesapeake Bay and the North Atlantic Ocean

“…Although solar-forcing cannot be ruled out as a triggering mechanism, the SST variability discussed above was not synchronous across the North Atlantic for the MCA-LIA interval. In fact, west-to-east "provincialism" in North Atlantic climate has been observed over several timescales: in Holocene millennial-scale sea-ice and surface salinity (de Vernal and Hillaire-Marcel, 2006) and SST in the North Atlantic (Keigwin and Pickart, 1999;deMenocal et al, 2000), 20th century decadal sea-ice fluctuations (Deser et al, 2000), and SST and deep water formation during the last few decades (Dickson et al, 2002). The temperature patterns presented above thus support paleoceanographic evidence (Newton et al, 2006) and model simulations (Renssen et al, , 2007Saenger et al, 2009) that suggest coupled oceanic-atmospheric processes, possibly involving deep-water formation in subpolar regions, may have influenced regional climate during the MCA-LIA.…”

“…The Labrador Current (LC), a branch of Labrador Sea Water formed near the shelf/slope break off maritime Canada, is an important distal source of MAB-Chesapeake Bay water. On decadal timescales, the strength of the LC is linked to i) North Atlantic Oscillation (NAO) decadal climate variability, ii) the Atlantic Meridional Overturning Circulation (AMOC) including Labrador Sea deep-water formation, and iii) wind-driven processes in the North Atlantic (e.g., Dickson et al, 2002). Thus, bay temperature variability should reflect that of its source water in the MAB, and more broadly, ocean-atmosphere Karlsen et al (2000), Cronin et al (2000), Zimmerman and Canuel (2002).…”

“…1 shows the major surface and deep ocean currents in the North Atlantic Ocean, their location with respect to the MAB, and the location of paleoceanographic records discussed below. Stable isotopic (Chapman and Beardsley, 1989;Houghton and Fairbanks, 2001) and oceanographic evidence (Petrie and Drinkwater, 1993;Dickson et al, 2002) shows that MAB water is the southern endmember of a 5000-km long, coastal current that flows southwest from subarctic regions (East Greenland Current) through the Labrador Sea, Scotian Shelf, and Gulf of Maine and into the MAB. The Labrador Current (LC), a branch of Labrador Sea Water formed near the shelf/slope break off maritime Canada, is an important distal source of MAB-Chesapeake Bay water.…”

Impact of freshwater from the Canadian Arctic Archipelago on Labrador Sea Water formation

“…As the North Atlantic is one of the few regions of the world ocean where deep waters are formed, this freshwater signal is being transferred into the deep oceans [Dickson et al, 2002]. Additionally, as a major source of deep waters for the world ocean and thus, a key player in the global meridional overturning circulation, the high latitude North Atlantic is potentially very sensitive to additions of freshwater (through freshwater's role as a stabilizing element in the buoyancy budget).…”

“…[2] A number of recent studies [Dickson et al, 2002;Curry et al, 2003] have suggested that the high latitudes of the North Atlantic Ocean have been freshening over the last several decades. As the North Atlantic is one of the few regions of the world ocean where deep waters are formed, this freshwater signal is being transferred into the deep oceans [Dickson et al, 2002].…”