Pathways of the Greenland Sea warming

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L10608, doi: /2007gl029974, 2007 Pathways of the Greenland Sea warming Waldemar Walczowski 1 and Jan Piechura 1 Received 12 March 2007; revised 23 April 2007; accepted 1 May 2007; published 30 May [1] Progressive warming of the West Spitsbergen Current (WSC) has been observed since During summer 2006 temperature and salinity of the core of Atlantic Water (AW) reached the highest ever observed by the Institute of Oceanology Polish Academy of Sciences (IOPAS) values. The structure of the WSC, the heat content and the extent of AW in the Fram Strait (FS) region has also changed. Temperature changes resulted from the upstream warming of the Norwegian-Atlantic Current (NwAC); the structure of the WSC and its heat content were modified by the northward advection of large mesoscale eddies observed within the western branch of the WSC in summer These changes may have large impacts on the Arctic Ocean (AO) climate and ecosystem. Citation: Walczowski, W., and J. Piechura (2007), Pathways of the Greenland Sea warming, Geophys. Res. Lett., 34, L10608, doi: /2007gl Introduction [2] Two branches of the NwAC, slope and offshore, carry warm, salty AW northward through the Norwegian Sea [Orvik and Niiler, 2002] (Figure 1). The eastern branch, called the Norwegian Atlantic Slope Current (NwASC), is a nearly barotropic flow related to the Norwegian shelf break. This current continues northward over the Barents Sea slope and along the West Spitsbergen shelf break as the WSC. The WSC carries only part (about 60%) of the AW transported by the NwASC; the other part, after passing northern Norway, branches eastward into the Barents Sea. Skagseth et al. [2004] claimed that the large-scale variability of the NwASC is a coherent mode extending from the shelf edge west of Ireland to the Fram Strait. They also describe this system as the main conduit of heat and salt between the North Atlantic Ocean and the Artic Ocean. [3] Considering northward transport of the AW, the WSC has been traditionally described as this barotropic flow along the Barents Sea and the West Spitsbergen shelf-break. The existence of a second, mostly baroclinic, western branch over the Mohns and Knipovich Ridges was postulated earlier [Walczowski et al., 2005], but its role in AW poleward transport was not considered. The western branch, which could be a continuation of the little known NwAC offshore branch, is an along-frontal baroclinic jet steered by the bottom topography of the Mohns and Knipovich Ridges. The dynamics of this flow are characterized by high variability and mesoscale activity, and eddies in the Arctic Front (AF) have been observed by IOPAS each summer. The part of AW carried by the western branch crosses the AF and joins Greenland Sea Gyre [Piechura and Walczowski, 1995], 1 Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland. Copyright 2007 by the American Geophysical Union /07/2007GL while the other fraction recirculates westward between 76 and 78 N. The bottom configuration causes the poleward flowing branches of the WSC to converge in the region of western Spitsbergen, at latitude of about 78 N. Continuing north, the current diverges again into three paths. The Svalbard and Yermak branches flow into to the AO through the FS (Figure 1), while the offshore branch recirculates westward and then southward [Manley, 1995]. The Svalbard Branch is fed by the along-slope core of the WSC, and recirculation is maintained by the western branch of the WSC. The postulated origin of the central, Yermak Branch, by the splitting of the Svalbard Branch due to bottom topography is not clear. Current investigations indicate that this central WSC branch may influence both, transport through the FS, and AW recirculation. [4] Since summer 2004, substantial increases have been observed in the temperature, salinity and heat transport of the WSC [Walczowski and Piechura, 2006; Beszczynska- Möller et al., 2007]. Holliday et al. [2007] found that the anomaly observed in had been formed in 1998 in the eastern subpolar gyre. Hakkinen and Rhines [2004] and Hátún et al. [2005] showed that the intensification of northward flow of warm, saline subtropical water was caused by changes in the subpolar gyre and increasing entrainment of AW into subarctic circulation. Increasing AW temperature, salinity, and heat transport by the NwASC was reported by Orvik and Skagseth [2005]. The time needed for the warm anomaly to travel from the Svinoy Section (63 N) to FS (79 N) was estimated at 18 months [Polyakov et al., 2005], which indicates that the mean speed of anomaly propagation was 3.8 cm/s. A similar value is derived using results from the Svinoy Section published by Orvik and Skagseth [2005] and IOPAS data for section N along the N parallel, which indicate a time lag of 18 to 21 months between the temperature increase in the Svinoy Section and the N section. This differs from the mean signal propagation speed from the Rockall Trough (57 N) to the Fram Strait, which was estimated based on information in the work of Holliday et al. [2007] at about 1.6 cm/s. 2. Data and Methods [5] The data used in the study were collected by IOPAS during cruises to the Nordic Seas. A detailed description of the data set and methods can be found in the work of Walczowski and Piechura [2006]. In this paper we also present data collected along N parallel during r/v Oceania cruises in summers and two CTD sections obtained by the Institute of Marine Research in Bergen. All anomalies were calculated in reference to the mean from the summers of Baroclinic calculations were applied to describe the structure of the currents. Although this method does not yield exact values of total L of5

2 Figure 1. Sketch of the AW currents in the Nordic Seas. fluxes, it represents the long-time averaged flows associated with water mass advection and variability. The temperature of and the heat stored in the AW layer were analyzed. Interannual variability of the heat flux to the atmosphere and seasonality of AW temperature was not considered. 3. Results [6] AW propagation pathways are represented by the mean baroclinic flow kinetic energy for the summers of (Figure 2). The western branch of the WSC, manifested as a strong flow over the underwater ridges, continues northward from the southwestern part of the study area. The eastern branch (core) of the WSC, in the southeastern part of study area is less visible, mostly due to the barotropic nature of the flow in this region and sparse covering by measurements. The core is more distinct over the shelf-break, north of the 74 N parallel. The distribution of mean currents also indicates that the western branch divides at the latitude of 73 N into two streams: one that flows northward over the ridges, and a second that joins the WSC core. Due to bottom topography, all the streams converge west of Spitsbergen at latitude of about 78 N. Very similar pathways were inferred by Jakobsen et al. [2003] from Lagrangian drifters. [7] Substantial summer-to-summer differences are noted in both flow magnitudes and patterns. In some years the western branch was relatively strong (2001), while in others eastern branch was dominating (2002). The summers of 2005 and 2006 were particularly characterized by an intensification of the northward flow. Since 2004, there have been signs of higher AW temperature and salinity propagation to the north towards the Fram Strait. Never-recorded by IOPAS, high AW salinity and temperature values in the WSC core west of Spitsbergen were observed in the summer of Unusually large and warm anticyclonic eddies in the western branch of the WSC carried a large amount of heat northward [Walczowski and Piechura, 2006] (Figure 3a). They further suggested that these structures would transport a large amount of heat into the AO through the Fram Strait. Indeed, in summer 2006 ice conditions in the region of northeastern Spitsbergen were unusual; in areas where AW remained at the surface, the sea ice edge was shifted towards the north and east. The winter of was curiously warm in Svalbard possibly due to the inflow of AW into the fjords and the release of heat into the atmosphere. The temperature of AW column recorded by a mooring situated in the Kongsfjorden [Cottier et al., 2005] increased from the end of January 2006 until March 2006 (F. R. Cottier, personal communication, 2006). This could be an effect of the communication of the heat anomaly observed in summer 2005 at 76 N (Figure 3a). The movement from latitude of 76 N to latitude of Kongsfjorden (79 N) within a six-month period indicates a mean propagation speed of about 2.1 cm/s. [8] The temperature of the AW core west of Spitsbergen was even higher in summer 2006 than it was in 2005, and heat anomalies shifted from the WSC western branch to the central part and over the Spitsbergen shelf break (Figure 3b). Because of this, warm AW extended over the shelf and penetrated the fjords. Horizontal distributions and water mass properties suggest that the heat anomaly observed in summer 2006 south of FS was the same as that observed in 2005 in the western branch at N. The hydrographic properties of the water in both places were very similar, and the mean anomaly propagation speed was 1.9 cm/s, a value similar to that estimated by Furevik at cm/s [Furevik, 2000]. [9] The mean northward propagation of the AW warming is clearly visible in the position of the summer isotherms. At 100 m the isotherm of 5 C (Figure 4) moved to the north Figure 2. Mean kinetic energy (cm 2 /s 2 ) of baroclinic currents at 100 dbar in the summers of of5

3 Figure 3. Heat content anomaly (10 9 J/m 2 ) of the AW layer in the summers of (a) 2005 and (b) more than 2 of latitude per year from 2004 to This corresponds to a summer-to-summer mean propagation velocity of the temperature signal of 0.8 cm/s. The horizontal temperature and baroclinic current distributions suggest that in 2004 warm AW entered the investigated area carried by the NwASC and was advected eastward to the Barents Sea and northward along the slope (Figure 4a). In 2005 the northward propagation of the AW warm signal intensified mostly due to large anticyclonic eddies in the western branch (Figure 4b). The AW layer heat anomaly observed in 2005 in the southwestern part continued northeastward, joined the core of the WSC, and then moved northward along the slope. Also in summer 2006 intensive AW inflow into investigated area along the western branch was observed (Figure 4c). The warmer fraction of the inflowing water turned northeastward between 72 and 73 N and continued northward along the slope. 4. Discussion and Conclusions [10] The data presented indicate that the temperature and heat content rise of the WSC is not only caused by the Figure 4. Distribution of temperature and baroclinic currents at 100 dbar in summers (a) 2004, (b) 2005, and (c) The 2 C and 5 C isolines are in bold. 3of5

4 Figure 5. Section along N parallel, between latitudes 04 E 15 E. (a)temperature at 200 dbar, summers (b) Hovmoeller plot of the heat content (GJ/m 2 ) of AW layer in summers Bottom topography has been marked. NwASC. The western branch of Atlantic inflow might also influence WSC temporal variability. The propagation of changes by the slope current system (NwASC, WSC core) is fast at about 3.8 cm/s, which is due to the coherent structure of these barotropic currents. Presented results indicate that the propagation of the anomaly by the western branch of the WSC may be two-fold slower, about cm/s. A northward shift of the 5 C isotherm towards the Fram Strait was slower due to cooling of the AW layer during its northward advection and observed summer-tosummer mean velocity was 0.8 cm/s. [11] The differing velocities of signal propagation among the various downstream branches that converge in Fram Strait will clearly impact the mean characteristics and dynamics of AW entering the AO. For example, Orvik and Skagseth [2005] showed that the mean NwASC temperature in the Svinoy Section increased in beginning of 2002 and then stabilized in If the overall signal transmission time between Svinoy Section and Fram Strait is 18 months, as postulated by Polyakov et al. [2005], then the mean AW temperature in Fram Strait should have stabilized in 2004, but this did not occur. Rising temperature and heat content of the AW layer at N parallel occurred in stages (Figure 5). In 2004 temperature has raised along whole section. In 2005 the core temperature decreased a bit while entire western part still continues warming. Finally, in 2006 warm anomaly moved from the western part over the slope, shifting the core over the shelfbreak. The mean AW temperature and heat content increase observed in 2005 and 2006 was carried by the western, highly baroclinic branch, which is slower than the barotropic current. Intensity of baroclinic currents and mesoscale activity along the AF depend of the front strength. Weakening and strengthening of the AF is correlated with increasing and decreasing of the NAO index [Schlichtholz and Goszczko, 2006]. [12] The specific pathways of AW in the southern part of the Nordic Seas, but especially the activity of the western branch of WSC, might also be very important for transport into the AO through the Fram Strait. Converging bottom topography at 78 N modifies pattern of AW circulation and inflow into AO. The core of AW supplied by the NwASC, after passing 78 N parallel follows Spitsbergen slope as the Svalbard Branch. Westward AW recirculation is maintained mostly by the western branch continuing over the AF. The present results indicate that the western branch of WSC entering the Greenland Sea over the ridges might also turn northeastward and next move northward along the slope. This path, which is parallel to the core flow, might pass the 78 N bottom narrowing and flow into the AO as the central, Yermak Branch (Figure 1); upstream propagation pathways might have a decisive impact on the southwestward recirculation or northward inflow of AW into the AO. [13] Similar to Holliday et al. [2007], the authors concluded that the unusually warm inflow into the Arctic Ocean will continue; however, the IOPAS upstream data further suggest that the local maximum of WSC warming has already occurred or should do so before summer 2007: in the southern part of the study area, the highest AW temperature in the summer of 2006 was already lower than 4of5

5 that in Since the increased heat inflow into the AO was mostly due to the higher AW temperature [Beszczynska- Möller et al., 2007], and upstream data show temperature decreasing already in 2006, we anticipate that in 2007 heat inflow into the AO will decrease. These short-term predictions were constructed based on the hydrographical situation in summer 2006 in the area between Norway and the Fram Strait. In contrast, the time series for locations far upstream that were presented by Holliday et al. [2007] suggest a renewed warming of AW inflow in subsequent years. [14] The International Polar Year (IPY) activity has begun in Intensive investigations of the AO and the subpolar seas will be conducted with icebreakers, moorings, drifting stations, automatic drifters, satellite observations, and other means. In this paper we have described the structure of the flow and the hydrographic situation upstream from the Fram Strait in the summer of 2006 and suggest that these conditions will influence the AO during the IPY. [15] Acknowledgments. We are grateful to the entire crew of the r/v Oceania who participated in the work at sea. Thanks are due to Kjell Arne Mork and the Institute of Marine Research, Bergen, for providing some hydrographic data. This research was supported by a grant from the European Union Fifth Framework Programme project ASOF-N (Arctic-Subarctic Ocean Flux Array for European Climate: North), contract EVK2-CT , and Sixth Framework Programme DAMOCLES (Developing Arctic Modelling and Observing Capabilities for Long-term Environment Studies), contract GOCE. References Beszczynska-Möller, A., U. Schauer, and E. Fahrbach (2007), Variations of the Atlantic inflow and related heat flux through Fram Strait in the last decade, CLIVAR Exch., 12(1), 7 8. Cottier, F. R., V. Tverberg, M. Inall, H. Svendsen, and C. Griffiths (2005), Water mass modification in an Arctic fjord through cross-shelf exchange: The seasonal hydrography of Kongsfjorden, Svalbard, J. Geophys. Res., 110, C12005, doi: /2004jc Furevik, T. (2000), On anomalous sea surface temperatures in the Nordic Seas, J. Clim., 13, Hakkinen, S., and P. B. Rhines (2004), Decline of subpolar North Atlantic circulation during the 1990s, Science, 304, Hátún, H., A. B. Sandø, H. Drange, B. Hansen, and H. Valdimarsson (2005), Influence of the Atlantic subpolar gyre on the thermohaline circulation, Science, 309, Holliday, N. P., S. Hughes, A. Lavín, K. A. Mork, G. Nolan, W. Walczowski, and A. Beszczynska-Möller (2007), The end of a trend? The progression of unusually warm and saline water from the eastern North Atlantic into the Arctic Ocean, CLIVAR Exch., 12(1), Jakobsen, P., M. Ribergaard, D. Quadfasel, T. Schrnith, and C. Hughes (2003), Near-surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: Variability from the mesoscale to interannual, J. Geophys. Res., 108(C8), 3251, doi: /2002jc Manley, T. O. (1995), Branching of Atlantic Water within the Greenland- Spitsbergen Passage: An estimate of recirculation, J. Geophys. Res., 100, 20,627 20,634. Orvik, K. A., and P. Niiler (2002), Major pathways of Atlantic water in the northern North Atlantic and Nordic Seas toward Arctic, Geophys. Res. Lett., 29(19), 1896, doi: /2002gl Orvik, K. A., and Ø. Skagseth (2005), Heat flux variations in the eastern Norwegian Atlantic Current toward the Arctic from moored instruments, , Geophys. Res. Lett., 32, L14610, doi: / 2005GL Piechura, J., and W. Walczowski (1995), The Arctic Front: Structure and dynamics, Oceanologia, 37(1), Polyakov, I. V., et al. (2005), One more step toward a warmer Arctic, Geophys. Res. Lett., 32, L17605, doi: /2005gl Schlichtholz, P., and I. Goszczko (2006), Interannual variability of the Atlantic water layer in the West Spitsbergen Current at 76.5 N in summer , Deep-Sea Res., Part I, 53, Skagseth, Ø., K. Orvik, and T. Furevik (2004), Coherent variability of the Norwegian Atlantic Slope Current derived from TOPEX//ERS altimeter data, Geophys. Res. Lett., 31, L14304, doi: /2004gl Walczowski, W., and J. Piechura (2006), New evidence of warming propagating toward the Arctic Ocean, Geophys. Res. Lett., 33, L12601, doi: /2006gl Walczowski, W., J. Piechura, R. Osinski, and P. Wieczorek (2005), The West Spitsbergen Current volume and heat transport from synoptic observations in summer, Deep-Sea Res., Part I, 52, J. Piechura and W. Walczowski, Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy 55, Sopot, Poland. (walczows@iopan.gda.pl) 5of5