# Influence of the advection of Labrador Sea Water on the decadal variability of the Eastern Subpolar North Atlantic

Christophe Herbaut, Marie-Noëlle Houssais, Anne-Cécile Blaizot, Sally Close

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## To cite this version:

Christophe Herbaut, Marie-Noëlle Houssais, Anne-Cécile Blaizot, Sally Close. Influence of the advection of Labrador Sea Water on the decadal variability of the Eastern Subpolar North Atlantic. Journal of Geophysical Research. Oceans, 2025, 130 (6), pp.e2024JC022269. 10.1029/2024JC022269. hal-05088455

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Submitted on 28 May 2025

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9 Influence of the advection of Labrador Sea Water on the decadal variability of the Eastern Subpolar North Atlantic

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14 C. Herbaut¹, M.-N. Houssais¹, A.C. Blaizot¹, S. Close².

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17 1: LOCEAN, Sorbonne Université, CNRS, IRD, MNHN

18 2: LOPS, CNRS - IRD - UBO - IFREMER

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26 Corresponding author: Christophe Herbaut (christophe.herbaut@locean.ipsl.fr)
28 **Key points**

29 Variations of the upper ocean heat content in the northeastern North Atlantic are related to the eastward propagation of Labrador Sea Water.

31 Warming of the northeastern North Atlantic in the mid-1990s is associated with a
32 southeastern expansion of the subpolar gyre.

33 **Abstract**

34 The upper ocean heat content of the eastern subpolar North Atlantic (ESPNA) displays large 35 decadal variations, in particular a warming starting in the mid-1990s followed by a cooling 36 trend from the mid-2000s to the mid-2010s. Using an eddy permitting ice-ocean model, we 37 find a relationship between the eastward propagation of dense water formed in the Labrador 38 and Irminger Seas, the barotropic horizontal circulation and the advective heat convergence in 39 the ESPNA. The time evolution of the 3-year average anomalies of the barotropic 40 streamfunction (BSF) shows a strong resemblance to that of the depth of the deep isopycnals. 41 In particular, the propagation of dense water drives a response of the BSF in the intergyre 42 gyre region. During the warming period, the advection of dense water is associated with a 43 southeastward expansion of the subpolar gyre and a reduction of the supply of cold water 44 from the western subpolar North Atlantic to the upper layers of the ESPNA. Sensitivity 45 experiments suggest that atmospheric forcing can also enhance the trends in the heat content, 46 but the latter cannot be related to trends of the local wind curl. However, circulation changes 47 induced by the wind curl on some specific years can amplify the heat content changes.

48 **Plain text summary**

49 The temperature of the upper northeastern North Atlantic Ocean can vary on periods on the 50 order of 10 years. Using a model, we show that these variations can be linked to changes in 51 the Labrador and Irminger Seas. In localized area of these regions, in winter, under large 52 surface heat losses, the surface waters become denser and mixed with deeper water forming 53 dense water. The eastward propagation of this dense water in the deep layers of the ocean 54 contributes the changes of the upper ocean currents which supply water into the northeastern 55 North Atlantic. The warming of this region between the mid-1990s and the mid-2000s, could 56 thus be explained by a reduction of the supply of cold water from the western North Atlantic.

57 **Introduction.**
The heat content of the thermocline layer in the subpolar North Atlantic (SPNA) displays large decadal to multidecadal variability (Williams et al., 2014; Nigam et al., 2018). In particular, a warming starting in the mid-1990s was observed in the central Labrador Sea (Lazier et al., 2002; Bersch et al., 2007), as well as in northeastern North Atlantic (Johnson and Gruber, 2007; Thierry et al., 2008) and even in the Nordic Seas (Holliday et al., 2008; Larsen et al., 2012). In the mid-2000s the temperature trend reversed in the eastern SPNA (ESPNA) (Holliday et al., 2015; Robson et al., 2016; Piecuch et al., 2017). This cooling period culminated in 2014-2015, coinciding with the exceptionally cold sea surface temperatures in the central SPNA (referred to as the cold blob) (Duchez et al., 2016; Josey et al., 2018).

On multi-annual to decadal timescales, the changes in the heat content of the ESPNA are mainly driven by changes in heat transport rather than surface heat fluxes (Williams et al., 2014; Moat et al., 2019; Volkov et al., 2022; Robson et al., 2012, Moat et al., 2024), which often tend to damp the oceanic response (Robson et al., 2012; Yeager, 2020). In particular, Desbruyères et al. (2019) show a link between the decline of the reconstructed meridional heat transport at 45°N and the downward trend of the ocean heat content (OHT) within the upper 0-1000m of the SPNA between 2006 and 2013. However, on shorter time scales, surface heat fluxes can be the main contributor to the heat budget. For example large heat losses in winter 2014 (Zunino et al., 2017) have triggered the cold blob anomaly which has persisted over 2 years (Duchez et al., 2016; Sanders et al., 2022).

A key driver of changes in the heat transport is the Atlantic Meridional Overturning Circulation (AMOC) (Zhang and Zhang, 2015; Buckley and Marshall, 2016; Ortega et al., 2021). In the SPNA, variations of the AMOC are mainly driven by buoyancy forcing (Yeager and Danabasoglu, 2014) through changes in the Labrador Sea densities (Li et al., 2019; Ortega et al., 2021). Propagation of density anomalies in the Western Boundary Current would modify the zonal density gradient and drive a AMOC response through thermal wind (Hirschi and Marotzke, 2007; Hodson and Sutton, 2012). The mid-1990s warming is therefore attributed to a strengthening of the AMOC following persistent large surface heat losses in the western SPNA (WSPNA) in the late 1980s and early 1990s (Robson et al., 2012). Similarly, Robson et al., (2016) explain the cooling trend of the mid-2000s by a delayed decrease of the meridional heat transport in response to weak convection in the Labrador Sea in the mid-1990s. Moat et al. (2024) put forward another process that can drive the heat transport convergence in the ESPNA: propagation of thermal Rossby waves across the basin. These
91 waves can modify the east-west pressure gradient and explain the simultaneous response of
92 the AMOC and heat convergence.

93 Changes in gyre extent and/or strength are also frequently invoked to explain variations of the
94 hydrographic properties in the ESPNA (Hátún et al., 2005; Häkkinen et al., 2011a; Herbaut
95 and Houssais, 2009; Koul et al., 2020). In these studies, the wind stress curl is the main driver
96 of the circulation changes. In particular, a shift of the wind stress curl along 46°N (Piecuch et
97 al., 2017) or/and a positive trend of the wind stress curl in the northern SPNA (Chafik et al.,
98 2019) could have led to the mid-2000s cooling of the ESPNA. The increased advection of
99 subtropical water in the ESPNA after 2016 (Desbruyères et al., 2021) is also attributed to gyre
100 circulation changes driven by the southward propagation of dense water anomalies along the
101 western boundary current (Chafik et al., 2023). Buoyancy forcing can also intensify gyre
102 circulation (Hogg and Gaven, 2020) and drive circulation changes in the intergyre region at
103 decadal timescales (Kim et al., 2020).

104 In fact, changes in AMOC and gyre circulation appear to be tightly linked in the SPNA
105 (Ortega et al., 2021). In a multi-model study, Ortega et al., (2021) showed that the variations
106 of the density in the Labrador Sea are a robust proxy of the changes of the AMOC and the
107 barotropic streamfunction (BSF) in the SPNA. The interaction of the flow of the dense water
108 formed in the WSPNA with the topography generates a bottom pressure torque, which is a
109 dominant term in the vorticity balance of the BSF and the AMOC (Yeager, 2015). Following
110 this idea, Yeager (2020), using a 1° resolution hindcast simulation, showed the role of the
111 interior pathways of the Labrador Sea Water (LSW) in the variations of the meridional
112 transport. The accumulation of dense water west of the mid-Atlantic Ridge (MAR)
113 strengthens the zonal gradient of sea surface height (SSH) and enhances the meridional
114 transport over the MAR. This process is reproduced in several coupled models (Kim et al.,
115 2024), particularly in low resolution models which overestimate the variability of the deep
116 water formation in the Labrador Sea, but also as a main contributor to the low frequency
117 variability in eddy resolving models (Yeager et al., 2021). Bersch et al. (2007), analyzing
118 hydrographic data, had already suggested that the eastward drainage of the LSW formed in
119 the first half of the 1990s was accompanied by a weakening of the cyclonic subpolar gyre
120 circulation and a westward shift of the southeastern extension of the gyre, leading to increased
121 northward advection of warm and saline water in the ESPNA.
In this study, following the ideas developed by Bersch et al. (2007) and Yeager (2015, 2020), we describe the impact of the eastward propagation of dense water formed in the western SPNA on the variations of the heat content in the northern SPNA. Using an eddy-permitting model, we will show that the eastward propagation of depth anomalies of the deep isopycnals is concomitant to anomalies of gyre circulation between 48° and 54°N and changes in the origin of the water feeding the northern SPNA.

The paper is organized as follows. In section 2, we describe the data and the model used in this study. The main mechanism driving the variability in the northern SPNA in our model is presented in section 3. Section 4 provides a discussion of our results and a conclusion.

## 2. Data and Methods

We used the monthly objective analysis of temperature and salinity fields from the EN4 product, version 4.2.1., bias correction g10 (Good et al., 2013), hereafter referred to as observations.

The sea ice-ocean model is based on NEMO (Nucleus for European Modelling of the Ocean) ocean model version 3.6 (Madec, 2008) coupled to the LIM3 sea ice model (Rousset et al., 2015). The equations are discretized on 75 vertical levels with thickness ranging from 1 m in the top layer to roughly 200 m at the deepest model level. Partial steps are used to improve the representation of the bottom topography. The domain encompasses the Arctic Ocean and the Atlantic Ocean, with open boundaries at 30°S in the Atlantic and 50°N in the Pacific, along which 5-day mean velocity and tracer distributions are prescribed from a global ¼° resolution hindcast simulation which has been run by the Drakkar group (https://www.drakkar-ocean.eu/). The model grid has a ¼° horizontal resolution varying from 10 km in the Arctic Ocean to 25 km at the equator.

The model is initialized from rest with initial temperature and salinity distributions from the PHC 3.0 global ocean climatology, updated from Steele et al., (2001). The model is spun-up from 1958 to 1978 using daily ERA40 datasets. The model is then integrated from 1979 to 2017, driven by 6-hourly surface atmospheric fields from the ERA-Interim reanalysis (Dee et al., 2011) (hereafter referred to as the control experiment). Regional corrections have been made to improve the air temperature at 2 m over the Arctic in order to remove unrealistic positive summer values. The corrections have been applied over the Arctic Ocean north of Fram Strait, where the ERA-Interim 2 m air temperature climatology (obtained by averaging
the 6-hourly fields between the 39 years of the reanalysis) has been replaced by the
climatology from the International Arctic Buoy Programme dataset (Rigor et al., 2000). The
air temperature anomalies (obtained by subtracting the ERA-Interim climatology from the
original fields) are kept unchanged. Precipitation is extracted from the DFS5 dataset (Dussin
et al., 2018), with a correction on snow precipitation: when 2-meter air temperature is below
zero, precipitation is considered as being solid.

Sensitivity experiments.

In order to disentangle the respective influence of the forcing and advective processes in the
spreading of the deep water formed in the WSPNA, two sensitivity experiments have been
performed in which the initial conditions have been modified relative to the control
experiment. The two sensitivity experiments were run over 18 years and forced by the same
2000-2017 atmospheric conditions as the control run. In the strong convection experiment the
initial state was changed from 1 January 2000 to 1 January 1995 of the control experiment
while in the medium convection experiment the initial state was taken on 1 January 1990 of
the control simulation. These two different initial states represent different oceanic conditions:
in 1995, the Labrador Sea is in a cold state after several years of intense convection due to
several years of large positive North Atlantic Oscillation (NAO) conditions, while 1990
represents conditions without strong deep convection. Note that the control experiment was
started in 1979, a year with weak convection in the Labrador Sea

3 Results

3.1 Variability of temperature in the northern North Atlantic.

An empirical orthogonal function (EOF) analysis has been performed on the depth averaged 0 to 500 m temperature over the North Atlantic domain (60°W-0°W, 40°N-65°N), in the control experiment (Fig 1). The loading pattern of the first mode shows the largest anomalies located in the northern part of the domain. This mode displays a low frequency variability, with a warming between 1995 and 2005, before a trend reversal in the mid-2000s. To compare the behavior of the model with the observations, the 0-500 m temperature averaged over the 1997-2009 warm period is plotted in Figure 2 for the model and the observations. As expected, the anomalies are stronger in the model since the objective analysis used in the EN4 product tends to smooth the data, but in both datasets we found positive anomalies in the northern ESPNA. Examining the time evolution of the temperature anomalies in the northern
184 ESPNA in the control experiment (defined as 37°W-18°W;53°N-62°N, see dotted box in
185 Figure 1), we find a reasonable agreement with the ENA4 dataset, even if the amplitude of the
186 anomalies is larger in the model (Fig. 2c).

187 **3.2 Heat budget in the northeastern SPNA**

188 To establish the origin of the temperature variations in the northern ESPNA, we analyzed the
189 model heat budget in this domain. Heat content change (hereafter dHC/dt), advective heat
190 convergence and surface heat fluxes are shown in Figure 3 for the control experiment. All the
191 components of the heat budget display a decadal variability. The time series have then been
192 low-pass filtered, using a Butterworth filter with a 6-year window. Based on the relative
193 strength of the advective heat convergence in relation to the heat fluxes, five main periods
194 may be distinguished. Between 1979 and 1990, dHC/dt is rather dominated by the surface
195 fluxes, but its low-frequency component remains rather moderate. The 1991-1997 period is
196 marked by a warming which can be attributed to the advective heat convergence, while the
197 surface fluxes anomalies remain close to zero. In the following period (1998-2005), the
198 warming can be still attributed to the advective heat convergence, but the surface loss
199 increases and tends to damp the warming. Between 2006 and 2010, the cooling is mainly
200 driven by the surface fluxes, while the cooling in the last period can be attributed to the
201 advective heat convergence, the surface fluxes rather acting as a damping term. In conclusion,
202 except between 2006 and 2010, the heat content changes are mainly the results of the ocean
203 transports.

204 **3.3 Origin of the decadal variability of the advective heat convergence in the northern**
205 **ESPNA.**

206 To examine the relationship between changes in the advective heat convergence and
207 circulation changes the BSF has been regressed onto the low-pass filtered advective heat
208 convergence of the northern ESPNA (see dotted box in Figure 1 and 2). Figure 4 shows that
209 positive anomalies of the heat convergence are preceded 9 years earlier by a strengthening of
210 the circulation in the Labrador Sea. At lag -6 years this negative BSF anomaly has extended
211 southwards before propagating eastward across the basin and spreading between 52°N and
212 55°N. We used the BSF to describe the variations of the gyre circulation, but similar results
213 are found with the SSH. To illustrate the link between BSF anomalies and advective heat
214 convergence, we have contrasted the period 1979-1990, associated with negative anomalies of
215 advective heat convergence with the period 2000-2007 when positive anomalies are found
216 (Fig. 3b). Compared with the period 1979-1990, the period 2000-2007 is marked by a
217 weakening of the gyre in the Labrador and Irminger Seas and the Iceland Basin accompanied
218 by its southern and southeastern extension southwest of the Rockall Plateau (compare the
219 location of the -25 BSF isopleth in magenta, between Figure 5c and Figure 5d). As a
220 consequence, in the first period, the Iceland Basin and the Irminger Sea are mainly fed by
221 cold water of subpolar origin (Fig. 5e), leading to negative anomalies of the advective heat
222 convergence in these basins (Fig 5a). In particular, the eastward current centered at 51°N
223 (west of 30°E) appears as a major source of cold water for the Iceland Basin (Fig 5e and g).
224 During the second period, a westward current forms between 53°N and 57°N (Fig 5f). The
225 Iceland Basin and the eastern Irminger Sea are then predominantly supplied by warm water
226 advected by this current. Furthermore, a latitudinal section at 36°W shows that the meridional
227 gradient of density between 50° and 53°N is reduced over the whole water column during the
228 second period (Fig. 6), leading to a weakening of the eastward component of the current
229 between 50° and 52°N (Figures 5g and 5h). This decrease of the eastward advection of cold
230 water can further contribute to the ESPNA warming.

231 To illustrate the time evolution of the southeastern extension of the gyre, a BSF index is
232 formed by averaging the BSF over the domain 30-24°W, 52-55°N (see dotted box in Figure
233 4). The period 2000-2007 appears indeed as a period of negative anomalies of the BSF (Fig.
234 7), which is consistent with an expansion of the SPG into this domain. More generally, the
235 time evolution of the BSF index matches the low frequency evolution of the advective heat
236 convergence in the northern ESPNA, confirming the link between the two quantities (Fig. 7).

237 The two aforementioned periods feature two distinct states of the BSF. To follow the time
238 evolution of the BSF between the first and second periods, we used 3-year averages (Fig. 8).
239 At the beginning of the 1990s, an acceleration of the cyclonic circulation is observed in the
240 Labrador and Irminger seas, accompanied by a thin band of negative BSF anomalies
241 developing south of the Reykjanes Ridge in 1991-1993. Three years later, the negative BSF
242 anomaly extends slightly southward off the Grand Banks. Between 1997 and 1999, the
243 cyclonic circulation slows down in the Labrador and Irminger Seas, the negative BSF
244 anomaly off the Grand Banks spreads eastward, while a retreat of the SPG is observed in the
245 ESPNA (positive BSF anomalies form east of the MAR). During the last two 3-year periods,
246 the negative BSF anomaly vanishes in the Labrador and Irminger seas, while it crosses the
247 MAR and propagates eastward.
Changes in the bottom pressure torque due to variations in the thickness of the Labrador Sea Water (LSW) layer can drive gyre circulation changes (Yeager, 2015; 2020). We first examined the time evolution of the deep isopycnals characterizing the dense water formed in the Labrador Sea in the model. Figure S1 shows the evolution of the density in the central Labrador Sea at 1000 m. Compared with observations (Yashayaev and Loder (2016), the LSW formed in the model is denser, but its density increases in the early 1990s and the formation of a lighter product in the beginning of the 2000s are well represented. To illustrate the spreading of the dense water formed in the early 1990s, we followed the depth of the 27.84 isopycnal (Fig. 8), which can be considered as the upper limit of the dense water (Fig. S1). Similar patterns to those of the BSF are seen between 48°N and 55°N during the period 1991-2005. Dense water is gradually drained out of the Labrador Sea and spreads eastward, with the maximum of the isopycnal depth anomalies west of the MAR reaching 300 m in 1999 (Fig. 9). This maximum is found 3 years later east of the MAR. A similar propagation is found for the depth anomalies of the 27.80 and 27.82 isopycnals. In contrast, we do not observe the signature of an eastward propagation of depth anomalies in the lighter isopycnal range (not shown), nor a link between these anomalies and the BSF anomalies. The spreading of the dense LSW east of the MAR, which leads to changes in the isopycnal slopes and pressure gradient, coincides with the southeastward expansion of the gyre (Fig. 7).

With the shutdown of deep convection in the WSPNA after the mid-1990s until the late 2000s (Fig. 9), positive anomalies of the 27.84 isopycnal depth corresponding to a deepening of this isopycnal begin to form in the Labrador and Irminger Seas in the early 2000s (Fig 8h). The supply of deep water off the Grand Banks is gradually reduced, and the 27.84 isopycnal deepening signal propagates eastward between 2006 and 2014 (Fig 8j,n,p) (a similar evolution also occurs for the 27.82 and 27.80 isopycnals (not shown)). As a consequence, the meridional gradient of the deep isopycnals increases around 52°N leading to a stronger eastward current at this latitude (compare Fig. 10d and Fig. 5h). The enhanced eastward transport then supplies the northern ESPNA with cold water (Fig 10b) and contributes to the negative anomalies of advective heat convergence in this area (Fig. 10a).

To test if the anomalies of the BSF and advective heat convergence in the ESPNA are not simply the response of local atmospheric forcing instead of a response to the advection of dense water, we compare the strong convection experiment with the control experiment over the period 1995-2012. These two experiments are initialized from the same ocean state on 1 January 1995 but are then forced with different surface forcing fields over that period.
Analyzing the period between years 6 to 13 in the sensitivity experiment, which corresponds to the 2000-2007 period of the control experiment, the pattern of circulation resembles that obtained in the control experiment (compare Figure 11 and Figure 5, right panels), with a southeastward expansion of the gyre, a reduced cyclonic circulation in the Iceland Basin and a westward current in the southern Iceland Basin. Positive anomalies of advective heat convergence are also found in the northern ESPNA. However, the intensity of the response is weaker than in the control experiment. Altogether, these results suggest that the ocean state at the beginning of 1995 contributes to the warming of the northern ESPNA over the first half of the 2000s, but they also highlight the role of the atmospheric forcing between 1995 and 1999 in enhancing this warming.

The qualitative similarity of the responses between the *strong convection* experiment and the control experiment is confirmed by some quantitative diagnostics. In both experiments, the eastward barotropic transport at 36°W computed between 50 and 53°N (Figures 5 and 11 for the definition of the section), which provides cold water to the ESPNA, decreases between year 5 and 12 (corresponding to the 1999-2006 period in the control experiment) (Fig. 12a). Further east, the BSF index (dotted box in Figure 4), which can describe the southward expansion of the SPG, displays simultaneous large negative anomalies between year 7 and 12 (Fig. 12b). The signature of the propagation of dense water across the basin is also observed in the *strong convection* experiment: the evolution of the 27.84 depth isopycnal east of the Charlie Gibbs Fracture Zone (CGFZ) is similar between the two experiments (Fig. 12c). The influence of the ocean state between 1990 and 1994 on the circulation changes is also indirectly confirmed by the results of the *medium convection* experiment. In this experiment, the strengthening of the deep convection which occurred in the early 90s is lacking in the absence of the atmospheric forcing representative of that period. No uplift of the dense isopycnals near the ridge is found, nor are a southward expansion of the SPG or an extended period of large weakening of the eastward transport (Fig 12a,b,c).

### 3.4 Impact of the wind forcing

We examined to what extent the wind forcing could have contributed to the low frequency variability of the ocean heat content, in particular whether it could have been the main driver of the mid-2000s temperature trend reversal. The surface wind curl computed over the ESPNA domain (hereafter referred to as the wind curl index) shows an increase over the 2005-2015 period, which is marked in particular by large values in 2009, 2011 and 2015, but the trend is not significant (Fig. 13a). The regression of the advective heat convergence on the
wind curl index displays negative anomalies in the ESPNA, which are mainly located in the southern part of the domain (Fig. 13b), suggesting that the 2009, 2011 and 2015 events of large positive wind curl anomalies may have partly contributed to the 2005-2015 cooling trend there. The composites of the mean 0-500 m velocity based on the wind curl index give a hint of a southward shift of the NAC east of 30°W during large positive wind curl (compare Figures 13c and 13d). However, since the trend in the wind curl remains weak, it does not seem to be the main source of the 2005-2015 cooling.

The interannual variability of the advective heat convergence in the northern ESPNA is dominated by a major negative event in 2013-2015, which led to the 2015 cold blob. We have estimated the respective contributions of the low frequency mode of the eastward advection of LSW and the wind curl to this cold event. Remembering that, in the northern ESPNA, the wind curl variations are mainly driven by the East Atlantic Pattern (EAP) or the NAO (Marshall et al., 2001; Häkkinen et al., 2011b; Foukal and Lozier, 2017), we have computed the regression of the advective heat convergence onto the first and second principal components of the wind curl (which are associated with the NAO and the EAP, respectively) and the depth of the 27.84 isopycnal in the box west of the CGFZ (see Figure 9 for the time series) (Fig. 14a,b,c). For the 2013-2015 period, the ratio of the associated regressions on the total advective heat convergence averaged over the northern ESPNA suggests that the changes in the 27.84 isopycnal depth can contribute to 55% of the total changes of the advective heat convergence while that of the wind forcing does not exceed 40%. The latter is dominated by the response to the EAP in 2014 and to the NAO in 2015.

## Discussion and conclusion

The role of the interior pathways of the dense water formed in the Labrador Sea on the ocean meridional transport has been highlighted in earlier studies (Yeager, 2020). Our model analysis further details this process by showing that the eastward advection of this dense water is associated with a southward expansion of the subpolar gyre and a reduced eastward advection of cold water in the 50-54°N latitudinal band, modifying the source of water advected into the northern ESPNA. The advection of dense water leads to variations in the depth of the deep isopycnals on the order of 200-300 meters east of CGFZ, which are consistent with the changes observed by Bersch et al. (2007).

We showed that the response of the BSF to the propagation of the dense water across the SPNA mainly occurs between 48°N and 54°N in the intergyre gyre area described by
Marshall et al., (2001). However, this response has to be distinguished from the intergyre- gyre, which is mainly driven by NAO-type wind stress curl anomalies and emerges in our analysis as the second mode of variability of the BSF. The response found in our study is to be paralleled with the response to persistent positive NAO-like surface heat flux anomalies over the Labrador Sea as was found in a coupled model by Kim et al. (2020).

The results of a sensitivity experiment (*strong convection* experiment) which was initialized at the end of a strong convection period in the Labrador Sea (1995) but subject to atmospheric forcing fields initialized in 2000 suggests a substantial contribution of the atmospheric forcing during the period 1995-1999 to the warming of the ESPNA. The principal component of the annual mean wind curl, which can be interpreted as the NAO wind curl, displays large value in 1995 and 1996 but also in 1997. In 1995-1996, the negative NAO conditions (on annual mean, see Hurrell et al., 2018) have been suspected to explain the rapid warming observed in the ESPNA, through the increased contribution of subtropical waters (Thierry et al, 2008). Our analysis indeed shows that large positive anomalies of the advective heat convergence occured in the ESPNA and along the NAC (Fig. 15 and Fig 3a) in 1995-1997. Additionally the 1995-1997 wind curl may have reinforced the uplift of dense water off the Grand Banks (Fig. 8d,f) and then enhanced the dynamical response to the propagation of the 27.84 isopycnal depth anomalies.

Our results suggest a weak impact of the wind curl on the 2005-2015 cooling trend of the northern ESPNA. The lack of wind influence may be due to the absence of a trend in the wind curl over this area. In contrast, Chafik et al. (2019) observed a significant trend of the wind curl over the Irminger Sea, to the west of our study area. This curl trend could have slowed down the advection of warm water toward the Irminger Sea, contributing to a cooling trend west of our study area. However, we show that the wind curl partially contributed to the negative anomalies of the advective heat convergence during the 2013-2015 cold period which led to the 2015 cold blob. Still the main drivers of the advective heat convergence have been the circulation changes associated with the eastward advection of the LSW. In particular the persistent negative anomalies of the advective heat convergence over 2013-2015 were the main contributors to the cold blob (Fig. 3a) while the large surface heat losses which occurred in winter 2014, as described by Duchez et al. (2016) and Sanders et al. (2022), but turned positive afterwards, did not contribute much to the cooling over this period (Fig 3a).
This study is based on simulations performed with an eddy permitting model. A common flaw of non eddy resolving models is the overestimation of the deep water formation in the Labrador Sea. The LSW is indeed denser in the model than in the observations (Fig. S1 and S2), due in particular to an overestimated salinity of the LSW in the model, and the March mixed layer is too deep with a weak interannual variability in the Labrador Sea (Fig S3). Nevertheless, the characteristics of the Subpolar Mode Water (SPMW) formed over the Reykjanes Ridge are in agreement with observations, with interannual variations of its density range between 27.5-27-6 and 27.4-27.5 (Petit et al., 2021) and the formation of a lighter mode of SPMW in the range 27.3-27.5 in the Iceland Basin (Petit et al., 2020). Thus, despite this flaw, as aforementioned, the variations of the deep isopycnals in the ESPNA appear reasonable. The impact of the spreading of dense water on the horizontal circulation should nevertheless be investigated in simulations using higher resolution models, where a better representation of advection of heat and salt by eddies may limit the formation of dense water compared to eddy permitting model.

In this study, we can observe alternating propagation across the SPNA of positive and negative anomalies of the 27.84 isopycnal depth, suggestive of a decadal cycle (20-25 years). This cycle would be consistent with the propagating signals of SSH described by Volkov et al. (2022), based on 1993-2020 satellite measurements. This propagation across the SPNA could be one component of a more complex mechanism of an internal mode of decadal variability, including feedbacks with the atmosphere via the NAO or EAP (e.g. Menary et al., 2015; Arthun et al., 2021), and the westward advection by the mean current of temperature anomalies from the Iceland Basin into the Irminger and Labrador Seas (Menary et al., 2015; Volkov et al., 2022; Passos et al., 2024).

The low frequency variations of the heat content of the subpolar gyre can enhance the predictive skills of the sea ice cover in the Arctic (Yeager et al., 2015) and have potential for skillfull predictions of the climate over northwestern Europe, in particular the air temperature over Norway (Arthun et al., 2017). The relationship that we found with the advective heat convergence in the northern ESPNA could therefore increase the range of these predictions.

Lastly, after several years of weak or moderate convection, strong deep convection occurred in winter 2014/2015 with oceanic conditions resembling those observed in the mid-1990s (de Jong and de Steur, 2016; Yashayaev and Loder, 2016). It would be interesting to follow the
409 spreading of these new dense waters and to examine if it can be related to the warming of the
410 SPNA observed since the mid-2010s (Desbruyères et al., 2021).

411

412 Acknowledgements

413 This work was granted access to HPC resources of CINES under allocation A0120100239 attributed
414 by GENCI. We would like to thank the reviewers for their useful comments which helped improve the
415 manuscript.

416

417 Open Research

418 The temperature and salinity from the EN4 product can be downloaded from
419 https://www.metoffice.gov.uk/hadobs/en4/. The model outputs can be found on zenodo
420 (Herbaut et al., 2024, https://doi.org/10.5281/zenodo.10630713).

421

422

423

424

425
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<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

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602 Figure 1: First empirical orthogonal function (EOF) of the depth-averaged (0-500m)

603 temperature over the period 1979-2017 for the control experiment. (a) Loading pattern, (b)

604 principal component. LS: Labrador Sea, IS: Irminger Sea, IB: Iceland Basin, RP: Rockall

605 Plateau, GB: Grand Banks, CGFZ: Charlie Gibbs Fracture Zone.
<figure><img src="image_5.png" /><figcaption>Figure 2: Anomalies of the annual depth-averaged (0-500m) temperature for the period 1997-2009. Spatial distribution of the anomalies in (a) the observations and (b) the control experiment. (c) Time evolution of the anomalies averaged over the northern ESPNA (domain delimited by the dotted lines in (a) and (b)) for the observations (blue) and the control experiment (red).</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>Figure 3: Contributions to the heat budget of the northern ESPNA (domain delimited by the dotted line in Figure 2) over the period 1979-2017 based on (a) annual mean values, (b) 6-year low-pass filtered times series. Contributions are from the heat content change (black), the advective heat convergence (blue) and the surface heat fluxes (red).</figcaption></figure>
<figure><img src="image_7.png" /></figure>

Figure 4: Regression of the barotropic stream function (BSF) onto the low-pass filtered time series of the advective heat convergence in the northern ESPNA over the period 1979-2017 when the BSF leads by (a) 9 years, (b) 6 years, (c) 3 years, and (d) are in phase. The black line defines the 95% significance level. The dotted lines define the domain where the BSF index is computed (see explanation in text).

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<figure><img src="image_8.png" /><figcaption>Figure 5: Distribution of the mean fields averaged over the period 1979-1990 (left panels) and 2000-2007 (right panels): (a)-(b) advective heat convergence, (c)-(d) depth-averaged (0-500 m) velocity superimposed on the BSF (color shading), (e)-(f) 0-500 m depth-averaged velocity superimposed on the (0-500 m) depth averaged temperature (g)-(h) meridional vertical section at 36°W (defined by the dotted red lines in (c) and (e)). Also added in (g)-(h)</figcaption></figure>
639 are the isopycnals (red contours) and the isotherms (black contours, c.i: 0.5°C); in (c)- (d) is

640 the -25 BSF isoline (magenta contour). In (g) and (h), the magenta tick marks define the

641 latitude range where the zonal transport is computed (see explanation in text).
<figure><img src="image_9.png" /><figcaption>Figure 6: Vertical section of the density along 36°W for (blue) the period 1979-1990 and (red) the period 2000-2007.</figcaption></figure>

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<figure><img src="image_10.png" /><figcaption>Figure 7: Normalized time series of the anomalies of (black) the low-pass filtered advective heat convergence in the northern ESPNA (domain delimited by the dotted line in Figure 2), (blue) the depth of the 27.84 isopycnal, with a reversed sign, east of the mid-Atlantic Ridge (see the green box east of the MAR in Figure 8) and (red) the BSF averaged over the domain 30-24°W; 52-55°N (with a reversed sign).</figcaption></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<caption>Figure 8: 3-year average of (left) the BSF and (right) the anomalies of the 27.84 isopycnal depth, from 1991-1993 to 2012-2014.</caption>
<figure><img src="image_27.png" /></figure>

660 Figure 9: Anomalies of the 27.84 isopycnal depth (red) west of the mid-Atlantic Ridge

661 (MAR), (blue) east of the MAR and (black) averaged over the Labrador Sea (see the green

662 boxes east and west of the MAR in Figure 8).
<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

663

Figure 10: (a) Advective heat convergence, (b) 0-500 m depth-averaged velocity superimposed on the BSF (color shading), (c) 0-500 m depth-averaged velocity superimposed on the 0-500 m depth averaged temperature and (d) vertical section along 36°W crossing the zonal current, for the period 2009-2014. Also added in (d) are isopycnal (red) and isotherm (black, c.i : 0.5°C) contours. In (b) the magenta contour defines the -25 BSF isoline.

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<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>
<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /><figcaption>Figure 12: (a) Eastward transport between 50 and 53°N through a section at 36°W (see the magenta tick marks in Figure 10d), (b) BSF averaged over the domain 30-24°W; 52-55°N, and (c) anomalies of the 27.84 isopycnal depth for (red) the strong convection experiment and (green) the medium convection experiment over the 17 years of integration, and (blue) the control experiment over the period 1995-2012. Note that over this 17-year time period the control and strong convection experiments are run with same initial conditions (year 1 = 1995) but different time evolutions of the surface forcing while the medium convection experiment is run with same time evolution of the surface forcing (2000-2017) as the strong convection experiment but with different initial conditions (year 1 = 1990).</figcaption></figure>

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<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /></figure>

693 Figure 13: (a) Wind curl index (calculated as the wind curl average over the northern

694 ESPNA). (b) Regression of the advective heat convergence on the wind curl index. (c)

695 Positive and (d) negative composites, based on the wind curl index, of the 0-500 m depth-

696 averaged velocity superimposed on its modulus.
<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /><figcaption>Figure 14: Regression of the advective heat convergence on (a) the PC1 of the annual wind curl, (b) the PC2 of the annual wind curl and (c) the anomalies of the 27.84 isopycnal depth west of the CGFZ, over the period 1979-2017.</figcaption></figure>
<figure><img src="image_46.png" /></figure>

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Figure 15: Anomalies of the advective heat convergence averaged over for the period 1995-

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1997.

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