Combined influence of oceanic and atmospheric circulations on Greenland Sea Ice

. 12 The amount and spatial extent of Greenland Sea (GS) ice are primarily controlled by the sea ice export across the Fram Strait 13 (FS) and by local seasonal sea ice formation, melting, and sea ice dynamics. In this study, using satellite passive microwave 14 sea ice observations, atmospheric and a coupled ocean-sea ice reanalysis system, TOPAZ4, we show that both the 15 atmospheric and oceanic circulation in the Nordic Seas (NS) act in tandem to explain the SIC variability in the western GS. 16 Northerly wind anomalies associated with anomalous low SLP over the NS reduce the sea ice export in the western GS due 17 to westward Ekman drift of sea ice. On the other hand, the positive wind stress curl strengthens the cyclonic Greenland Sea 18 Gyre (GSG) circulation in the central GS. An intensified GSG circulation may result in stronger Ekman divergence of 19 surface cold and fresh waters away from the western GS. Both of these processes can reduce the freshwater content and 20 weaken the upper ocean stratification in the western GS. At the same time, warm and saline Atlantic Water (AW) anomalies 21 are recirculated from the FS region to western GS by a stronger GSG circulation. Under a weakly stratified condition, 22 enhanced vertical mixing of these subsurface AW anomalies can warm the surface waters and inhibit new sea ice formation, 23 further reducing the SIC in the western GS.


Introduction 25
The strength of the Atlantic meridional overturning circulation partly depends on freshwater availability in the GS (Serreze 26 et al. 2007;Eldevik & Nilsen 2013;Buckley & Marshall 2016). The freshwater content in this region is largely driven by the 27 amount of sea ice therein (Aagaard & Carmack 1989). Sea ice in GS is also important in determining shipping routes 28 (Instanes et al. 2005;Johannessen et al. 2007), as well as to the regional marine ecosystem due to its impact on the light 29 availability (Grebmeier et al. 1995). Most of the sea ice in the GS is exported from the central Arctic Ocean across the Fram 30 Strait (FS) and is largely controlled by the ice-drift with the Transpolar Drift (Zamani et al. 2019). Anomalous sea ice export 31 6 index crosses the 0.75 and -0.75 mark respectively. The 0.75 threshold was chosen to consider only the sufficiently 123 strong/weak gyre circulation periods. Throughout the article, all regression and correlation analysis were performed with the 124 detrended time series for the corresponding variables. Freshwater content was calculated using the following formula 125 − where, S is salinity and the reference salinity is chosen as 34.8 psu. 126

127
The standard deviation of winter-mean DJF SIC, in both observation and TOPAZ4, showed high variability along the MIZ in 128 western GS and the Odden region in central GS (Fig. 2). Note that, the TOPAZ4 reanalysis data exhibits a more confined 129 MIZ than observations, which is a known model deficiency (Sakov et al. 2012). The sea ice model (Hunke and Dukowicz, 130 1997), used in TOPAZ4, has a narrower transition zone between the pack ice and the open ocean. Although assimilation of 131 the sea ice observations does slightly improve the position of MIZ in TOPAZ4 compared to observation, the sharp transition 132 in a narrow band still remains, which could have resulted in higher standard deviations in a narrow MIZ of TOPAZ4 as 133 observed in Fig. 2b. However, as we will find in the next section, the sea ice response to the atmospheric and oceanic 134 processes explained in the study can be significantly found in both the observation and TOPAZ4 with slightly higher signals 135 along the MIZ in TOPAZ4. Thus the higher signal-to-noise ratio in TOPAZ4 should not affect the qualitative aspects of the 136 processes and their influence on SIC, which is the main objective of the study.  Fig. 2) in western GS where the standard deviation of the SIC is found to be 145 maximum both in TOPAZ4 and observations. Also we will show in the next section that SIC response to the processes 146 described here is most profound in this region. Hereafter we refer to this region as western GS for simplicity. Fig. 3 shows 147 the spatio-temporal patterns of sea surface temperature (SST) and salinity (SSS) in western GS as found in TOPAZ4 and 148 EN4. Although the temporal evolution of these parameters are well captured in TOPAZ4, compared to observation, the 149 westward extension of the warm and saline waters was found to be less in TOPAZ4. This indicates that the front between the 150 cold and fresh waters along the Greenland shelf and the warm and saline waters in the western GS is slightly shifted towards 151 the east in TOPAZ4 compared to observation. This could be a reason for the fact that higher standard deviation of SIC is 152 8 found slightly toward the east in TOPAZ4 than observations (Fig. 2). In western GS, both the surface and subsurface 153 temperature in TOPAZ4 was found to be colder compared to observations (Fig. 4). The negative biases in TOPAZ4 were 154 more profound in the subsurface for both temperature and salinity. Xie et al., (2017) also found a similar result with 155 TOPAZ4 and attributed it to sparse observations. Using the potential density difference between 200m and the surface as an 156 indicator of the stratification, we found that TOPAZ4 has weaker stratification compared to observations (Fig. 4e). 157 Consistent with the cold bias in TOPAZ4, winter-mean SIC in TOPAZ4 is higher than the satellite observation in the 158 western GS (Fig. 4f). However, we found a strong correlation (r=0.9) between the SIC in observation and TOPAZ4. This 159 indicates that the interannual variability of SIC, which is the focus of the study, is quite consistent in both TOPAZ4 and 160 observation.  The regression map of winter mean SIC on the gyre index showed significant negative SIC in the western GS (Fig. 5). The 176 spatial pattern of the regression coefficients closely resembles the standard deviation of winter mean SIC in the GS, as 177 shown in Fig. 2. This indicates that a considerable amount of the SIC variability in GS can be associated with GSG 178 circulation. However, it should be noted that the atmospheric forcing in the NS can influence both the GSG circulation 179 (Aagaard 1970 To elucidate the possible influence of atmospheric circulation pattern associated with GSG circulation on the SIC variability 187 in the GS, linear regression of the sea level pressure anomalies on the gyre index was calculated and shown in Fig. 6. The 188 large-scale atmospheric circulation shows a positive NAO-like pattern associated with a strong GSG circulation, but with 189 centres of actions north of their usual locations (Fig. 6). The GSG circulation responds to the anomalous wind stress curl 190 induced by the low SLP anomaly patterns in the NS (Chatterjee et al. 2018 its influence on the Nordic Seas circulation. Also note that the low correlation could be due to the fact that the equatorward 195 pole of NAO doesn't exhibit much significant regression patterns in Fig. 6. GS associated with increase in GSG strength (Fig. 7b). This indicates that the anomalous northerly winds during a strong 203 GSG circulation would lead to Ekman drift of sea ice which tends to push the sea ice towards the Greenland coast and reduce 204 the mean southward sea ice velocities in this region (Fig. 7a). This could lead to reduced sea ice export in this region and 205 result in low SIC. anomaly for the strong GSG circulation years was found to be ~1 o C higher than the same during weak GSG circulation 216 years. The warm anomalies further extend eastward with the JMC towards the central GS and could potentially affect the sea 217 ice formation in the Odden region. Further, we found significant positive correlation (r=0.7, p<0.01; Fig 8b) between gyre 218 index and temperature advection (in upper 400m) in the western GS (marked region in Fig. 8a), where maximum GSG 219 influence on SIC is found (Fig. 3a). This suggests that a strong GSG circulation recirculates the warm AW anomalies into 220 the western GS from the FS. This is consistent with earlier study indicating an increased oceanic heat content in the western 221 GS due to a stronger GSG circulation (Chatterjee et al., 2018). However, it should be noted that the recirculated AW in the GS still remains dense enough to be in subsurface (Schlichtholz 228 & Houssais 1999; Eldevik et al. 2009) and needs to be vertically mixed to have an impact on the sea ice. We found that the 229 upper ocean stratification in the western GS strongly covaries with GSG circulation strength (Fig. 9a). The analysis shows 230 that a weakening of the stratification in the upper part of the water column coincides with a stronger GSG circulation and 231 vice versa (Fig. 9a). Further, warm and saline signatures in the upper ocean can be found during strong GSG circulation, 232 indicating enhanced vertical mixing of the AW in the western GS (Figs. 9b,c). This is further confirmed by significant 233 positive correlation (r=0.7, p<0.01) between surface salinity anomaly and gyre index (Fig. 8b) Anomalous winds in the Nordic Seas are known to influence the SIC in the GS through Ekman drift of the sea ice (Germe et 258 al., 2011). During time-periods with anomalously low SLP over NS, anomalous northerly winds and associated Ekman drift 259 towards the Greenland coast that can reduce the sea ice export in the western and central GS (Fig 8b). Enhanced Ekman 260 divergence due to a strengthened GSG circulation can further lead to reduced freshwater and sea ice in the western GS (Fig.  261   11). We found that these can lead to weakening of the upper ocean stratification in the western GS (Fig. 9a). At the same 262 time, a stronger GSG circulation recirculates the warm and saline subsurface AW anomalies from the FS into the western GS 263 (Fig 8a). These AW anomalies can warm the surface waters by enhanced vertical mixing in a weakly stratified condition 264 ( Fig. 9) and can cause further reduction of SIC by inhibiting new sea ice formation or even melting the sea ice from bottom. 265 Although our study doesn't show bottom melting of the sea ice, this can be realized from the findings by Ivanova et al. 266 (2011) which showed enhanced bottom melting in this region during positive NAO periods. Thus, the SIC variability in the 267 western GS responds to simultaneous influences from the atmospheric and oceanic circulation (Fig. 10) find a significant positive trend in the GSG circulation strength during the study period (Fig. 12). The response of GSG 288 circulation to this altered atmospheric forcing can further be realized with increased GSG strength (Fig. 1c)  results show that the salinity anomalies and intensified convection in the GSG can be induced by a stronger GSG circulation 294 (in response to the atmospheric forcing) which helps in recirculation of AW anomalies in the GS. Thus we propose t 295 atmospheric forcing over the NS imposes a positive oceanic feedback (Fig. 13). The low SLP anomaly over the NS 296 strengthens the GSG circulation. The Ekman divergence pushes the freshwater and sea ice from the GS interior towards the 297 coast. Enhanced AW recirculation due to a stronger GSG and weakened stratification due to reduced freshwater allows the 298 warm and saline AW anomalies to get vertically mixed and increase the temperature and salinity in the central GS. The 299 increased salinity further helps in a stronger GSG circulation, completing the feedback loop. The findings of the study thus 300 highlight that interaction between large scale atmospheric and oceanic circulation in NS is crucial for understanding the 301 North Atlantic and Arctic oceanic connectio 302 303 304 305 recirculated AW on inducing intensified convection in the GSG through surface salinity anomaly. Consiste results show that the salinity anomalies and intensified convection in the GSG can be induced by a stronger GSG circulation (in response to the atmospheric forcing) which helps in recirculation of AW anomalies in the GS. Thus we propose t atmospheric forcing over the NS imposes a positive oceanic feedback (Fig. 13). The low SLP anomaly over the NS strengthens the GSG circulation. The Ekman divergence pushes the freshwater and sea ice from the GS interior towards the AW recirculation due to a stronger GSG and weakened stratification due to reduced freshwater allows the warm and saline AW anomalies to get vertically mixed and increase the temperature and salinity in the central GS. The n a stronger GSG circulation, completing the feedback loop. The findings of the study thus highlight that interaction between large scale atmospheric and oceanic circulation in NS is crucial for understanding the North Atlantic and Arctic oceanic connections.
A proposed positive oceanic feedback induced by atmospheric forcing in NS. recirculated AW on inducing intensified convection in the GSG through surface salinity anomaly. Consistent with this, our results show that the salinity anomalies and intensified convection in the GSG can be induced by a stronger GSG circulation (in response to the atmospheric forcing) which helps in recirculation of AW anomalies in the GS. Thus we propose that the atmospheric forcing over the NS imposes a positive oceanic feedback (Fig. 13). The low SLP anomaly over the NS strengthens the GSG circulation. The Ekman divergence pushes the freshwater and sea ice from the GS interior towards the AW recirculation due to a stronger GSG and weakened stratification due to reduced freshwater allows the warm and saline AW anomalies to get vertically mixed and increase the temperature and salinity in the central GS. The n a stronger GSG circulation, completing the feedback loop. The findings of the study thus highlight that interaction between large scale atmospheric and oceanic circulation in NS is crucial for understanding the Funding (information that explains whether and by whom the research was supported) 321

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Conflicts of interest/Competing interests (include appropriate disclosures) 323

Authors declare no Conflicts of interest/Competing interests 324
Availability of data and material (data transparency) 325 All the data used here are freely available on respective data portals (links provided in the 'Acknowledgements' section) 326 Code availability (software application or custom code) 327 All the codes are available on reasonable request to the corresponding author.