Revisiting the Causal Connection between the Great Salinity Anomaly of the 1970s and the Shutdown of Labrador Sea Deep Convection

The Great Salinity Anomaly (GSA) of the 1970s is the most pronounced decadal-scale low-salinity event observed in the subpolar North Atlantic (SPNA). Using various simulations with the Community Earth System Model, here we offer an alternative view on some aspects of the GSA. Specifically, we examine the relative roles of reduced surface heat flux associated with the negative phase of the North Atlantic Oscillation (NAO) and extreme Fram Strait sea ice export (FSSIE) in the late 1960s as possible drivers of the shutdown of Labrador Sea (LS) deep convection. Through composite analysis of a long control simulation, the individual oceanic impacts of extreme FSSIE and surface heat flux events in the LS are isolated. A dominant role for the surface heat flux events for the suppression of convection and freshening in the interior LS is found, while the FSSIE events play a surprisingly minor role. The interior freshening results from reduced mixing of fresher upper ocean with saltier deep ocean. In addition, we find that the downstream propagation of the freshwater anomaly across the SPNA is potentially induced by the persistent negative NAO forcing in the 1960s through an adjustment of thermohaline circulation, with the extreme FSSIE-induced low-salinity anomaly mostly remaining in the boundary currents in the western SPNA. Our results suggest a prominent driving role of the NAO-related heat flux forcing for key aspects of the observed GSA, including the shutdown of LS convection and transbasin propagation of low-salinity waters.

Using a first-order autonomous dynamical system to evaluate residence time of the Greenland freshwater anomaly in the Subpolar Gyre

<p>Increasing Greenland discharge has contributed more than 5000 km<sup>3</sup> of surplus fresh water to the Subpolar North Atlantic since the early 1990s. The volume of this freshwater anomaly is projected to cause freshening in the North Atlantic leading to changes in the intensity of deep convection and thermohaline circulation in the subpolar North Atlantic. This is roughly half of the freshwater volume of the Great Salinity Anomaly of the 1970s that caused notable freshening in the Subpolar North Atlantic. In analogy with the Great Salinity Anomaly, it has been proposed that, over the years, this additional Greenland freshwater discharge might have a great impact on convection driving thermohaline circulation in the North Atlantic with consequent impact on climate. Previous numerical studies demonstrate that roughly half of this Greenland freshwater anomaly accumulates in the Subpolar Gyre. However, time scales over which the Greenland freshwater anomaly can accumulate in the subpolar basins is not known. This study estimates the residence time of the Greenland freshwater anomaly in the Subpolar Gyre by approximating the process of the anomaly accumulation in the study domain with a first order autonomous dynamical system forced by the Greenland freshwater anomaly discharge. General solutions are obtained for two types of the forcing function. First, the Greenland freshwater anomaly discharge is a constant function imposed as a step function. Second, the surplus discharge is a linearly increasing function. The solutions are deduced by utilizing results from the numerical experiments that tracked spreading of the Greenland fresh water with a passive tracer. The residence time of the freshwater anomaly is estimated to be about 10–15 years. The main differences in the solutions is that under the linearly increasing discharge rate, the volume of the accumulated Greenland freshwater anomaly in the Subpolar Gyre does not reach a steady state. By contrast, solution for the constant discharge rate reaches a steady state quickly asymptoting the new steady state value for time exceeding the residence time. Estimated residence time is compared with the numerical experiments and observations.</p>

Gulf Stream Excursions and Sectional Detachments Generate the Decadal Pulses in the Atlantic Multidecadal Oscillation

Decadal pulses within the lower-frequency Atlantic multidecadal oscillation (AMO) are a prominent but underappreciated AMO feature, representing decadal variability of the subpolar gyre (e.g., the Great Salinity Anomaly of the 1970s) and wielding notable influence on the hydroclimate of the African and American continents. Here clues are sought into their origin in the spatiotemporal development of the Gulf Stream’s (GS) meridional excursions and sectional detachments apparent in the 1954–2012 record of ocean surface and subsurface salinity and temperature observations. The GS excursions are tracked via meridional displacement of the 15°C isotherm at 200-m depth—the GS index—whereas the AMO’s decadal pulses are targeted through the AMO tendency, which implicitly highlights the shorter time scales of the AMO index. The GS’s northward shift is shown to be preceded by the positive phase of the low-frequency North Atlantic Oscillation (LF-NAO) and followed by a positive AMO tendency by 1.25 and 2.5 years, respectively. The temporal phasing is such that the GS’s northward shift is nearly concurrent with the AMO’s cold decadal phase (cold, fresh subpolar gyre). Ocean–atmosphere processes that can initiate phase reversal of the gyre state are discussed, starting with the reversal of the LF-NAO, leading to a mechanistic hypothesis for decadal fluctuations of the subpolar gyre. According to the hypothesis, the fluctuation time scale is set by the self-feedback of the LF-NAO from its influence on SSTs in the seas around Greenland, and by the cross-basin transit of the GS’s detached eastern section; the latter is produced by the southward intrusion of subpolar water through the Newfoundland basin, just prior to the GS’s northward shift in the western basin.

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

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.

Impact of Great Salinity Anomalies on the Low-Frequency Variability of the North Atlantic Climate

In this paper, it is shown that coherent large-scale low-frequency variabilities in the North Atlantic Ocean—that is, the variations of thermohaline circulation, deep western boundary current, northern recirculation gyre, and Gulf Stream path—are associated with high-latitude oceanic Great Salinity Anomaly events. In particular, a dipolar sea surface temperature anomaly (warming off the U.S. east coast and cooling south of Greenland) can be triggered by the Great Salinity Anomaly events several years in advance, thus providing a degree of long-term predictability to the system. Diagnosed phase relationships among an observed proxy for Great Salinity Anomaly events, the Labrador Sea sea surface temperature anomaly, and the North Atlantic Oscillation are also discussed.