Interruption of the Gulf Stream in 2014 caused the 2014-16 North Atlantic Cold Fresh Anomaly and reciprocal warming/ salinification of the North American coastal region

doi:10.21203/rs.3.rs-1731069/v1

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Interruption of the Gulf Stream in 2014 caused the 2014-16 North Atlantic Cold Fresh Anomaly and reciprocal warming/ salinification of the North American coastal region

https://doi.org/10.21203/rs.3.rs-1731069/v1

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We present evidence that the previously reported 2014-2016 North Atlantic cold fresh anomaly (CFA) and warming/ salinification of the North American coastal region are reciprocal effects of an interruption of Gulf Stream (GS) mass transport near 45oW. In February-March 2014, sudden cooling and reduced salinity near the Milne seamounts (45oN, 40oW) was quantitatively reciprocated by warming and increased salinity near the Tail of the Grand Banks (TGB, 43oN 50oW), creating coincident dipoles of heat and salt anomaly. The magnitude of these changes is consistent with a temporary interruption of about 70-75% of GS heat and salt transport close to where it branches to form the North Atlantic Current (NAC) and Azores Current (AC) near 40-43oN 45oW. The negative heat and salt anomalies near 40°W subsequently migrated northeastwards to constitute the CFA (centred on 50oN 30oW) while the reciprocal positive anomalies near 50°W spread westwards and southwards into the North American coastal region. The quantitative reciprocity of heat and salt anomalies in the CFA and North American coastal region can thus be explained by an interruption of mass transport between the GS and NAC. Although there has been much speculation regarding the possibility that global warming might block transport of heat and salt in the GS (possibly leading to extreme climate changes) this is the first evidence that a short-lived interruption of GS transport has already occurred.

There has been recent interest in the unusual short-term cooling and freshening of the subpolar North Atlantic (SPNA) between 2014 and 2016 (1–4) which contrasted sharply with rising temperatures in the rest of the global ocean during this period (5). In this work we refer specifically to the 2014-16 feature as the cold fresh anomaly (CFA) because we believe that as pointed out by Josey et al. (2), this short-term cold anomaly is quite distinct from the long-term pre-2014 cold anomaly (so-called ‘warming hole’) originally described by Drijfhout et al. (6) and subsequently investigated by others (7–14). Generally, this difference between these two ‘cold blobs’ has not been recognised sufficiently, leading to conflation of the pre-2014 long term cold anomaly with the short term 2014-16 CFA and potential confusion in this area. Some studies of the 2014-16 CFA have been confined either to heat anomalies (1, 2) or salt anomalies (3) although Bryden et al. (4), examined both parameters in parallel, which is what we have endeavoured to do here.

Heat and salt transport from the subtropics to the subpolar SPNA are thought to occur largely via the Gulf Stream (GS) and its extension, the North Atlantic Current (NAC) but no specific changes in the transmission of heat and salt between these currents have been identified which might illuminate the mechanism of cooling and freshening in the SPNA in 2014–2016. There is little evidence for a marked change in flow in the GS at 38^oN between 1993 and 2013 (16) and although a small reduction in meridional heat transport at 26°N was apparent from 2009–2012 (17) anomalies between 2014 and 2016 appeared to stable (17) or slightly positive (18). Bryden et al. (4) proposed that changes in the GS only occurred north of 40°N (and consequentially east of 60°W) where enhanced recirculation of GS waters may occur resulting in cooling and freshening of the SPNA. However, Grist et al. (1) and Josey et al. (2) emphasised the likely role of extreme atmospheric influences in cooling the SPNA in 2013–2014 while Holliday et al. (3) explained the freshening as a consequence of unusual wind patterns changing ocean circulation so as to introduce cold fresh Arctic water into the SPNA.

East of the Tail of the Grand Banks (TGB, 43°N 50°W) the GS branches to form the NAC (about two thirds of GS flow) and the Azores Current (AC, about one third) (19–22) although recirculation pathways return some GS water westwards (23). There is some uncertainty about the precise location of the branching point but because the AC appears to originate near 45^oW (21, 22) this longitude should also define the eastern end of the GS and the origin of the NAC (Fig. 1).

The TGB region has fundamental significance as the major site where the cold (< 4^oC) fresh(< 33.5psu) Labrador Current moving south through between the Grand Banks and Flemish Cap (Flemish Pass) meets the warm (> 15^oC) salty (> 36.0 psu) waters of the GS moving eastwards (19,20 and Fig. 1). Near the TGB a large proportion of LC flow undergoes a radical retroflection which entrains it gradually into the NAC, decreasing the temperature and salinity of the NAC compared with the GS and possibly adding momentum to the combined current (20). The route of the NAC as far as the Charlie-Gibbs Fracture Zone (52^o-53^oN) lies mainly over water at least 4000m deep, largely avoiding the Grand Banks (24) and two seamount groups: the Newfoundland group centred on 43.5^oN 45^oW (here abbreviated NEWF) and the Milne group near 45^oN 40^oW (Fig. 1). The northern branch of the NAC subsequently passes east of Flemish Cap and along the line of the Charlie-Gibbs Fracture Zone (CGFZ) (24, 25) while branching further as it negotiates the Mid-Atlantic Ridge (26) and enters the northeastern SPNA.

Here we present evidence that the CFA originates as the consequence of a large but transient interruption of heat and salt delivery from the GS to the NAC near 45°W in early 2014 and this heat and salt ‘missing’ from the NAC and the SPNA is recirculated into the GS region leading to reciprocal increases of heat and salt near the North American coast and further south.

Salinity and ocean heat content (OHC) data were based on the UK Met Office EN4 version 2.2 analysis (27) including the modifications proposed by Gouretski et al. (28) and were obtained from https://www.metoffice.gov.uk/hadobs/en4/ and are © Crown Copyright, Met Office, [2013], http://www.nationalarchives.gov.uk/doc/non-commercial-government-licence/version/2/. Data were accessed through Climate Explorer (http://climexp.knmi.nl) which was also used to generate latitude versus longitude plots and Hovmöller (time versus latitude or longitude) plots.

Figure 2 compares the 0-400m anomalies of ocean heat content (OHC) and salinity in the North Atlantic in July 2015 when the CFA was strongest. It identifies the CFA as the region centred near 50^oN 30^oW which exhibits deep almost coincident negative anomalies of OHC (a) and salinity (b) and a region near the North American coast which is close to the path of the GS (24) and shows deep positive anomalies of heat and salinity. These findings are similar to those described by Bryden et al. (4) and with respect to salinity, resemble those reported by Holliday et al. (3). We note further areas of positive anomaly in the region south of 35^oN both near the American coast and further west.

Latitudinal changes in ocean heat content (OHC) and salinity in the top 700m at 70^oW (highlighting the GS region positive anomalies shown in Fig. 2) and at 30^oW (highlighting the negative anomalies in the CFA) are shown in Fig. 3. Figure 3a shows that at 70^oW between 38^oN and 44^oN (intersecting the GS) there was a sudden onset of pronounced warm anomalies reaching 5GJ/m² from winter 2012 and this became more sustained from 2014, strengthened in 2015-16 and faded by 2017. There was also an increase in OHC anomaly at lower latitudes down to 20^oN which was first observed in 2015 but continued thereafter (not shown). In approximately the same time frame that these warming changes were seen in the GS region, a pronounced cold anomaly developed at 30^oW and 45^o-55^oN corresponding to the CFA (Fig. 2) mainly in the period 2014-16 but with small cooling episodes in the winters of 2012 and 2013 (b). It is important to note that the results for salinity anomalies (c,d) were very similar to those for OHC and particularly for the CFA region (b,d). This evidence confirms the correspondence between warming/increased salinity in the GS region and cooling/decreased salinity in the CFA region previously indicated in Fig. 2 and in (4). It also shows that the largest anomalies (positive and negative) occurred between 2014 and 2017.

This apparent inverse relationship between the CFA and GS regions in terms of salinity and temperature anomalies was further examined using EN4 data for 0-1000m along a line between 36°N 75°W and 52^oN 19^oW over the period from January 2014 (when changes were first seen) to July 2015 (when CFA anomalies were greatest) (Fig. 2). This line (shown in red in Fig. 2a) was chosen because it intersects the GS between 75^oW and 55^oW (24), the region near the TGB at 42^oN 50°W (20, 22), the NAC near 40^oW, east of Flemish Cap (24) and the CFA epicentre at 50^oN 30^oW (Figs. 2,3).

In Fig. 4 there were three constant features in terms of raw temperature (column 1) and salinity (column 3) and in all such data between 2010 and 2019 (not shown). The first of these was a wedge of very warm (> 15^oC) salty (> 36 psu) water between 36^oN 75^oW and 42^oN 52^oW which is likely to represent the GS (24). The second was a very cold (< 5^oC) fresh (< 34.6 psu) column of water at about 43^oN 50^oW extending from close to the surface down to at least 1000m and which is likely to correspond to the Labrador Current (LC) near where it intersects the GS close to the TGB (19,20, Fig. 1). The third was a warm (10^o-14^oC) salty (35.2-35.7psu) layer near 46^oN 40^oW extending from the surface to about 500m depth which is likely to correspond to the NAC (24). In what follows we refer to these three features as GS, LC and NAC regions respectively.

Beginning in February 2014 and climaxing in March 2014 there were pronounced changes in the distribution of temperature and salinity (columns 1 and 3 respectively) and their anomalies (columns 2 and 4 respectively). The temperature in the LC region rose from about 5^oC to 9^oC and salinity from 34.7 to 34.9psu while the NAC region dropped from about 14^oC to about 11^oC and from 35.7 psu to 35.2 psu. These changes were reflected in the temperature and salt anomalies (columns 2 and 4 respectively) for February and March which were strongly positive (about + 3^oC and 0.4psu) near 50^oW but strongly negative (-3^oC and − 0.35psu) near 40^oW. During February and March these adjacent anomalies formed a positive/negative dipole separated by a null region at about 44^oN 45^oW although the development of the positive anomaly slightly preceded that of the negative anomaly. By August 2014 the LC and NAC regions had largely been restored to their original state in January 2014 (columns 1 and 3) and the considerable anomalies of temperature and salinity at 50^oW and 40^oW had mostly faded away. However, there were indications that the positive anomalies had moved westwards towards the North American shelf and the negative anomalies had moved progressively eastwards from about 40^oW in March to near 30^oW in July 2014 and within the CFA region (see Fig. 1). To add more precision to the positions of the warm/saline and cold/fresh anomalies at 50°W and 40°W respectively, latitude-longitude plots were constructed for salinity and OHC at 0-400m (Fig. 5).

Figure 5 confirms the development in February and March 2014 of a dipole composed of the positive warm/salty anomaly at 42^oN and 55-60^oW (close to the TGB) and the cold fresh anomaly centred on 45^oN, 41^oW close to the Milne seamounts (see Introduction) corresponding to a cooling and freshening of the NAC region. For simplicity we refer subsequently to these components of the dipole as ‘TGB’ and ‘Milne’ to give them the precision of a geographic reference point without implying any functional connection between the geographical points and these anomalous heat and salinity features. From April onwards there was rapid attrition of the original dipole as the centre of the warm/salty anomaly moved southwestwards back into the GS region and the centre of the cold/fresh anomaly moved northeastwards into the CFA region (Fig. 5). By July 2015 (see Fig. 2) the negative anomalies of heat and salt corresponding to the CFA near 50^oN 30^oW and the positive anomalies in the GS region near the American coast had strengthened further.

Using the information from Figs. 4 and 5 we examined quantitative changes in anomalies of OHC (Fig. 6a) and salinity (Fig. 6b) at the epicentres of ‘TGB’ (42°N 50°W) and ‘Milne’ (45°N 41°W) and anomalies of total heat (Fig. 6c) and salt content (Fig. 6d) in wider regions corresponding to the warm and cold components of the Feb/March dipole. The wider areas chosen were 40°-45°N 60°-45°W (‘TGB’, 0.67 million km²) and 40°-47°N 45°-35°W (‘Milne’, 0.62 million km²). For both heat (a,c) and salinity (b.d) there was good correspondence between the time series for the epicentres (a,b) and those for the wider area (c,d). There was also good correspondence between the heat (a,c) and salinity (b,d) anomalies time series at ‘TGB’ and ‘Milne’ for 2013–2016 (Fig. 5) and this close correspondence also applies to the longer period from 2000 to 2020 (not shown). It is expected that two regions relatively close together on the path of the GS/NAC would have similar time series for OHC and salinity anomalies.

This is why the events of February-April 2014 were so exceptional, with transitory large increases of heat and salt at ‘TGB’ accompanied by reciprocal decreases at ‘Milne’ in both the punctate (a,b) and the wider (c,d) areas, consistent with the changes seen in Figs. 4,5. It is of crucial significance that the increases at ‘TGB’ and the decreases at ‘Milne’ were so similar in magnitude because a temporary interruption of GS flow near ‘NEWF’ is the only mechanism that can readily explain why heat and salt accumulate at ‘TGB’ while ‘Milne’ is deprived of a closely equivalent amount of heat and salt. There is no indication of another similar event in the period between 2014 and 2016 (Fig. 6) or indeed at any other time between 2000 and 2020 (not shown). Almost identical results were obtained using 0-700 m data but with increases of about 20% in the values compared with 0-400m.

The increase in heat content at ‘TGB’ between January and February 2014 in Fig. 6c is about 1.5ZJ so the rate of change of energy (power) during this 1month period (about 2.6 million seconds) is about 0.6 petawatt. This value is about 50% of previous estimates of the entire power of the Gulf Stream at 26^oN (15, 16) and 75% of the power at 45^oN (15) implying that in February 2014 about three quarters of the normal heat transfer capacity of the GS was retained within the ‘TGB’ region centred on 50°W and did not reach the NAC. The corresponding deficit in heat reaching the NAC at ‘Milne’ (− 1.4 ZJ) in February 2014 is approximately reciprocal to the increase at ‘TGB’ (+ 1.5ZJ). This implies relatively small transfers to the atmosphere, deeper ocean levels and lateral regions outside the designated areas. The corresponding changes in salt content in January-February 2014 were ± 50 Gtonnes in 1 month or about 25000 tonnes/sec. Total NAC transport of fresh water is about − 1 Sv (million m³/sec) at 44°N 45°W (18) which is equivalent to 35000 tonnes/sec of salt assuming a standard salinity of 35psu. Thus about 70% of total salt transport in the NAC appears to be blocked in early 2014, quite similar to the result for the extent of the putative heat block. The changes in early 2014 which we associate with a hiatus in transport near ‘TGB’ were slightly earlier (perhaps half a month) than the changes at ‘Milne’ (Figs. 3–6).

Figure 6 highlighted the reciprocal change in heat and salt distribution between ‘TGB’ and ‘Milne’ in a few months in early 2014. Figure 7 is a similar assessment of the heat and salt budget but over a longer time scale and much wider area, comparing the GS region (25^oN-45^oN 75^oW-45^oW, 5.1 million km²) with the CFA region (40^oN-60^oN 45^oW-20^oW, 4.8 million km²) (see rectangles outlined in Fig. 2a). It is apparent from Fig. 7a that the time series of heat anomalies in the CFA region shows largely negative values (average anomaly about − 5 ZJ between 2014 and 2017) which approximately match the positive values of those in the GS region (+ 7 ZJ). Consistent with the results of Figs. 4–6, the time series for total salt content (Fig. 7b) is very similar to that for total heat content (Fig. 7a) for the CFA region (and to a lesser extent for the GS region).

Although the unprecedented sudden event in early 2014 seen in Fig. 6 is also apparent in Fig. 7, there are subsequent changes which continue to decrease the heat and salt anomaly in the CFA region in 2015 and 2016 before the final disappearance of the anomaly in 2017. It is difficult to relate these individual changes in the CFA directly to the increases in anomalies in the GS region although overall there is a good correspondence between the minimum values for the CFA region in 2015-16 and the maximum values for the GS region in this time frame.

The salinity data in Fig. 7 allows an estimate to be made of the total fresh water which is equivalent to the maximum total salt deficit in July 2014 (about 200 GT from Fig. 7b). The volume of fresh water to be added to this amount of salt in order to reach the reference salinity of 35 psu (35g/l) is about 5700 km³ of fresh water. Results for 0-700m depth were very similar to the 0-400 m data and gave a value of about 7100 km³ of fresh water which agrees well with the estimate of 6600 km³ of Holliday et al. (3) for the fresh water equivalent of the 2014-16 salt deficit in the CFA.

This work provides evidence that the 2014–2016 North Atlantic cold fresh anomaly (CFA) was balanced by reciprocal warm, salty anomalies in the GS region on the same time scale (Figs. 2,3,7). Such observations recall the findings of Grist et al. (1) Robson et al. (8) and Smeed et al. (18) with regard to temperature anomalies, Holliday et al. (3) who considered salinity data since 1950, and Bryden et al. (4) who compared anomalies of OHC and salt in 2014-16 using 2007–2009 data as a control. The latter authors showed an approximately inverse relationship between the reductions of salt and heat centred on 46°N 30°W (equivalent to our CFA) and rises in these parameters near the North American coastal region in 2014-16. They suggested the possibility that the cooling of the subpolar gyre since 2008 was due to a hiatus in delivery of warm saline water from the subtropics. However, there has been no marked alteration in the magnitude of heat transport in the GS measured at 26°N-40°N (16–18) so any reduction in heat and salt content anomaly in the CFA and equivalent rises in these parameters in the GS region demonstrated either previously or in the present work (Figs. 2,4,7) for 2014-16 cannot be traced to changes in the GS flow west and south of 40^oN.

The distinctive feature of our work is that we have tentatively identified a site at 40–43°N 45°W (NEWF in Fig. 1) where adjacent dipoles of heat and salt anomaly appeared suddenly in early 2014 and that event can most easily be explained by a major but temporary interruption of mass transport between the GS and NAC. This unusual feature subsequently evolved into a wider dipole composed of the positive anomalies near the North American coast and negative anomalies in the SPNA (CFA) which have been previously recognised (1–4). Thus our findings support important aspects of the work of Bryden et al. (4) but point to a hiatus in heat and salt transport in the GS near 40°-43°N 45°W which establishes the initial heat and salt anomaly dipole.

In Feb-March 2014 there was a large accumulation of heat and salinity near ‘TGB’ which corresponds to the point where the Labrador Current meets the GS and a matching cooling and freshening centred on 45^oN, 40^oW (‘Milne’) corresponding to the path of the NAC (Figs. 3–5). The key feature of these observations is the high degree of reciprocity between the short-lived positive anomalies of heat and salt at ‘TGB’ and negative anomalies at ‘Milne’ which is difficult to explain except by an interruption of mass transport at a point between these two sites, possibly corresponding to the null region of the dipole (40^o-43°N 45^oW near ‘NEWF’, Figs. 1, 4,5). This null region is close to the site where the GS bifurcates to form the NAC and AC (21, 22) which itself represents an implicit major interference with GS flow occurring under normal circumstances. It is also close to the semi-permanent anticyclonic vortex known as the Mann eddy (42°N 44°W) which is known to influence the path of the NAC northwards (24). Whether the Mann eddy or the Newfoundland seamounts have a significant influence on branching of the GS and/or the putative GS interruption in 2014 is unknown. We have attempted to follow salt and heat changes in the AC in 2014 but the results were ambiguous because the changes were relatively small and were obscured by the larger changes appearing at ‘TGB’ and ‘Milne’.

Subsequently, heat and salt anomalies initially elevated near ‘TGB’ were retroflected into the GS region (Figs. 3–5), possibly through known recycling pathways (23). Meanwhile, the cooling and freshening at ‘Milne’ was transmitted northeastwards in the normal branching of the NAC (26) so that by 2015 the epicentre of the CFA was near 50^oN, 30^oW, 1000 km from ‘Milne’ (Figs. 2,4,5,7). In effect, the heat and salt deficit represented by the CFA is found as reciprocal surpluses in the GS region (Fig. 7) so the initial heat and salinity dipole between 50°W and 40°W in early 2014 (Figs. 4,5) became a wider and more diffuse dipole between the CFA and the GS region (Fig. 2,4,7).

This apparent hiatus in transport of heat and salt near the GS terminus at 45^oW in February-March 2014 appears to represent 70–75% of normal GS flow (see Results) so it constitutes a major and very unusual event, not seen anywhere else in the EN4 record (not shown). The CFA continued to become colder and fresher until summer 2015 and the heat and salt anomalies persisted until late 2016 while the GS region became warmer and saltier until 2017 (Fig. 7). From Fig. 6 it appears that the maximum heat and salt anomaly in the initial dipole was about ± 1.4 ZJ and ± 50 Gtonnes of salt in areas of about 0.6 million km² whereas in Fig. 7 the much larger areas (about 5 million km²) of the GS and CFA region show maximum anomalies of about ± 6 ZJ and ± 200 GTonnes of salt. Thus only about 25% of the heat and salt deficit eventually accumulated in the CFA can readily be accounted for by the initial dipole in March 2014. Of course, these are fast-moving anomalies and more detailed studies involving further sections similar to Fig. 4 are required to properly establish the budgets for heat and salt transport from the original dipoles (Figs. 4,5) to the wider dipoles between the CFA and North American coastal region (Fig. 2).

It is important to emphasise that the maximum salt deficit in the CFA in July 2015 (equivalent to about 6000-7000km³ of fresh water) agrees very well with the estimate obtained by Holliday et al. (3) who described it as ‘the largest freshening event for 120 years’ in the eastern SPNA’. This underlines that the scale of the heat and salinity changes which began in early 2014 are commensurate with an initial temporary block of the GS close to its branching point near 45^oW.

These findings are consistent with the hypothesis that the CFA is the result of a transport hiatus in the GS near its terminus near 45^oW which affects heat and salt equally (see Fig. 3) and thus is likely to represent an interruption of mass transfer from the GS into the NAC. It is unclear why there should be a temporary accumulation of positive anomalies at ‘TGB’ and negative anomalies at ‘Milne’ as a consequence of a block of mass transport near the GS terminus but may reflect the fact that currents are faster between 50^oW (‘TGB’) and 40^oW (‘Milne’) than further east or west (29, 30). Thus arrival of negative anomalies at ‘Milne’ would have been faster than their departures eastwards leading to a temporary accumulation.

It is difficult to fit our findings into other schemes which envisage the CFA is the result of acute atmospheric cooling (1, 2) or the influence of an extraneous source of fresh water (3). A distinctive feature of our hypothesis is that it does not require an extraneous source of wind or water to explain either the CFA or the warming and increased salinity of the North American continental shelf; it needs only a temporary block of mass transport in the GS to explain the negative anomalies in the CFA and the obverse positive anomalies in the GS region near the American coast. Even supposing that the cooling and freshening of the CFA were simply due to the incorporation of more cold fresh LC water into the NAC, this would not explain why the GS region accumulates the heat and salt which is missing from the CFA. (Figs. 2–4). Furthermore, LS flow near the TGB is only about 5% of GS flow (20) and is unlikely to produce a change as radical as the 2014 event simply by mixing more LS water into the GS. Whatever the precise mechanism of formation of the heat/salt dipole centred near 45°W in early 2014, we do not exclude the possibility that it could depend on sudden and extraordinary local changes in atmospheric conditions, although there are no obvious indications of these in the available data (not shown). However, it would not be surprising if the large changes in sea temperature near the initial dipole in early 2014 had unusual atmospheric effects. It has already been suggested that the subsequent evolution of the CFA might have had consequences for European climate (31) and it would be interesting to look for changes in the climate of the American eastern coastal region corresponding to the warming of that region in 2014-16. There are indications that in the longer term, heat and salt ‘missing’ from the CFA might appear further south and east of the American coastal region (Figs. 2,3).

If the early 2014 heat and salt dipole is due to a hiatus in transport near the GS terminus, the mechanism of such a blockade is obscure. This region is close to the primary site of interaction between the GS and the LC (19, 20) so it is possible that some extraordinary clash between these contrasting currents in early 2014 caused the transport hiatus. Either a southward movement or increased flow of the LC or a northward movement of the GS might increase interaction between the two currents making it more likely that GS flow would be interrupted. Indeed, there is recent evidence for a slight northwards migration of the GS towards the TGB since 2008 and this is associated with a pronounced warming and increased salinity along the North American coastal region (32) which we would explain in terms of retroflection of GS flow, perhaps via established recycling pathways (23). This observational evidence is supported by modelling of future trends which predict a northward movement of the GS (7, 10). Of course, these possible movements of the interface between the cold fresh northern waters and the warm salty southern waters are likely to be characterised by local events rather than wholesale uniform changes and it is the local events which will dictate the anomalies of heat and salt which are measured. Increased LC flow is a likely consequence of the currently enhanced melting of the Greenland ice cap and may therefore represent a long-term factor which could impact on GS flow near ‘TGB’.

In recent years there has been intense speculation in the scientific literature and even in the popular press regarding the possibility and climate consequences of a block in the GS precipitated by increasing global temperatures. Our findings are evidence for a transient block in the GS in early 2014 near 45^oW which caused large changes in the distribution of heat and salt across the North Atlantic that are likely to have had widespread effects on climate (5, 31). Future such events should be characterised by the dipolar anomalies of heat and salt described here and may become more frequent in a warming world.

Acknowledgments

Richard Allan received support from the Natural Environment Research Council SMURPHS project (NE/N006054/1).

Grist, J.P., Josey, S.A., Jacobs, Z.L., Marsh, R., Sinha, B. and Van Sebille, E. (2016) Extreme air–sea interaction over the North Atlantic subpolar gyre during the winter of 2013–2014 and its sub-surface legacy. Clim. Dyn., 46 (11), 4027-4045. (doi:10.1007/s00382-015-2819-3).
Josey, S A., Hirschi, J.J-M., Sinha, B., Duchez, A., Grist, J.P. and Marsh, R. (2018). The recent Atlantic cold anomaly: causes, consequences and related phenomena. Ann. Rev. Marine Sci. 10, 10 (1). 475-501. doi:10.1146/annurev-marine-121916-063102.
Holliday, N.P., Bersch, M., Berx, B. et al. (2020) Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Commun. 11, 585. doi.org/10.1038/s41467-020-14474-y.
Bryden, H.L., Johns, W.E., King, B.A., McCarthy, G., McDonagh, E.L., Moat, B.I. and Smeed, D.A. (2020). Reduction in Ocean Heat Transport at 26°N since 2008 Cools the Eastern Subpolar Gyre of the North Atlantic Ocean. J. Climate, 33, 1677–1689. doi:10.1175/JCLI-D-19-0323.1.
Yin, J., Overpeck, J., Peyser, C., & Stouffer, R. (2018). Big jump of record warm global mean surface temperature in 2014–2016 related to unusually large oceanic heat releases. Geophys. Res. Letts, 45, 1069– 1078. doi.org/10.1002/2017GL076500.
Drijfhout, S., van Oldenborgh, G.J. and Cimatoribus, A. (2012) Is a decline of AMOC causing the warming hole above the North Atlantic in observed and modelled warming patterns? J. Climate 25, 8373-8379. doi: 10.1175/JCLI-D-12-00490.1.
Rahmstorf, S, Box, J. Feulner, G., Mann, M., Robinson, A., Rutherford, S. & Schaffernicht, E. (2015). Exceptional twentieth-Century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change. 5. 10.1038/nclimate2554.
Robson, J., Ortega, P. & Sutton, R. A reversal of climatic trends in the North Atlantic since 2005. Nature Geosci. 9, 513–517 (2016). doi.org/10.1038/ngeo2727.
Caesar, L., Rahmstorf, S., Robinson, A. et al. (2018) Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556, 191–196. doi.org/10.1038/s41586-018-0006-5.
Caesar, L., McCarthy, G. D., Thornalley, G. J. R., Cahill, N., Rahmstorf, S. (2021): Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nat. Geosci. 14, 118–120. doi.org/10.1038/s41561-021-00699-z.
Boers, N. (2021) Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Clim. Change 11,680-688. doi.org/10.10.38/s41558-021-01097-4.
Keil, P., Mauritsen, T., Jungclaus, J. et al. (2020). Multiple drivers of the North Atlantic warming hole. Nature Climate Change 10, 667–671. doi.org/10.1038/s41558-020-0819-8.
Gervais, M., Shaman, J. and Y. Kushnir, Y. (2018). Mechanisms governing the development of the North Atlantic Warming Hole in the CESM-LE Future Climate Simulations. J. Climate, 31, 5927–5946. doi:10.1175/JCLI-D-17-0635.1.
Allan, D. and Allan, R. P. (2019) Seasonal changes in the North Atlantic cold anomaly: The influence of cold surface waters from coastal Greenland and warming trends associated with variations in subarctic sea ice cover. J. Geophys. Res. Oceans, 124, 9040-9052. doi:10.1029 /2019JC015379.
Li, L., Lozier, M.S. & Li, F. Century-long cooling trend in subpolar North Atlantic forced by atmosphere: an alternative explanation. Clim. Dyn. 58, 2249–2267 (2022). doi.org/10.1007/s00382-021-06003-4
Rossby, T., C. N. Flagg, K. Donohue, A.Sanchez-Franks, and J. Lillibridge (2014),On the long-term stability of Gulf Stream transport based on 20 years of direct measurements, Geophys. Res. Lett.,41,114–120, doi:10.1002/2013GL058636.
Liu, C., Allan, R.P., Mayer, M. et al. (2020) Variability in the global energy budget and transports 1985–2017. Clim. Dyn. 55, 3381–3396 (2020). doi.org/10.1007/s00382-020-05451-8.
Smeed, D. A., Josey, S. A., Beaulieu, C., Johns, W. E., Moat, B. I., Frajka-Williams,E., et al. (2018). The North Atlantic Ocean is in a state of reduced overturning. Geophys. Res. Letts, 45, 1527–1533. doi.org/10.1002/2017GL076350.
Klein, B., and Siedler, G. (1989), On the origin of the Azores Current, J. Geophys. Res., 94( C5), 6159– 6168, doi:10.1029/JC094iC05p06159.
Caldeira, R.M.A. and Reis, J.C. (2017) The Azores Confluence Zone. Front. Mar. Sci. | doi.org/10.3389/fmars.2017.00037
Krauss, W., Käse, R. H., and Hinrichsen, H.-H. (1990), The branching of the Gulf Stream southeast of the Grand Banks, J. Geophys. Res., 95(C8), 13089– 13103, doi:10.1029/JC095iC08p13089.
Fratantoni, P.S. and McCartney, M.S. (2010) Freshwater export from the Labrador Current to the North Atlantic Current at the Tail of the Grand Banks of Newfoundland. Deep-Sea Res. I 57(2) 258-283. doi:10.1016/j.dsr.2009.11.006.
Jacobs, Z. L., Grist, J. P., Marsh, R., Sinha, B., & Josey, S. A. (2020). The major role of air-sea heat fluxes in driving interannual variations of Gulf Stream transport. J.Geophys.Res.: Oceans, 125, e2019JC016004. doi.org/10.1029/2019JC016004.
Rossby, T. (1996), The North Atlantic Current and surrounding waters: At the crossroads, Rev. Geophys., 34( 4), 463– 481, doi:10.1029/96RG02214.
Bower, A.S. and von Appen, W.-J. (2008) Interannual variability in the pathways of the North Atlantic Current over the Mid-Atlantic Ridge and the impact of topography. J. Phys. Oceanog. 38, 104-120. doi:10.1175/2007JPO3686.1.
Stendardo, I., Rhein, M., & Steinfeldt, R. (2020). The North Atlantic Current and its volume and freshwater transports in the subpolar North Atlantic, time period 1993–2016. J. Geophys. Res. Oceans, 125, e2020JC016065. doi.org/10.1029/2020JC016065.
Good, S. A., M. J. Martin and N. A. Rayner, 2013. EN4: quality-controlled ocean temperature and salinity profiles and monthly objective analyses with uncertainty estimates, J. Geophys. Res. Oceans, 118, 6704-6716. doi:10.1002/2013JC009067.
Gouretski,V. and Reseghetti, F. (2010) On depth and temperature biases in bathythermograph data: development of a new correction scheme based on analysis of a global ocean database. Deep-Sea Res. I, 57, (6) 812-833. doi.org/10.1016/j.dsr.2010.03.011.
Fratantoni, D. M. (2001), North Atlantic surface circulation during the 1990's observed with satellite-tracked drifters, J. Geophys. Res., 106( C10), 22067– 22093, doi:10.1029/2000JC000730.
Reverdin, G., Niiler, P. P., and Valdimarsson, H. (2003), North Atlantic Ocean surface currents, J. Geophys. Res., 108( C1), 3002, doi:10.1029/2001JC001020,.
Duchez, A., Frajka-Williams, E., Josey, S.A., Evans, D.G., Grist, J.P., Marsh, R., McCarthy, G.D., Sinha, B., Berry, D.I. and Hirschi, J.J-M. (2016). Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave. Environ. Res. Letts, 11(7). doi.org/10.1088/1748-9326/11/7/074004
Gonçalves Neto, A., Langan, J.A. & Palter, J.B. Changes in the Gulf Stream preceded rapid warming of the Northwest Atlantic Shelf. Commun Earth Environ 2, 74 (2021). doi.org/10.1038/s43247-021-00143-5.

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Interruption of the Gulf Stream in 2014 caused the 2014-16 North Atlantic Cold Fresh Anomaly and reciprocal warming/ salinification of the North American coastal region

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