Mechanisms behind the Temporary Shutdown of Deep Convection in the Labrador Sea: Lessons from the Great Salinity Anomaly Years 1968–71

Abstract

From 1969 to 1971 convection in the Labrador Sea shut down, thus interrupting the formation of the intermediate/dense water masses. The shutdown has been attributed to the surface freshening induced by the Great Salinity Anomaly (GSA), a freshwater anomaly in the subpolar North Atlantic. The abrupt resumption of convection in 1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here oceanic and atmospheric data collected in the Labrador Sea at Ocean Weather Station Bravo and a one-dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail. These results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet. The analysis herein shows that the shutdown was initiated by the GSA-induced freshening as well as the mild 1968/69 winter. After the shutdown had begun, however, the continuing lateral freshwater flux as well as two positive feedbacks [both associated with the sea surface temperature (SST) decrease due to lack of convective mixing with warmer subsurface water] further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air-sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surface water. In 1972 convection resumed because of both the extreme atmospheric forcing and advection of saltier waters into the convection region.

Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

“…A shutdown of deep convection in the Labrador Sea from 1969 to 1971 is well documented 35 and the stability of Labrador Sea convection has been the subject of a number of studies 36,37 . Perhaps as a consequence of the cooling in the Greenland region starting in 1970, the GIS subsequently was closer to mass balance for three decades until AD ∼2000 (ref.…”

Extinction of the northern oceanic deep convection in an ensemble of climate model simulations of the 20th and 21st centuries

“…Such a combination has been reported by Lazier (1980) to have caused the shutdown of LSDC in the late 1960s and early 1970s, and which coincided with both a freshwater anomaly and low NAO winter conditions. Indeed, many authors have linked past and presentday shutdowns of LSDC to pronounced freshwater anomalies and persisting low NAO winter conditions (Gelderloos et al 2012), but it remains unclear to what extent each of these two mechanisms actually contribute.…”

“…Deep convection in the Greenland Sea starts to decline in the 2020s, until complete extinction in 2100. As a response to the extinction of deep convection in 1 3 of the AMOC, primarily through deep convective mixing in the Labrador Sea Kuhlbrodt et al 2007;Gelderloos et al 2012). Deep convection in the Nordic Seas is also reported to play a significant role, for instance, Langehaug et al (2012) could link the variability of the AMOC to the variability of the overflows across the Greenland-Scotland Ridge.…”

“…Indeed, the AMOC is primarily sustained by mechanical energy input through winddriven upwelling, gyre circulation, and wind and tidal vertical mixing (Kuhlbrodt et al 2007;Medhaug and Furevik 2011). Therefore, it is likely that the potential extinction of deep convection in the northern hemisphere would not necessarily lead to a collapse of the AMOC (Marotzke and Scott 1999;Kuhlbrodt et al 2007;Gelderloos et al 2012). However, many studies show the important role of deep water formation in setting the strength and the variability Abstract We study the variability and the evolution of oceanic deep convection in the northern North Atlantic and the Nordic Seas from 1850 to 2100 using an ensemble of 12 climate model simulations with EC-Earth.…”

Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean

“…Note that the great salinity anomalies, in spite of their similar magnitudes, had considerably different impacts on the convection intensity 29 : whereas the GSA70, in conjunction with a series of mild winters, effectively shut down deep convection for three consecutive years 8,9 , there was no obvious impact of the last anomaly during the phase of harsh winters with very strong convection in the early 1990s. Although we cannot predict the absolute year-to-year evolution of convection in the future (nor hindcast inter-annual variations beyond 2007), the difference between MELT and CNTR does provide a useful means of isolating the meltwater impact ( Fig.…”