Rapid Cooling and Increased Storminess Triggered by Freshwater in the North Atlantic

Recent winters have been unique due to the rapid and extreme cooling of the subpolar North Atlantic. Here, we present a novel view on its causes and consequences. Combining in‐situ observations with remote sensing and atmospheric reanalysis data, we show that increased freshening of the subpolar region gives rise to a faster surface cooling in fall and winter. Large freshwater events, in particular, result in pronounced cold anomalies with sharp temperature gradients that promote an enhanced storminess. The storms reinforce the cooling by driving stronger heat losses and modulating the surface flow. Consistent with this mechanism, past freshwater events have been followed by cold anomalies in winter of approximately −2°C and increases in the North Atlantic Oscillation index of up to ∼0.6 within 3 years. We expect that future freshwater discharges into the North Atlantic will amplify the cold anomaly and trigger an enhanced wintertime storminess with far‐reaching climatic implications.


Introduction
Recent winters have been characterized by a rapid and extreme cooling of the subpolar North Atlantic, which has been unprecedented in over 30 years and stands in marked contrast to the warming observed over most of the Earth's surface (Josey et al., 2018). Given the importance of the North Atlantic sea surface temperature (SST) for the large-scale weather and climate (Czaja & Frankignoul, 2002;Sutton & Dong, 2012), it is critical to understand the causes and effects of this anomaly.
Previous studies have attributed the cooling to a slowdown of the Atlantic overturning circulation due to increased freshwater fluxes from Greenland (Caesar et al., 2018;Rahmstorf et al., 2015). This idea is motivated by paleoclimate records, suggesting that past cooling events were caused by a freshwater-forced shutdown of deep ocean convection in the subpolar North Atlantic and a subsequent collapse of the overturning (Barber et al., 1999;Clark et al., 2001Clark et al., , 2002. However, observations show that this buoyancy-driven mechanism cannot easily explain the recent heat transport into the subpolar region (Lozier et al., 2019).
The influences of the cold anomaly on the climate are likewise uncertain. Yet, earlier studies have found that increased SSTs in the subpolar region initially trigger a transient baroclinic response in the atmosphere, forcing enhanced ocean heat losses (Deser et al., 2007;Kushnir et al., 2002). After a few weeks, a barotropic equilibrium response emerges that is associated with reduced ocean heat losses and thus represents a positive feedback to the SST anomaly (Deser et al., 2007;Kushnir et al., 2002).
However, we hypothesize that freshwater modulates this atmospheric response by strengthening the stratification, allowing for a faster adjustment of the surface to the lower air temperatures in fall and winter. By eroding the SST anomaly during the transient response, the faster surface cooling interferes with the setup of ©2020. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. the equilibrium response and instead increases the meridional SST gradient, key source of baroclinic instability (Hoskins et al., 1985). To test this hypothesis, we investigate the chain of mechanisms initiated by freshwater. The main data products involved in this study are listed in Table 1, while a detailed data description is provided in the supporting information.

Detection of Surface Freshwater
Motivated by earlier studies that discovered a significant anticorrelation of the North Atlantic Oscillation (NAO) in summer with the melting over Greenland (Hanna et al., 2013) and the Arctic sea ice export (Haine et al., 2015), both potential sources of freshwater, we use the mean NAO in July and August, multiplied by −1, as an index ("F NA ") to describe the freshwater variability in the subpolar North Atlantic (Figure 1a).
Based on a scaling analysis of the surface mass balance, we find that the surface freshening associated with F NA can be estimated from (1) where both sides have been regressed on F NA , α and β are the thermal and haline expansion coefficients, SSS is the surface salinity, and Δ refers to the change from summer to winter (see supporting information for details).
The inferred surface freshening is most pronounced in the western subpolar region, off the coast of Newfoundland, from where it expands eastward into the central gyre, following the mean geostrophic flow (Figure 1b). Both F NA and the inferred freshening are characterized by a significantly positive trend over the investigated period and their correlation is largest on time scales above 5 years ( Figure 1c). Thus, F NA identifies periods of increased freshwater rather than individual years.   10.1029/2020GL087207

Influence of Surface Freshwater on the SST
The surface mass balance states that an enhanced surface freshening strengthens the stratification, which allows for an increased surface cooling in fall and winter before the freshwater is mixed down. This freshwater-induced surface cooling is one order of magnitude larger than the cooling resulting from the anomalous surface heat flux associated with F NA .
Hydrographic observations from the Labrador Sea over the period 2002-2018 show that an increased surface freshening leads not only to an anomalous cooling relative to the summer (Figure 2a) but also to a cold surface anomaly relative to the climatological mean (Figures 2b and 2c). While the positive subsurface temperature and salinity anomalies are expected from the increased advection of heat and salt in negative NAO periods (Reverdin, 2010;Sarafanov, 2009), the cold surface anomaly can only be explained by the freshwater-induced surface cooling.
However, the hydrographic observations were acquired over a time with an overall elevated freshening ( Figure 1a). When considering the full period of satellite observations, the anticorrelation between the freshening in summer and the SST in the subsequent winter only holds for positive freshwater anomalies (Figures 3a-3c), reflecting the transition from the regime where the surface heat fluxes control the SST anomaly to the regime where the freshwater dominates.
The freshwater-induced cold anomaly is most pronounced in the eastern subpolar region (Figures 3a and 3b), where the correlation of the SST, multiplied by −1 (the cold anomaly index "CAI"), with the positive F NA values amounts to ∼0.57, which increases for higher values of F NA (Figure 3c). The associated heat flux anomaly is directed upwards and thus cannot account for the cold anomaly. On the contrary, the cold anomaly weakens the heat loss to the atmosphere by reducing the air-sea temperature contrast (Figure 3d).
Southeast of the cold anomaly over the Gulf Stream, the SST is increased (Figure 3a), giving rise to a sharper meriodonal SST gradient, key source of baroclinic instability in the atmosphere (Hoskins et al., 1985). Thus, we next investigate the interaction of the cold anomaly with the atmosphere.

Atmospheric Feedbacks
Regressing the sea level pressure (SLP) onto the cold anomaly, we find that the cold anomaly is accompanied by an anomalous low over and southeast of Greenland (Figure 4a), representative of a positive NAO. In addition, the standard deviation of the 2-to 6-day band-pass-filtered SLP is increased (Figure 4b), implying an enhanced storminess (Blackmon, 1976;Blackmon et al., 1977;Ulbrich et al., 2008). Consistent with the increased cyclonic activity, the heat loss over the Labrador Sea, south of the sea ice edge, is amplified (Figure 4c), and the wind stress curl under the low is higher (Figure 4d).
A high wind stress curl has been identified as an important driver of the subpolar gyre circulation, which retains cold, fresh polar water in the interior gyre (

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Geophysical Research Letters data confirm that the obtained wind stress curl is associated with a stronger cyclonic circulation, implying an increased advection of polar water off the coast of Newfoundland toward the anomaly, hence reinforcing it (Figure 4e). The correlation between the surface flow pattern and the cold anomaly is ∼0.80.

Summary of the Feedback Mechanism
Closer inspection of the timing of the correlations reveals that the cold anomaly emerges in the winter immediately following a high F NA summer and occurs in the same winter with the enhanced wind stress curl ( Figure 4f). It then continues to intensify and peaks 1 year after the wind stress curl reaches its maximum (Figure 4f), as expected from the subpolar gyre response (Lohmann et al., 2009). We sum up: 1. Increased freshening of the subpolar region gives rise to a faster surface cooling. 2. After large freshwater anomalies, a distinct cold anomaly appears. 3. The cold anomaly promotes an enhanced storminess. 4. The resulting higher wind stress curl strengthens the subpolar gyre circulation. 5. The stronger subpolar gyre circulation reinforces the cold anomaly.
This chain of events is supported by the change in the sign of the heat flux anomaly, which is initially downward due to the reduced air-sea contrast ( Figure 3d) and then upward and driven by the atmospheric forcing ( Figure 4c). However, the discrepancy between the two heat flux anomalies also reveals a nonlinearity in the direct relation between freshwater and the atmosphere: There exists a threshold for the freshwater, after which the sign of the heat flux anomaly reverses, reflecting the transition from the ocean-driven to the atmospherically driven heat flux anomaly.
A composite of the largest freshwater events over the investigated period, included in the supporting information, shows the transition of the heat flux anomaly from being positive in January and February to negative in March. It shows that the most negative NAO summers were followed by a positive NAO in winter. A regression on these large freshwater events reveals, in addition, a high sensitivity of the atmospheric response to small variations in the freshwater when the freshwater concentration is already high, consistent with the nonlinear relation between F NA and the SST (Figures 3c and S7-S10).

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Implications of Freshwater Events for the North Atlantic Climate
Considering that the observational record is still short with regard to the occurrence of large freshwater events, we inspect the evolution of these events individually. Two large freshwater events are the Great Salinity Anomaly in 1969-1972(Lazier, 1980 and the recent freshening (Holliday et al., 2020)  Additional salinity anomalies have been reported for the 1980s and 1990s (Belkin et al., 1998). Although the sparse sampling prevents the determination of the exact starting dates, compilations of historical salinity data suggest that they appeared in the Labrador Sea in 1980 (Reverdin et al., 1997;Yashayaev & Loder, 2016) and 1993 (Yashayaev & Clarke, 2006;Yashayaev & Loder, 2016), consistent with increased values of F NA (Figure 1a). Both events were followed by a reduced SLP and a negative SST anomaly in the subpolar region within 3 years ( Figure 5).

Geophysical Research Letters
The detailed evolution of the cold anomaly differed between the investigated freshwater events, which we attribute to variations in the freshwater volumes, their pathways (Belkin et al., 1998), and the strength of the surface fluxes, mixing freshwater down. Also, the event in 1980 preceded an increase in the winter NAO of ∼0.46, similar to the other events, whereas the event in 1993 occurred in a phase, in which the

Geophysical Research Letters
winter NAO was already high (Figure 5a). Thus, the subpolar gyre circulation was already intensified (Belkin, 2004) and the maximum SST anomaly was reached sooner (Figure 5g).
Examining the winter NAO more closely, we detect another period of increase starting in 1986 (Figure 5a), concurrent with an elevated value of F NA (Figure 1a). The following winters were characterized by a strongly reduced SLP and a distinct cold anomaly, exceeding approximately −2°C in the Labrador Sea (Figures 5d  and 5e). We conclude that, despite differences in their amplitude and detailed evolution, all freshwater events show a link with an enhanced wintertime storminess and the emergence of a cold anomaly. Conversely, all major NAO increases in the last 50 years show a link with a freshwater event.

Conclusion
Combining in situ observations with remote sensing and atmospheric reanalysis data, we have shown that enhanced freshening of the subpolar North Atlantic gives rise to an increased surface cooling in fall and winter. Over the last four decades, the freshwater-induced surface cooling has been characterized by a significantly positive trend, reflecting a growing discrepancy between the summer and winter SSTs.
Superimposed on the trend, individual strong freshwater events have triggered pronounced cold anomalies in winter. By increasing the meridional SST gradient, cold anomalies in the subpolar region promote the development of cyclones, which reinforce the cooling by modulating the subpolar gyre circulation. In agreement with this chain of events, past freshwater events were followed by cold anomalies of approximately −2°C and an enhanced wintertime storminess, reflected in increases of the NAO of up to ∼0.6 within 3 years.
Our findings suggest that the cold anomaly in winter can be explained by an enhanced surface freshening and the resulting regional atmosphere-ocean interactions, without the need for a buoyancy-driven slowdown of the large-scale overturning circulation. Thus, this study reconciles the proposed inconsistencies between the recent strong convection in the Labrador Sea and the reduced northward heat transport (Lozier et al., 2019). However, the results do not exclude the possibility of a buoyancy-driven slowdown of the overturning on longer time scales.