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North Atlantic, Largest Freshening Event for 120 Years Caused by Ocean Circulation in Eastern Subpolar Waters

The Atlantic Overturning Circulation carries heat and carbon to the north, and from the surface to the deep ocean, both of which makes it important to the climate system. For the subpolar North Atlantic a prerequisite is the high salinity, and strong freshening could herald a slowdown. In this paper Holliday et al., show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, and the magnitude of this freshening had never before been seen in the 120 years since measurements began. Unusual patterns of winds was the cause, as they drove major changes in the circulation, which included slowing of the North Atlantic current and the diversion of Arctic freshwater from the western boundaries into the eastern basins. They found that routing of freshwater of Arctic origin links intimately conditions of the North West Atlantic shelf and slope region with subpolar basins. They suggest this reveals the importance of atmospheric forcing of intrabasin circulation in determining the salinity of the subpolar North Atlantic.

The high salinity of the upper waters of the North Atlantic Ocean that flow northwards is an essential condition for the formation of deep, cold, dense waters at high latitudes, as part of the meridional overturning circulation (MOC) (Buckley & Marshall, 2016; Ferreira et al., 2018). It has been shown by models that the addition of freshwater to the upper layer of the subpolar North Atlantic (SPNA, 47-65^oN, 0-60^oW) could reduce the salinity sufficiently that the atmospheric cooling has the result of only a cold, fresh light upper layer. This, in turn, potentially weakens deep convection in the Nordic Seas (65-80^oN, 25^oW – 20^oE) and the SPNA, and the density of the deep western boundary currents, which leads to a reduction in the MOC and the associated transport of heat (Gelderloos, Straneo & Katsman, 2012; Böning et al., 2016; Yang et al., 2005).

The upper 1,000 m of the SPNA acquired an extra 6,600 km³ of freshwater during 2012-2016; a rate of change and volume that had not been observed since the late 1960s. In 2012-2016 the distribution of the additional freshwater was not uniform over the SPNA, and tracing the development of the signal and its propagation during the modern period when the ocean was well observed has allowed the identification of the mechanism that led to its development. In the North Atlantic the ocean acquires its high salinity signal by the combination of saline water from the Indian Ocean (Beal et al., 2011), MOC processes in the South Atlantic (Ferreira et al., 2018), and freshwater removal from the surface in the subtropics by evaporation and subsequent atmospheric transport to the Pacific Ocean (Ferreira et al., 2018). The addition of salt and the removal of freshwater over long timescales is balanced approximately by introduction of freshwater from the Arctic Ocean by way of the shallow Labrador Current (LC) and the East Greenland Current (including meltwater from large ice sheets), the importing of freshwater from the Southern Ocean by the South Atlantic subtropical gyre, and SPNA net precipitation. The southern and eastern boundary currents of the subtropical gyre circulation is formed by the North Atlantic Current (NAC [Rossby, 1996]). Northeast of the Grand Banks and Flemish Cap, where the western boundary current of the subtropical gyre (the Gulf Stream) veers sharply to the east is the NAC formation zone. As the NAC crosses the North Atlantic it widens, and separates into branches to the east of the Mid-Atlantic Ridge that flow into the Iceland Basin, the Rockall Trough and southwards to where it re-joins the subtropical gyre (Rossby, 1996). The net effect of the processes of salt and freshwater to the SPNA being subject to temporal variation, is the salinity change that has been observed over interannual and decadal timescales (Reverdin et al., 1997; Hátún et al., 2005; Häkkinen, Rhines & Worthen, 2011; Holliday et al., 2015; Chafik et al., 2016; Friedman et al., 2017). From the late 1960s to the mid-1990s the SPNA underwent a period of freshening (Friedman et al., 2017; Curry & Mauritzen, 2005), that was followed by a decade of increasing salinity (Holliday et al., 2008). The Great Salinity Anomaly is a rapid freshening of the upper layer in the late 1960s has been associated with a reduction in the of winter convection in the Labrador Sea in subsequent years (Gelderloos, Straneo & Katsman, 2012; Dickson et al., 1988). That event, as well as the following prolonged period of low SPNA salinity has been shown to originate from increased precipitation and river runoff in the Arctic that was subsequently transported to the south (Peterson et al., 2006). Interannual variability is superimposed on decadal changes, with notable salinity minima in the 1980s and 1990s (Belkin et al., 1998).

According to Holliday et al. there is a hypothesised link between varying export of freshwater from the Arctic and the salinity of the SPNA (Peterson et al., 2006). In the SPNA increasing salinity (lowered freshwater content) that occurred from the mid-1990s to the late 2000s coincided with the accumulation of freshwater in the Arctic Ocean (Rabe et al., 2014; Polyakov et al., 2013). At the same time, transport of fresh Arctic water into the SPNA by way of the Canadian Arctic Archipelago and LC was low compared to the long-term (70 years) mean transport (Florindo-Lopez, 2019).

In order to explain the long term centennial to decadal scale changes in the North Atlantic, the large-scale circulation changes have been invoked (Delworth & Zeng, 2012); A weaker MOC is associated with decreased transport of ocean heat and convergence and reduced SPNA heat storage and basin-wide sea surface temperature (SST) (Knight, Folland & Scaife, 2006; Smeed et al., 2018; McDonagh et al., 2015). In the 20^th century there is evidence that periods of low SPNA salinity arose in part from a reduction in northwards transport of salt from the subtropics by the MOC, which was associated with changes in wind forcing (Häkkinen, 2002). When the MOC slows down there is less transport of freshwater southwards to the deeper layers and so imported Arctic water is returned in the SPNA gyre, even with a constant import of Arctic water (Mauritzen & Häkkinen, 1999). It is argued by a recent analysis that at 26^oN a reduced MOC after 2018 led to subsequent cooling and freshening of the eastern SPNA (Bryden et al., 2019). During 2014-2016, however, the convergence of transport of freshwater between 2 MOC observational arrays (RAPID at 26^oN (Bryden et al., 2019) and OSNAP at 53-60^oN) appears to be too low to account for a large change in freshwater storage in the SPNA storage of freshwater.

Interaction with the atmosphere is also important; net precipitation changes contribute to changes in salinity (Josey & Marsh, 2005). The first leading mode of the atmospheric  variability, the North Atlantic Oscillation (Delworth & Zeng, 2012) (NAS), is a key driver of change and its associated local wind stress patterns, heat loss and net precipitation can have a cumulative effect over several years (Eden & Willebrand, 2001). The second leading mode of atmospheric variability, the East Atlantic Pattern (Barnston & Livezey, 1987), is believed to regulate the circulation of the subpolar gyre, and the leakage of subtropical waters into the SPNA (Häkkinen, Rhines & Worthen, 2011).

Atmospheric forcing (particularly the NAO and associated wind stress curl) within the SPNA region can alter the regional salinity distribution through changes in the zonal spread of water masses and shifts in the location of the NAC in the Newfoundland Basin and the Iceland Basin/Rockall Trough region (Hátún et al., 2005; Eden & Willebrand, 2001; Barnston & Livezey, 1987; Bersch, 2002; Holliday, 2003; Chafik et al., 2019). The NAC forms a boundary zone, (Subpolar Front between Arctic-influenced cool, freshwaters of the western and central SPNA, and the subtropical-influenced warm, saline waters.

In spite of the conceptual link between the salinity of the SPNA and the transport of freshwater from the Arctic and salt from the subtropics, there is a lack of detailed knowledge of the processes and the way in which they change over time. In this paper Holliday et al. examined the mechanisms that determine the tropical variability of the SPNA upper ocean salinity (0-1,000 m) through a freshening event that was extraordinarily strong that was observed from 2012 to 2016, the fastest and greatest change in the eastern SPNA in 120 years. They showed the unusual winter wind patterns driving major changes in the circulation of the ocean.

Discussion

Holliday et al. have shown that the subpolar North Atlantic underwent a freshening, in the period 2012-2016, on a basin-scale that was more rapid and of a larger magnitude than any changes that had been observed in the previous 5 decades. Also, in the eastern basins the salinity reached levels that were lower than has been observed in any records for the past 120 years. The primary cause of this massive, rapid increase in freshwater content of the region was the large scale changes in circulation of the ocean that were driven by atmospheric forcing.

The 2012-2016 event does not share the same characteristics as the Great Salinity Anomaly, of which much has been written about the causes, in the late 1960s and the early 1970s. Most notable is that the 2012-2016 event was not evident in the Labrador Sea: there was enhanced freshwater export during the Great Salinity anomaly through the Fram Strait, and the wind pattern (negative NAO and East Atlantic Pattern) forced additional freshwater of the Greenland Shelves to spread directly over the Labrador Sea. Therefore the processes that drove the 2012-2016 event are different from those of the Great Salinity Anomaly that occurred 50 years earlier.

Holliday et al. have shown that in the winters of 2012-2016 anomalously strong wind stress curl increased the convergence of freshwater in the subpolar region by rerouting the LC-Arctic to the east off the Newfoundland shelf, by shifting the baroclinic subpolar front to the southern branch of the NAC and by extending the southern branch further to the east. The linkage that has been identified between the wind stress curl pattern and changes in the characteristics of the NAC is consistent with earlier studies which suggested that a stronger cyclonic wind stress curl over the SPNA resulted in cooler, fresher conditions. They had no direct evidence of this being related to a reduction of  penetration of warm, saline subtropical water  into the region as has previously been argued (Häkkinen, Rhines & Worthen, 2011), but they concluded that a new mechanism (redistribution of Arctic water) is important in generating the extreme anomalies that were observed  in 2012-2016, It has been shown by studies that the inclusion of subarctic water into the NAC to the east of the Grand Banks favoured by a weak MOC, and this process has been linked with an intensification of the circulation of the subpolar gyre (Hátún et al., 2005; Foukal & Lozier, 2017) and interior cooling and freshening of SPNA (Häkkinen, Rhines & Worthen, 2013; Piecuch et al., 2017). In this study nuance is added to the picture, during the 2012-2016 period, the increased speed of the gyre is restricted to the southern branch of the NAC, and not the northern branch, which slowed when the baroclinic front moved zonally. It is illustrated by these results that these dynamical changes, which are initiated by the wind stress curl and buoyancy forcing, are reflected directly in the freshwater pathways and therefore, freshwater variability in the subpolar region on a larger scale. Consequently, the exceptionally strong atmospheric forcing that was reported for the winters 2014-2016 and which caused significant loss of heat (Josey et al., 2018) also contributed to a redistribution of freshwater in the North Atlantic that was exceptional.

Some clarity around a recent debate has been provided by Holliday et al. in which the concept of the expansion and contraction zonally of the subpolar gyre (and possibly associated with a spin up or slowdown) in response to atmospheric forcing, was called into question recently (Foukal & Lozier, 2017; Hátún &Chafik, 2018). A lack of clarity in the definition of the gyre and the subpolar front and its relationships with the branches of the NAC is at the centre of the debate. The front forms a boundary between the cold/freshwater Arctic/subpolar water masses, and the warm/saline subtropics-dominated water masses, and is connected dynamically to the baroclinic current cores of the NAC due to the geostrophic equation. Holliday et al. have shown that the subpolar front can shift location; meridionally in the Newfoundland Basin, and in the eastern basins, zonally. The location depends on where (and how much) LC-Arctic water mixes with and modifies the water that was originally warm and saline in the NAC (Desbruyeres, Thierry & Mercier, 2013). Also, it depends on the zonal reach of the southern branch, which does not form part of a closed streamline and was, therefore, not considered to be part of the gyre by (Foukal & Lozier, 2017). A view of the gyre expanding and contracting is given by defining the extent of the gyre by the location of the spread of cold/fresh water, i.e., the location of the polar front by its baroclinic current. In contrast, a view of the gyre that does not change shape or size, results from defining the gyre by the location of its northern NAC branch alone, which forms a closed, mainly barotropic streamline.

The results of Holliday et al. confirms that both interpretations are consistent with the observations, but the expanding subpolar gyre is defined more clearly as the expanding spread of subpolar water masses, a zonal shift of salinity and density isolines, and a zonal shift of the baroclinic NAC current. Attempts to find a link between the size and strength of the gyre itself may be confounded by the zonal shift of the subpolar front and the subsequent impact on the speed of the NAC branches. A high sensitivity of the dynamics of the subpolar gyre and large scale hydrographic characteristics in interannual changes in the atmospheric circulation patterns, is revealed by the freshwater event that was investigated that was the largest for almost 5 decades across the SPNA as a whole, and in the Iceland Basin, for 120 years. Potential changes in the atmospheric forcing in the future, such as the NAO (Bacer, Christoudias & Pozzer, 2016) and associated patterns of wind stress curl will have direct consequences for the basin-wide SPNA freshwater content and the properties that are received downstream, and for the distribution of hydrographic properties within the SPNA and on the NWACSS. The diversion of oxygen-rich and nutrient-rich LC water of Arctic origin into the NAC has had profound consequences for the NWACSS as well as for the eastern basins, the Iceland basin and Rockall Trough). Since 2012 the NWACSS has experienced a rapid increase in marine heatwaves as well as reduced oxygen levels associated with the flooding of the Gulf Stream water onto the shelf, as well as serious impacts on local ecosystems (Claret et al., 2018; Pershing et al., 2015). The ecosystems of the eastern basins, in contrast, have been shown to be stimulated into increased productivity by the arrival of fresh subpolar water in pulses that were substantially weaker than the event described in this paper (Hátún et al., 2017). Understanding the impact of the freshening in 2015 in the Rockall Trough and the Iceland Basin will be an important next step.

In the Iceland Basin and the Rockall Trough the freshwater anomaly is now propagating into the Irminger and Labrador Seas along the pathway of the subpolar circulation, and into the Nordic Seas (Holliday et al., 2018; Arthun, 2017). It has been shown that historical salinity anomalies have taken 4 to 6 years to propagate from 50^oN,30^oW to Svalbard, it therefore might be expected that the Atlantic waters there would be freshening from 2018 onwards. The extent of deep water convection (Gelderloos, Straneo & Katsman, 2012; Böning et al., 2016; Yang et al., 2016) is impacted by changes in salinity and stratification, which also contribute to changes of density of overflow waters and subpolar deep western boundary currents (Dickson et al., 2002) and hence the MOC (Stouffer et al., 2006). This impact of eastern Atlantic salinity anomalies highlights the importance of understanding, and simulating correctly, interactions between the dynamics of the North Atlantic Ocean and the atmospheric circulation for predictions of future climate.

Holliday, N. P., et al. (2020). "Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic." Nature Communications 11(1): 585.