Upper ocean heat content in the Nordic seas

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi:10.1029/2007jc004674, 2009 Upper ocean heat content in the Nordic seas Daniela Di Iorio 1 and Caitlin Sloan 1 Received 4 December 2007; revised 12 January 2009; accepted 26 February 2009; published 25 April 2009. [1] Seven years of temperature profiles are obtained from the Argo float project and are used to study the vertically integrated heat content for the Nordic seas which is broken up into the Norwegian Basin, Greenland Sea, Lofoten region, and Iceland Plateau. The World Ocean Atlas 2005 (WOA05) is used as the climatological baseline for temperature and hence heat content. Temporally and spatially averaged temperature profiles within the Norwegian and Iceland basins show good agreement between the Argo and WOA05 data sets. For the Greenland and Lofoten basins, the WOA05 data show cooler surface temperatures (by 1 2 C) but still lie within 1 standard deviation of each other over the 7-year period. The Argo float data show circulation within each of the basins that is counterclockwise, and some of the floats eventually disperse toward the outer edges of the basins. Vertically integrated heat content from 0 to 1200 m shows similar characteristics as the WOA05 results including the timing and strength of the seasonal variability. The anomalous heat content for the Greenland and Norwegian sea basins is calculated in depth bins of 400 m. The Greenland Sea shows a warming trend over the 7 years of study, which corresponds to a temperature increase of (4.1 ± 0.3) 10 2 C/yr. The Norwegian Sea basin shows interannual variability in heat content for the surface layer (0 400 m) that may be related to the North Atlantic Oscillation (NAO). During an NAO+ phase, the Norwegian Basin shows a warm phase, and during an NAO phase, the Norwegian Basin is in a cold phase; the correlation is small (r = 0.32) but different from zero correlation only at the 90% level, implying that the variability in the AHC may be due to (1) local atmospheric forcing and hence changes in heat loss, (2) atmospheric pressure differences that are not resolved by the NAO index, and (3) exchange of heat with the basin boundaries. Citation: Di Iorio, D., and C. Sloan (2009), Upper ocean heat content in the Nordic seas, J. Geophys. Res., 114,, doi:10.1029/2007jc004674. 1. Introduction [2] The Greenland, Iceland and Norwegian Seas connect the North Atlantic and Arctic Oceans and are bounded by Greenland to the west, Greenland-Iceland-Scotland Ridge to the south, and Norway and Spitsbergen to the east with limited connection to the Barents Sea (Figure 1). The Mohn and Knipovich midocean ridges separate the Greenland and Boreas basins (the Greenland Sea) from the Norwegian and Lofoten basins (Norwegian Sea). The Iceland Plateau to the northeast of Iceland defines the Iceland Sea which is bounded by the Jay Mayen fracture zone to the north and the Jan Mayen and Kolbeinsey Ridges to the east and west respectively. This region of the Nordic seas together with the Arctic Ocean is responsible for the transformation of warm, salty Atlantic water into colder, denser intermediate and deep waters which eventually leave the region as overflows across the Greenland-Iceland-Scotland Ridge [see, e.g., Meincke et al., 1997]. 1 Department of Marine Sciences, University of Georgia, Athens, Georgia, USA. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2007JC004674 [3] The Nordic seas play an important role in the Atlantic meridional overturning circulation (AMOC). Newly formed dense intermediate water masses overflow across the Greenland-Scotland ridge (GSR) to form the dense North Atlantic Deep Water. This region is also one of the few areas in the world where dense water formation takes place by open ocean convection. During cold winter conditions the Greenland Sea deep waters will be renewed from time to time by deep convective vertical mixing; during warmer winter conditions that predominantly arise during a positive phase of the North Atlantic Oscillation (NAO) [Hurrell, 1995], convection would be limited to intermediate depths [Visbeck and Rhein, 2000]. [4] A steady warming of the Greenland Sea bottom waters have occurred since the early 1980s [Watson et al., 1999; Walter, 2004] that is at least partly due to inflow of warmer water from the Eurasian Basin in the Arctic Ocean [Meincke et al., 1997]. During the Greenland Sea Project [GSP Group, 1990] convection was only sporadically observed and confined in the upper 1000 m of the water column. This lack of deep reaching convection implies a warming of the water column and Bonisch et al. [1997] document continuous warming from 1981 1994 between 200 and 2000 m and Karstensen et al. [2005] document that 1of11

Figure 1. Bathymetry of the Nordic seas using TerrainBase, with major sea basins and ridge systems identified. Yellow lines outline the Greenland Sea, Norwegian Basin, Lofoten region, and Iceland Plateau areas. the Greenland Sea Intermediate Water changed to warmer and fresher mode during the 1990s. According to Alekseev et al. [2001] averaged temperature profiles in the central Greenland Sea for the 1980s showed a significant subsurface (200 400 m) warming and then a rapid warming of the water column below 500 m during the 1990s. This increase in heat content in the upper layers was explained either by reduction of the heat loss to the atmosphere or the increased volume and temperature of the AW inflow from the North Atlantic. [5] According to Isachsen et al. [2007], the bulk of the intermediate and deep waters in the Nordic seas are formed along the advective path of the Norwegian Atlantic Current (NwAC) and hence in the Norwegian Sea (Norwegian and Lofoten basins). The warming of the Norwegian Sea along the major pathways of the Atlantic Inflow ranging from the eastern subpolar gyre to Fram Strait has been documented by Furevik [2001] for measurements in the 1980s and 1990s and more recently by Holliday et al. [2008] for measurements spanning a 30-year period (1978 2006). The Atlantic inflow encompasses the NwAC of which the western branch, originates as a major pathway over the Iceland- Faroe Ridge, continues eastward into the Norwegian Sea, then turns to the northwest toward the Mohn Ridge along the Voring Plateau and then either recirculates back into the Norwegian Basin or follows ridges (as a polar front jet) toward Spitsbergen and Fram Strait where it can recirculate back into the Greenland Sea [Orvik and Niiler, 2002; Jakobsen et al., 2003; Mauritzen, 1996a; Nost and Isachsen, 2003]. The eastern branch originates through the Faroe- Shetland Channel which then follows the Norwegian coast and then splits with some flow turning into the Barents Sea and the rest becoming the West Spitsbergen Current (WSC) that flows into the Arctic which after modification can circulate back to the Nordic seas eventually forming upper Norwegian Sea Deep Water. [6] Thus our objective is to quantify the anomalous upper ocean heat content for the Greenland and Norwegian Sea for the past 7 years using the Argo float data to quantify whether warming is continuing in the upper ocean water masses which are an important component of the AMOC and compare to atmospheric conditions via the North Atlantic Oscillation index. Section 2 describes the Argo data and the World Ocean Atlas 2005 (WOA05) climatology with section 3 showing significant results in terms of circulation, vertically integrated heat and anomalous heat content. Section 4 summarizes the results and discusses the Norwegian Basin results to the North Atlantic Oscillation. 2. Methods [7] The Argo float data obtained from the Coriolis Operational Oceanography web site (www.coriolis.eu.org) are used for our potential temperature profiles. We used data collected within the latitude band of 60 N and 80 N and longitude band of 24 W and 18 E, outlining the part of the Nordic seas which consists of the Greenland, Norwegian and Iceland Seas (see Figure 1). Each major basin is outlined with yellow polygons which generally follow the ridge system and the 500 m continental slope contour. The Greenland Sea, in our analysis, contains the Greenland basin, most of the Boreas basin and the West Iceland Basin, 2of11

Table 1. Final Number of Argo Profiles Used in the Nordic Regions That Were Within the Depth Bins Shown a Region 0 400 m 400 800 m 800 1200 m 0 1200 m Total Greenland Sea 975 (96.8) 1006 (99.9) 1005 (99.8) 973 (96.6) 1007 Norwegian Basin 1218 (94.6) 1270 (98.6) 1185 (92) 1135 (88.1) 1288 Lofoten Region 1105 (97.3) 1117 (98.3) 1058 (93.1) 1041 (91.6) 1136 Iceland Plateau 229 (99.1) 231 (100) 228 (98.7) 226 (97.8) 231 Total 3527 (96.3) 3624 (99.0) 3476 (94.9) 3375 (92.2) 3662 a The total number of profiles for each region is the last column. The numbers in parenthesis represent the percentage of profiles used. and is identified with a polygon following the Kolbeinsey Ridge, Jan Mayen Fracture Zone, Mohn Ridge and Knipovich Ridge systems along the eastern side and the Greenland continental slope along the west. The Norwegian Sea is broken up into the Norwegian and Lofoten basins which are partly separated by the Voring Plateau. The Norwegian Basin is outlined with Mohn Ridge to the north, the Jan Mayen Ridge to the west, the Iceland-Scotland Ridge to the southwest, and the continental slope boundary toward the east. The Lofoten region consists of the Lofoten basin and the west Spitsbergen region bounded by the ridge systems to the west and the continental slope to the east. Finally the Iceland Plateau region which identifies the Central Iceland Sea, is separated by the Norwegian Basin by the Jan Mayen Ridge, separated from the West Iceland Basin by the Kolbeinsey Ridge and bounded by Iceland continental slope to the south. [8] Data were collected from 23 April 2001 to 30 April 2008 with the number of Argo floats increasing with time in this area to a total of 49 different unique float identifications over the 7-year period. The profiles that were classified as good data only which had the quality control (QC) flag of 1 provided by the organization were used in our analysis. This QC value means that all real time QC tests, which are documented in the Argo manual by Wong et al. [2008], were passed. Some of these controls include a pressure increasing test, detection of spikes, gradient steepness checks and a gross temperature sensor drift test. It should be noted that none of the profiles used here were from SOLO floats with FSI CTD (Argo Program WHOI) which have a recently discovered pressure offset [Willis et al., 2007]. [9] Missing values in the temperature profiles were replaced with a linearly interpolated value only if the depth separation between the nearest values was less than 100 m. The number of profiles in each of the basins that contained good data between the depth intervals of 0 400, 400 800, 800 1200 and 0 1200 m are summarized in Table 1 together with the percentage of profiles used, shown in parenthesis. The number of profiles vary depending on whether missing values could be corrected within each depth interval. The depth interval 400 800 m had the greatest number of good profiles (at most 2% discarded in total) as most of the uncorrected missing values were either at the surface (at most 5.4% discarded as will be explained) or the profile did not extend to 1200 m (a maximum of 8% discarded). [10] Each profile of potential temperature was then linearly interpolated onto a grid having a constant depth increment of 10 m for easy vertical integration. Those profiles that do not extend to within 10 m of the surface were also eliminated for the 0 400 m and 0 1200 m analysis. This is because there is a sharp and shallow summer thermocline in the Greenland and Iceland Seas as will be shown. Values of temperature for the surface (0 m) were given the same values as those at depths within 10 m; all the profiles used for the 0 400 and 0 1200 m intervals typically had measurements at 4 8 m from the surface. [11] The temperature profiles were sorted by time and basin using the polygons in Figure 1, and their locations are shown in Figure 2. These locations correspond to the Argo s position, after profiling the water column, during which time they were on the surface for 6 12 hours for communication and data transfer purposes. Each subplot represents 455 days which corresponds to 1 year and 2 months (a way to fit 7 years of data evenly over 6 subplots). [12] The profiles were then averaged over the summer (August/September) and winter (February/March) months for the 7 years of data and averaged over space (within each polygon). The mean temperature profiles along with their depth-dependent standard deviations are shown in Figure 3. Temperature characteristics in the Norwegian and Lofoten basins show warm Atlantic water, which flows in primarily across the Iceland-Scotland Ridge, and forms the western and eastern branch of the Norwegian Atlantic current (NwAC). The thickness of this water is 500 600 m in the Norwegian Basin consistent with the results of Mauritzen [1996a]. As the Atlantic Water enters the Lofoten basin, it fills a larger portion of the water column extending down to 800 m depth [Nilsen and Falck, 2006; Mauritzen, 1996a]. According to Orvik [2004] this deepening of Atlantic Water in the Lofoten basin is due to a reduced speed in the polar front jet (western branch of the NwAC along the eastern Mohn Ridge) due to its encounter with a deep counter current associated with the Lofoten basin cyclonic gyre. Because of conservation of volume the reduced speed will cause a deepening of the AW. The seasonal difference in surface temperature for the Norwegian and Lofoten basins is approximately 6 C and 4 C respectively. [13] In the Greenland and Iceland Seas the summer surface waters are warm and a very shallow thermocline is apparent compared to winter time when there is little stratification. This highly variable upper layer is modified through atmospheric exchange and lateral intrusions from the basin perimeter: the East Greenland current along the continental slope carries a mixture of Atlantic Water (AW) recirculated in Fram Strait, AW recirculated in the Arctic Ocean, and Polar waters [Mauritzen, 1996a]. From the averaged time series this upper layer is 100 m deep in the Iceland Sea and 300 m in the Greenland Sea. Below the Greenland Sea upper layer is the Greenland See Intermediate Water which is a relatively homogenous patch of 3of11

Figure 2. Argo float locations processed for the Greenland Sea (red), Iceland Plateau (yellow), Norwegian Basin (green), and the Lofoten region (blue) for the 7 years of data. The locations correspond to the Argo s position, after profiling the water column, during which time they were on the surface for 6 12 hours for communication and data transfer purposes. Note that each subplot spans 1 year 2 months. cold water affected by winter convection [Karstensen et al., 2005]. In the early 1990s this layer consisted of a temperature maximum at about 700 m in 1992 and then subsequently deepened to 1500 m in 2000. Over the course of this 7-year data set no temperature maximum was observed in the individual profiles extending from 700 to 1200 m. [14] The World Ocean Atlas 2005 (WOA05) [Locarnini et al., 2006] was used for the monthly climatological fields of temperature at standardized depths on a 1 grid within the Nordic seas. Spatial (within each polygon) and temporal (during the summer and winter months) averages of temperature, together with the calculated depth-dependent standard deviations are also shown in Figure 3 for comparison to the Argo float profiles. One significant difference in temperature that is observed in the 0 200 m depth of the Greenland Sea (and is also apparent for the Iceland Plateau region) is that the WOA05 shows cooler mean temperatures during summer. During winter a shallow temperature maximum layer is more pronounced with the WOA05 data. Another difference is in the Lofoten region with a 2 degree difference in the surface layer. Despite these differences the mean Argo temperature profiles still remain within 1 standard deviation of the WOA05 climatology. [15] Finally, the Argo data were then sorted in terms of their platform identification so that each drifter could be tracked in time and thus give a representative circulation pattern for each basin. The Argo floats are designed to drift for approximately 9 days at its cruising depth (typically 1000 m) and then fall to a depth of 2000 m to start its profile to the surface, sampling pressure, temperature and conductivity. Data including its location is transmitted via satellite over a period of 6 to 12 hours while at the surface and then the float sinks to its cruising depth to start the cycle over again. 3. Results 3.1. Circulation [16] Representative drifter tracks, which is a subset of all the drifter data used, for each of the basins are shown in Figure 4; a strong dependence on topographic steering is evident. The general circulation results here are very much in agreement with the surface circulation outlined by Poulain et al. [1996] and more recently by Jakobsen et al. [2003] for their Lagrangian studies. Each of the basins have counterclockwise circulation cells (cyclonic gyres) and the Argo drifter paths show limited connections between the basins possibly because the drifters tend to follow bathymetric contours when submerged at depth. As the drifters are on the surface for 6 12 hours and at depth for 9 days we do not separate surface from deep ocean trajectories, which can be done following the methods of Park et al. [2005]. 4 of 11

Figure 3. Time- and space-averaged temperature profiles (thick line) during the winter (February March) and summer (August September) together with depth-dependent standard deviations (thin line, winter; dotted line, summer) for the Greenland, Norwegian, Lofoten, and Iceland areas using the (left) Argo and the (right) WOA05 profiles. [17] In the Iceland Plateau region (yellow), drifters circulate in counterclockwise submesoscale eddies and some flow out of the basin and into the Norwegian Basin following the Iceland-Faroe ridge slope and presumably flowing with the East Icelandic Current (EIC). In the Norwegian Basin the Argo drifters (green) very rarely left the basin. On some occasions they would pass into the Lofoten basin following the 1500 m contour along the Voring Plateau and then get incorporated into the Norwegian Atlantic slope current (NwASC); most would head northwest toward Mohn Ridge and circulate cyclonically while some would cross the ridge separating the Norwegian and Lofoten basin and follow Mohn and then Knipovich Ridges forming the western branch of the Norwegian Atlantic current (NwAC) [Orvik et al., 2001]. This splitting of the currents at these locations is modeled by Nost and Isachsen [2003] when considering the sum of the observed thermal wind shear using climatological density fields and modeled bottom geostrophic currents. Their results show that there must be nonzero bottom flows in order to create the cyclonic circulations in the Norwegian Basin. [18] In the Lofoten basin, drifters (blue) would circulate counterclockwise and some would get incorporated into the western NwAC at Mohn Ridge which would then turn and head north; the eastern branch of the NwAC flows along the continental slope and then forms the West Spitsbergen current (WSC) flowing northward. It is these currents that are responsible for keeping the waters ice free at these latitudes [Aagaard et al., 1987; Bourke et al., 1987]. Along 5of11

Figure 4. Representative drifter paths selected for the Greenland Sea (red), Iceland Plateau (yellow), Norwegian Basin (green), and the Lofoten region (blue), with asterisks corresponding to the start of the drifter path. the WSC some drifters cross the Knipovich Ridge and enter into the Greenland Sea as part of the return Atlantic flow [Bourke et al., 1988]. Similarly drifters in the Greenland Sea (red) cross the Knipovich Ridge and get incorporated into the WSC which then follows the ridge and circulates back into the Greenland basin via the East Greenland current (EGC) with some of the drifters circulating back into the Greenland Sea via the Jan Mayen current toward Mohn Ridge; some of the drifters cross the ridge and became incorporated into the NwAC and WSC. Other drifters in the Greenland Sea moving with the EGC would presumably leave the basin through Denmark Strait: but this was not observed with this data set. [19] These observations show that in the Greenland and Norwegian basins the cyclonic circulation paths occur over a wide range of scales and the drifters tend to remain in the deepest parts of the Greenland/Boreas basins. One characteristic of the float circulation is that some would disperse to the perimeter of the basin. This can be seen in the Greenland, Lofoten and Norwegian Basin following the drifters but also in Figure 2 for the profile locations. It should be noted, however that there are still drifters in the center of the basins. The perimeter of the basins are generally warmer than in the center of the basins and this will show up as variability in the temperature and resulting heat content. 3.2. Vertically Integrated Heat Content [20] For each regional area outlined in color in Figure 2, we calculate the upper ocean (0 1200 m) heat content. From the Argo and WOA05 data, the vertically integrated heat content Q ¼ Z 0 1200 rðt; S; 0Þc p ðt; S; 0ÞTðÞdz z is shown in Figure 5 for the Greenland Sea, Norwegian Basin, Lofoten region and the Iceland Plateau. The density of water (r) is dependent on potential temperature (T in C) and salinity (S), c p is the specific heat and both are evaluated at the surface. Note that since we are using potential temperature in C our heat content is relative to 0 C. The color of the measurement corresponds to the depth of the ocean at the Argo location using the international bathymetric chart of the Arctic Ocean (IBCAO) 1 by 1-min geographic grid [Jakobsson et al., 2008]. Plotting the result with depth information gives evidence as to whether there are changes in heat content that are dependent on the drifters position inside the basin. For the Greenland Sea and Norwegian Basin significantly more measurements are in waters deeper than 2750 m and those few measurements from where the ocean is shallower than 2500 m (red and yellow) show up as increased scatter in the time series. [21] In the Lofoten region (see Figure 1), which includes the West Spitzbergen Current region (which is bounded by the Knipovich Ridge and the eastern continental slope), there is a large spread of heat content with generally higher values than the climatological baseline (note the change in ð1þ 6of11

Figure 5. Vertically integrated heat content from 0 to 1200 m for each Nordic regions (dots) together with the spatially averaged vertically integrated heat content from the WOA05 climatology (solid lines). Color of the dots corresponds to the IBCAO 1 by 1-min geographic grid interpolated at the depth at the Argo location. vertical scale corresponding to a range of 25 10 9 J/m 2 whereas the other graphs only span 15 10 9 J/m 2 ). The lower value of 5 10 9 J/m 2 for the integrated heat content corresponds to those drifters that are between the Knipovich Ridge and the continental slope. This is also confirmed by the depth information: as the drifters move northward toward Fram Strait more of them encounter depths less than 2500 m which implies that they are dispersed along the continental slope in shallower and generally colder areas. According to Piechura et al. [2001] and Saloranta and Haugan [2004] significant cooling occurs in the heat transported by the WSC, and thus we expect a large difference between the Lofoten basin (higher heat content) and the WSC (lower heat content). According to these studies heat loss along the WSC is partly due to transfer of heat to the atmosphere and partly due to lateral exchange with colder neighboring coastal waters. According to Mauritzen [1996b], as Atlantic water transits through the entire Norwegian Sea cooling of the NwAC can be explained by an atmospheric heat loss of approximately 70 W m 2 along its northly passage. Because of this inhomogeneity we do not consider the Lofoten region further for anomalous heat content measurements. [22] Finally, since the time series for the Iceland Plateau region is shorter than that for the Greenland Sea and Norwegian Basin, we also neglect its anomalous heat content as a function of depth. With continued monitoring in the Iceland Sea it will be interesting to compare whether the trends in the Greenland Sea are also evident in the Iceland Sea. [23] The WOA05 climatological heat content for these regions represents a spatial average (within each region) and the 12-month time series is repeated over the 7-year span. Comparisons with the Argo measured heat content show similar characteristics and the timing of the seasonal variability is somewhat consistent. Table 2 shows the mean difference in heat content from when it is maximum in summer and when it is minimum in winter using weekly (7-day) averaged Argo measurements and using the monthly WOA05 climatology. The Lofoten region is not included 7of11

Table 2. Seasonal Variability in the Vertically Integrated Heat Content From 0 to 1200 m Calculated as the Mean Difference Between the Maximum Heat Content in Summer and the Minimum in Winter for Both the Argo and WOA05 Data a Region Argo (10 9 J/m 2 ) WOA05 (10 9 J/m 2 ) Greenland Sea 1.29 1.14 Norwegian Basin 3.37 2.07 Iceland Plateau 1.70 1.56 a The result is not calculated for the Lofoten region. because of the large variability in the Argo measurement. The Greenland Sea area has the smallest seasonal change in heat content, the Iceland Plateau region has the next largest difference and then the Norwegian Basin has the greatest seasonal change in heat content (even compared to the climatological baseline) possibly because of interannual variations that would cause a warmer summer and/or colder winter (as will be discussed). 3.3. Anomalous Heat Content [24] The anomalous heat content (AHC) as a function of depth is defined as the difference between the observed Argo heat content and the climatological baseline. Vertically integrating we calculate the total AHC for the Greenland and Norwegian basins broken up into depth bins of 400 m. Following the method of Ivchenko et al. [2006], the AHC was calculated by taking the difference between the observed (Argo) values and the WOA05 climatological values linearly interpolated in time and space to the Argo profile. This method produced an AHC in the 0 400 m range that had seasonal variability, very much like the authors reported in their study for the North Atlantic. This implies that the climatological profile was generally warmer in winter and cooler in summer consistent with the lower seasonal range shown in Table 2. This could be due to the fact that the WOA05 climatology and the Argo profiles reflect the hydrography for different periods of time. As a result, we decided to use the spatially averaged climatological heat content (shown in Figure 5) for each basin (since we are dealing with fairly small areas) and use these values as the baseline when computing the AHC. We found that this method preserves the trends and interannual variability, and removes the seasonal variability. [25] Figure 6 shows the AHC in the Greenland basin for the upper (0 400 m), middle (400 800 m) and lower (800 1200 m) depth bins all plotted to the same scale. Superimposed on the heat content values (dots) are the Figure 6. Anomalous heat content for the Greenland Sea between depth intervals of (top) 0 400 m, (middle) 400 800 m and (bottom) 800 1200 m. Dots correspond to the individual profiles, solid line represents weekly (7-day) averaged values, and the dashed line is a least squares fit to the result. 8of11

Figure 7. Anomalous heat content for the Norwegian Basin between depth intervals of (top) 0 400 m, (middle) 400 800 m and (bottom) 800 1200 m. Dots correspond to the individual profiles, the solid line represents weekly (7-day) averaged values, and the dashed line is a zero reference. weekly (7-day) averaged quantities (solid line) together with a least squares slope (dashed line). For all depth intervals there is a trend toward increasing heat content in the Greenland Sea. The variability in the integrated AHC, which is more pronounced in the surface, is presumably associated with warmer waters toward the perimeter of the basin and colder waters in the deep center (which can be seen in Figure 5), that is averaged out in the WOA05 baseline. According to Wadhams et al. [2004] deep convective chimneys in the Greenland Sea show a doming of the isotherms with colder core temperatures than the surrounding waters. From the circulation pattern of the drifters small coherent vortices in the deep Greenland Sea at 75 N 0 W are visible and could coincide with those convective chimneys observed from March 2001 to May 2003. Using the lower depths between 800 1200 m where the variance in the integrated AHC is smallest, the increasing heat content corresponds to an increase in temperature of (4.1 ± 0.3) 10 2 C/yr, where the confidence interval is calculated at the 95% level. [26] Figure 7 shows the AHC for the Norwegian Basin between depth intervals of 0 400, 400 800 and 800 1200 m with their weekly averaged quantities (solid line). Contrary to the Greenland Sea there is no general trend toward increasing heat content that can be detected with this shorter time span of 6 years. For the surface layer the AHC is indicative of interannual variability with a period of about 4 years, that is more pronounced in the surface layer than in the mid and lower depth layers, possibly associated with variability in the inflow of Atlantic Water that fills this 0 400 m depth range (see Figure 3) or possibly because of local changes in atmospheric fluxes over the basins [Furevik, 2001]. [27] At the start of the time series there is a warming trend that continues until January 2005. After this warming the AHC shows a short period of cooling until January 2006. By January 2008 another warming episode is seen but not as high as the previous one and then a cooling trend as of April 2008. These results are consistent with observations made by Holliday et al. [2008]. Maximum temperature anomalies were observed at ocean weather station (OWS) M in 2004; this maximum warm anomaly was also in Fram Strait in 2006. There was also a decrease in temperature anomalies in 2005 at OWS M and then an increase in 2006. 4. Discussion and Conclusions [28] This study gives estimates of the changing anomalous heat content over the past 7 years for the Greenland basin, over the past 6 years for the Norwegian Basin and the differences associated within these basins. The Argo float 9of11

Figure 8. North Atlantic Oscillation index (bars) calculated as a 3-month moving average of the monthly indices. Data obtained from the NOAA Climate Prediction Center. Superimposed is the 7-day average of the upper Norwegian Basin anomalous heat content normalized by 1.5 10 9 J/m 2 (thin line). data clearly depicts the seasonal variability in heat content for the Greenland and Iceland Seas and the Norwegian Basin; this seasonal variability is closely represented by the spatially averaged WOA05 climatological data. By using a spatially averaged climatological heat content, as opposed to a linearly interpolated value at the Argo location, we were able to remove an observed seasonal cycle (not shown) from the anomalous heat content. The Greenland Sea shows increasing heat content over the 1200 m depth range for the past 7 years that corresponds to a temperature rise of 0.04 C/yr. [29] The Norwegian Basin shows a maximum AHC in the surface layer during the year 2004 which is consistent with the findings in the International Council for the Exploration of the sea cooperative research report [ICES, 2005]. In this annual climate summary, the working group document that the core of Atlantic Water along the Norwegian continental slope was 0.5 0.8 C warmer than normal. In fact the observed change in AHC for the upper layer of the Norwegian Basin corresponds to a change of 0.6 C. [30] Many researchers have tried to link interannual variability in the North Atlantic and Nordic seas in terms of atmospheric variability parameterized by the North Atlantic Oscillation (NAO) because it has a significant impact on oceanic [Schlichtholz and Goszczko, 2005; Orvik et al., 2001; Mork and Blindheim, 2000; Mauritzen et al., 2006] and ecological [see, e.g., Skogen et al., 2007] conditions. According to Mauritzen et al. [2006] the inflow of Atlantic water to the Nordic seas increases immediately in response to an increase in the NAO. Orvik et al. [2001] document a high inflow of Atlantic water through the Faroe Shetland channel (eastern branch of the NwAC) that coincides with a high NAO+ index, implying that variabilities of the Atlantic inflow are related to the westerly wind field of the North Atlantic. During a positive phase (NAO+), a deepened Icelandic low causes strong westerlies over the eastern North Atlantic which then forces a large northward transport in the North Atlantic and a subsequent strong Atlantic inflow to the eastern Norwegian Sea. The western branch of the NwAC, however, was found to have a negative correlation between salinity and the NAO (Nilsen and Nilsen [2007], and consistent with the findings of Blindheim et al. [2000]), but temperature did not show any significant changes with the NAO suggesting atmospheric heat fluxes dominant influence in the upper layers. Furevik [2001] also indicates that a reduced oceanic heat loss to the atmosphere (during times of NAO+ when warmer atmospheric conditions exist) is important. [31] In Figure 8 we show the 3-month moving average of the monthly NAO indices obtained from the NOAA/ Climate Prediction Center (Entry ID: NOAA_NWS_CPC_ NAO) and try to see if there are similarities with the upper layer (0 400 m) AHC for the Norwegian Sea. Heating and cooling trends may correspond to the NAO positive and negative phases respectively over an approximate periodic cycle of 4 years. Just before January 2003 the AHC has a cold phase which corresponds to a strong negative NAO. Between January 2004 and 2005 the NAO becomes positive and a warm phase is seen in the AHC. After January 2005 the NAO begins a negative phase until January 2007 and the AHC moves to a cold phase. After January 2007 the NAO is more strongly positive and the heat content recovers from the previous cold phase. A correlation coefficient of r = 0.32 is obtained that is different from 0 correlation only at the 90% confidence level. The significance test was carried out using the effective degrees of freedom described by Emery and Thomson [1998] because of autocorrelation in the data sets due to low-frequency oscillations. Since the correlation is weak we suggest that the variability in the AHC may be due to (1) local atmospheric forcing and hence changes in heat loss, (2) atmospheric pressure differences that are not resolved by the NAO index, and (3) exchange of heat with the basin boundaries. [32] Acknowledgments. This work was supported by the NSF CA- REER grant (OCE0449578) as part of the educational component to increase data processing and research skills in MARS4100 Physical Processes of the Ocean at the University of Georgia of which Sloan was a student. We thank the anonymous reviewers for their detailed comments, which have greatly improved this manuscript. 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(Available at: http://www.coriolis.eu.org/ cdc/argo_rfc.htm) D. Di Iorio and C. Sloan, Department of Marine Sciences, University of Georgia, 250 Marine Sciences Building, Athens, GA 30602, USA. (daniela@uga.edu) 11 of 11