Decadal variations of ocean climate in the Norwegian Sea observed at Ocean Station Mike (66 N 2 E)

Transcription

1 ICES mar. Sei. Symp., 195: Decadal variations of ocean climate in the Norwegian Sea observed at Ocean Station Mike (66 N 2 E) Tor Gammelsrød, Svein Østerhus, and Øystein Godøy Gammclsrød, T., Østerhus, S., and Godøy, Decadal variations of ocean climate in the Norwegian Sea observed at Ocean Station Mike (66 N 2 E). - ICES mar. Sei. Symp., 195: Temperature and salinity observations obtained at Mike since 1948 are presented. Average vertical profiles arc established. The evolution of the vertical structure of the "Great Salinity Anomaly, when it passed the Norwegian Sea in the late 197s, is discussed. A recent marked temperature increase at 2 m depth is related to the cessation of the formation of deep water in the Greenland Sea during the 198s. Tor Gammelsrød, Svein Østerhus, and Øystein Godøy: Geofysisk Institutt, Universitetet i Bergen, 57 Norway. Introduction The weather ship station Mike" in the Norwegian Sea (66 N 2 E) has been occupied since Besides the standard meteorological routines, an oceanographic observation program has been carried out. This program consists of daily Nansen casts, including one deep station every week. The water depth is more than 2 m in this position, so the Mike data constitute the longest and most complete time series from the deep ocean. The Mike time series is of particular value for the study of climate variations on decadal time scales. According to the IPCC report (Houghton et al., 199). two of the basic reasons that climate fluctuations are only poorly understood are the lack of both homogeneous time series and oceanic observations. Here we meet these two needs simultaneously. The position in the Norwegian Sea (Fig. 1) is also strategic from the point of view of monitoring climate fluctuations: the upper few hundred meters are dominated by the warm, saline water of the Norwegian Atlantic current, which is the most important contribution of oceanic heat transport towards high latitudes in the Northern hemisphere, and at great depths the water is influenced by the bottom water formed in the adjacent Greenland Sea. So variations in key climate processes like poleward heat transport and bottom water formation may be recorded at Mike. Data and methods Water samples, taken with Nansen bottles equipped with reversing thermometers, are analysed for oxygen content onboard and salinity concentration ashore after each cruise, which usually lasts one month. The observation depths are corrected using unprotected thermometers. Temperature and salinity data are presented here. Mean values near the standard observation depths were constructed by averaging all the observations within a certain depth range. Because of the small vertical gradient at greater depths, this depth range was chosen from about 5 m in the upper layer to about 5 m near the bottom. The results are given in Table 1. There are more observations from the upper layers than from great depths. However, as the seasonal variability is confined to the upper 3 m, and the variations at great depths are slow, the observation frequencies are adequate for studies throughout the water column. Monthly averages were computed at the standard depths. Mean monthly averages for the whole observation period were calculated to study seasonal variations. These were subtracted from the monthly averages to form time series of anomalies. If observations from one month were missing we used the mean of the previous and subsequent months. In cases when several consecutive monthly values were missing, the 68

2 ARCTIC OCEAN Fr am SPITSBERGEN Greenland S e a Iceland Sea '^ICELAND Table 1. Dee Figure 1. Main current systems (schematic) in the Nordic Seas. The weathership station position "M is also shown. Depth (m) s.d. (m) Temperature ( C) s.d. ( C) Number of observations Salinity (psu) s.d. (psu) Number of observations

3 monthly averages for the actual months the year before and after were used. To study long-term trends we filtered the monthly anomalies using a squared Butterworth filter with a 1/4 power point at about four years (Roberts and Roberts, 1978). Results Mean vertical profiles 5 - temperature Average vertical profiles with the associated standard deviations are shown in Figure 2 (Table 1). The seasonal variations are confined to the upper 3 m (Gammelsrød and Holm, 1984) and explain the high variability in the surface layer. However, even from 3 m down to about 8 m the variability is high. This is probably due to meanderings and changes in the position of the Norwegian Atlantic Current (Fig.l). The stratification in the Norwegian Atlantic Current is dominated by the temperature. The salinity profile tends to destabilize the water column below the salinity maximum at 75 m depth. The salinity minimum at the surface layer is due to precipitation and the influence of fresh water from the Norwegian Coastal Current (Heiland, 1963). A salinity minimum, due to the influence of Arctic Intermediate Water, is usually found in the Norwegian Sea between 5 m and 8 m depth (Blindheim, 199). This minimum does not show up in the average profiles (Fig. 2) because the vertical resolution of the Nansen casts is too poor (Table 1). and it is also smoothed out in the averaging process. However, we were able to see the salinity minimum in many of the individual profiles, and it also appears in the average minimum profile (computed by subtracting the standard deviation from the mean), between 6 m and 8 m depth (Fig. 2). Time series J : 2 J salinity Monthly temperature and salinity anomalies at the surface layer, represented by the 5 m depth, and at 15 m and 2 m are shown in Figure 3. Note that the ordinates are different for the surface and the two deep time series. The 5 m is representative of the upper 3 m, as the variations here are more or less parallel (Gammelsrød and Holm, 1984). We notice the warm saline period in the early 196s, as well as the Great Salinity Anomaly (Dickson et al., 1988) in the second half of the 197s. This feature is believed to be caused by an increased freshwater transport in the East Greenland Current. Mysak and Power (1991) point out that the Great Salinity Anomaly was preceded by an above average runoff from North America into the western Arctic. This feature was traced in a cyclonic path in the northern North Atlantic from the Iceland Sea in passing the Norwegian Sea about 1 years later (Dickson et al., 1988). Figure 2. Average vertical profiles of temperature and salinity with associated standard deviations. The numbers are given in Table 1. The vertical structure and the time evolution of the Great Salinity Anomaly in the Norwegian Sea are seen in the isopleth diagrams (Fig. 4b). The relative fresh water of the Great Salinity Anomaly is accompanied by a low water temperature (Fig. 4a), but not enough to compensate for a deficit of the water density in the order of.5 a units (Fig. 4c). It may be observed (Fig. 4b) that the Great Salinity Anomaly arrived at the upper 2 m at Mike in 1975, was gradually deepening down to 5 m in 1979, but disappeared in the whole water column simultaneously in 198. In the deep ocean (15 m and 2 m, Fig. 3) the amplitudes of the fluctuations are small. Even at these depths the temperature during the 196s was above average, while the salinity was slightly below average. 7

4 Recent trends In the surface layer the temperature has leveled out at about average since the "Great Salinity Anomaly. The salinity seems to have a small negative trend (Fig. 3). In the deep ocean the temperature remained constant below the average at 15 m during the 198s. In contrast there was a definitive warming trend during the last decade at the 2 m level. The temperature at 2 m reached its absolute minimum value in 1981, increasing to the maximum of the whole period in 199; the temperature rise was.7 ±.2 C. The deep ocean salinities seem to have been above average during the last decade. Discussion Most of the bottom water in the Nordic Seas is believed to be formed by deep convection in the Greenland Sea (Helland-Hansen and Nansen, 199; Mosby, 197; Clarke et al., 199). The Greenland Sea Deep Water (GSDW) is observed to be colder, but also slightly less saline than adjacent bottom water masses (Aagaard et al., 1985). Decadal variations of the abyss temperature in the Greenland Sea (Clarke et al., 199) seem to be consistent with the variations at 2 m at Mike (Fig. 3a). Using chlorofluorocarbon (CFC) as a tracer and a box model, Schlosser et al. (1991) concluded that the production of GSDW was reduced from about.5 Sv to about.1 Sv some time between 1978 and Swift and Koltermann (1988) argue that Norwegian Sea Deep Water (NSDW) is formed by a mixing of about equal parts of GSDW and Eurasian Basin Deep Water (EBDW ). Rhein (1991), using freon-11 and freon-12 data, found that even after 1982 there was a transport of GSDW and EBDW into the Norwegian Sea. The average deep volume transport between the Greenland Sea and the Norwegian Sea is estimated at about 1 Sv (Smethie etal., 1988). The main deep water communication between the Greenland Sea and the Norwegian Sea takes place through a 2 m deep narrow canyon south in the Jan Mayen Fracture Zone, sometimes referred to as the Jan Mayen Channel (Smethie et al., 1988; Swift and Koltermann, 1988). Sælen (1986) took current measurements during two periods of 7 and 1 months' duration in 1981 and 1983/1984 in this Channel, and on both occasions calculated the transport to be about.1 Sv. This indicates that the reduction of GSDW production may be dated to before However, based on current measurements from 1982, Swift and Koltermann (1988) estimated the transport to be.9 Sv. Their current meters were situated east of the narrowest part of the Channel, so they may have overestimated the width of the current. Also, their current meter records are relatively short (about one month). Based on CTD measurements in 1989 and 199, Bourke et al. (1991) argue that the flow through the Channel had ceased in 199, and that the newly formed NSDW was only able to penetrate into the Norwegian Sea further north through the Mohn Ridge. However, direct current measurements in several of the channels in the Mohn Ridge during 1988/1989 showed negligible average transports (Foldvik, pers. comm.). The general impression is that the deep flow from the Greenland Sea to the Norwegian Sea continued during the 198s, but at a low and decreasing rate. The abyss circulations in the Greenland and Norwegian basins are believed to be cyclonic, steered by the bottom topography (Sælen, 1986). Thus changes in the water characteristics flowing into the Norwegian Sea abyss through the Jan Mayen Fracture Zone will arrive in the bottom layer at Mike about one year later, assuming an average current speed of 5 cm/s. The fact that the temperature minimum at 2 m at Mike" occurred in 1981, and the temperature has increased since then at this level, without a similar increase at 15 m, indicates that this change is due to horizontal rather than vertical advection, and that the GSDW production decreased prior to 198. The reason for the reduction in the GSDW formation during the last decade is not clear. The cessation seems to coincide with the arrival of the Great Salinity Anomaly in the Greenland Sea (Dickson et al., 1988; Schlossere? al., 1991). The reduced salinity will give rise to an increased stability, and therefore at first sight reduce the convection. However, it is demonstrated here (Fig. 4) that when passing the Norwegian Sea the salinity deficit was accompanied by low temperatures, partly compensating for the density reduction by the increased freshwater content. Besides, it has been argued by Aagaard and Carmack (1989) that the most effective way to form GSDW is to cool relative fresh water so that the surface temperature gets colder than the temperature in the water column below. Because of the greater compressibility of cold water, the convection, when started, will have a self-induced amplifier, which will increase the downward penetration of cold water. So it is unclear why the Great Salinity Anomaly should reduce the bottom water formation. Another possible explanation for the reduced bottom water formation is changes in the atmospheric forcing, i.e. increased temperatures. But observations from Jan Mayen and Svalbard show a cooling rather than a warming trend during the 198s (Nordli, 199; Hanssen- Bauer et al., 199). There is a tendency, though, showing the spring temperatures increasing, thus reducing the cooling season. This may be of importance for the bottom water formation, which is believed to take place in the spring after the destratification of the surface water by the winter cooling. Finally, we would like to mention that deep water temperature fluctuations of the magnitude reported here have been observed earlier: Aagaard (1968) found that the GSDW temperature in 1954 was.2 C warmer than in

5 5 5 m G 58 GØ G2 G4 GG G G G ØG m G 58 GØ G2 G4 GS G m G 88 9 Figure 3. Monthly means of (a) temperature and (b) salinity anomalies at 5 m (representing the surface layer -3 m), 15 m and 2 m. (Note that the scaling on the ordinates from the surface and deep layers are different.) 72

6 . 5 5 m GG G G G m G 58 GØ G2 64 G m G2 G

7 A CT, G 58 GØ G2 G4 GG G G G 88 9 Figure 4. Isopleth diagrams for (a) temperature and (b) salinity anomalies for the upper 1 m. (Temperature anomalies below -.25 C and salinity anomalies below -.5 are hatched.) (c) a, anomaly at 5 m depth.

8 36 J 35 CL Q o ' * Figure 5. Atmospheric C 2 concentrations have been observed at Station Mike during the last decade by NOAA (Conway el al., 199). Future work Mike, one of the last weathership stations still running, has a strategic position for climate watch. It is of the greatest importance that this observation series is continued in the same manner to secure homogenous time series. Other climate relevant parameters are now obtained at Mike. Since 198 N OA A has been carrying out registration of greenhouse gases like carbon dioxide (C 2) (Fig. 5) and methane (CH4) in the atmosphere (Conway et al., 199). The trends in the climate gases at Mike are comparable to land-based records from the Northern hemisphere (Keeling et al., 1989). Vertical ocean profiles of carbon (DIC, p C 2, C 14,C13) are now obtained regularly in cooperation with the Laboratory for Radioactive Dating (LRD). Trondheim. Tritium and helium profiles will be taken in collaboration with the University of Heidelberg, Germany. Some of the water samples from the deep ocean have been stored during the years. We hope to be able to construct a 4-year-long time series of radioactive carbon (C 14) using the historical water samples from Mike. Acknowledgments We thank the captains and crews on the weathership Polarfront for their positive attitude and for the skill they show in carrying out the Nansen Casts in a rough area. They should know that the work they do is of greatest importance in climate research. S.. was supported by the Nordic Council for Physical Oceanography and.g. by the Norwegian Council for Research and the Humanities (NAVF). References Aagaard, K Temperature variations in the Greenland Sea deep-water. Deep Sea Res., 15(3): Aagaard, K., Swift, J. H., and Carmack, E. C Thermohaline circulation in the Arctic Mediterranean Seas. J. Geophys. Res., 9(C3): Aagaard, K.. and Carmack, E. C The role of sea ice and other fresh water in the Arctic circulation. J. Geophys. Res., 94(C1): Blindheim, J Arctic Intermediate Water in the Norwegian Sea. Deep-Sea Res., 37(9): Bourke, R. H., Blythe, R. F., Paquette, R. G., and Stone, M. D On the deep and bottom waters of the Greenland Sea from summer 1989 and 199 data. Clarke, R. A., Swift, J. H., Reid, J. L., and Koltermann, K. P The formation of Greenland Sea deep water: double diffusion or deep convection? Deep-Sea Res., 37(9): Conway, T. J., Tans, P., and Waterman, L. S Trends '9 (a compendium of data on global change), pp Ed. by R. J. Sepanski and F. W. Stoss. Carbon Dioxide Information Analysis Center, Environmental Sciences Division, Oak Ridge National Laboratory, 286 pp. Dickson. R. R., Meincke, J., Malmberg, S-A., and Lee, A The Great Salinity Anomaly in the Northern North Atlantic Prog. Oceanogr., 2: Gammelsrød, T., and Holm, A Variations of temperature and salinity at Station M (66 N,2 E) since Rapp. P.-v Réun. 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