The Greenland Sea interannual variability by Jens Mei ncke Institut für Meereskunde, Universität Hamburg

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1 - J nternational Council for the Exploration of the Sea C.M C:17 Hydrography Committee The Greenland Sea interannual variability by Jens Mei ncke nstitut für Meereskunde Universität Hamburg Abstract Hydrographie data from summer 1986 to summer 1989 partly resolving the seasonal signal are analyzed with special emphasis on the interannual variability of winter convection. The maximum depths of convection were found to vary from 200 m (198687) via 1350 m (198788) to 1600 m ( ). Causes for these differences are found in (a) a variable cireulation status which allows occasional advection of Atlantic water into the interior of the conveetive gyre and (b) in a variable ice cover which affects heat exchange with the atmosphere and salt brine release during iee formation. ntroduction The Greenland Sea is one of the few major areas where eonveeti.ve renewal of intermediate and deep waters contributes to world ocean ventilation. Basin-seale eyelonie eirculation boundary currents advecting waters of Atlantic and Polar origin mixing across the fronts related to the boundary currents wintertime heat loss to the atmosphere ice formation and related brine release and sequences of penetrative plumes control the renewal. The quantitative description of these processes was the eentral aim of the international Greenland Sea Project (GSP) whieh carried out an intense field programme during the seasonal cycle (GSP-Group 1990 and Figure 1). This contribution focusses on the question how representative the cyele is for conveetive conditions in the Greenland Sea. nterannual variability is readily evident from Figure 2 showing differences in stratification measured at a fixed position in consecutive summers. question is crucially related to the role that convection in the Green This

2 land Sea has in the elimate system. Aagaard et al. (1985) have elarified that convection to intermediate (-= 1500 m) levels is - via the overflow across the Greenland-Seotland ridge system - central to the formation of North Atlantie Deep Water and its global spreading. Deep convection (exceeding 2000 m) is primarily important to the thermohaline cireulation internal to the high latitude oeean. Convective depths during winters 1987 to 1989 We consider data obtained along 75 N latitude which cuts right through the center of the Greenland gyre (Figure 3). n summer 1986 stratification was found to be 'normal' i.e. a warm fresh surface layer covered a nearly homogeneaus intermediate and deep water body. Arepetition of this section in March 1987 showed a warm and saline layer between 200 and 500 m which prevented convection from reaching deeper than 200 m during that winter. The explanation for these drastic changes can be deduced from observations by Quadfasel and Meincke (1987) which show two cyclonie gyres one over the Greenland and one over the Boreas Basin. From langer term trajections of satell ite-tracked drifters (MZEX-Group 1988) it appears that a double-gyre only exists intermittently. This is supported by numerical model simulations of the currents in the Nordic Seas (Legutke 1989) whieh indieate gyres within the individual basins only if the spati al sca le of the wind-stress curl corresponds to the si ze of the basins. Consequently the breaking up of the usual one-gyre eireulation over the Greenland-Boreas Basins into two separate gyres also breaks up the front between the Arctie andthe Atlantic waters and allows advection of saline and warm water into the interior of the convective region. During winter Rudels et al. (1989) observed a homogenization of the water eolumn in the central gyre to a depth of 1350 m (Figure 4). During the same winter a mooring was placed about 300 km to the west of this convection area. Figure 5 shows the time series of temperature at 1250 m depth. t is dominated by a lang term trend of stepwise cooling by 0.5 K. Conveetian in the eentral gyre is thought to be the source of this eoaling and we believe subsequent adveetion within the gyre system brought the eooled water. to the position of the mooring. A sequence of eooling events would lead to stepwise ehanges of temperature at the

3 -3 - mooring site. The temperature difference between summers 1987 and 1988 suggests that convection did not reach to 1250 m in winter but did so in winter 8788 quite consistent with the hydrographie observations mentioned for the section along 75 N in both years. For the demonstration of maximum convection depth during winter Figure 6 was choosen. t shows three profiles of temperature and salinity at 75 N 5 W obtained during FebruaryMarch The maximum convection depth was 1600 m confirmed by etd-profiles from the following summer as well as by oxygen and freon-11 and 12 measurements. n summary there was practically no convection in winter whereas convection in and in reached intermediate levels of 1350 and 1600 m respectively. Thus the intense GSP-survey eovered the renewal of upper deep water i.e. the source water for the overflow into the North Atlantic. No renewal of Greenland Sea Deep Water was observed. Discussion Asking for the reasons of the observed interannual variability of the maximum depth of convection it has already been argued that wind-induced circulation variability and related water mass advection into the convective gyre has prevented convection in winter A second candidate to affect convection is sea ice variability. One component of it is the insulation effect of the iee cover which prevents heat exchange between ocean and atmosphere and thus hampers convecti on. The second component i s i ce formation whi ch releases bri ne i nto the upper layers and thus initiates haline convection (Rudels 1990). To be effective several cycles of ice formation have to occur which implies melting phases in-between. The local importance of insulation and freezingmelting cycles are shown for the Greenland Sea in Figure 7 by means of the mean ice coverage and the variance. With reference to 75 N it is evident that the winter 1987 had maximum ice coverage and minimum variance whereas winters 1988 and 1989 showed less coverage and larger variance. This eomplies well with the observations shown for 75 N.

4 For the winter the effects of both the ice cover and the Atlantic water advection are additive with respect to hampering convection. Their co-existence allows to conclude that they operate on very different timescales since otherwise the presence of an ice cover separated by a cold and well-mixed and thus highly diffusive layer from a warm intermediate layer is unreasonable. Further insight into the relative magnitudes and time scales of variances in advection and ice cover are expected from the ongoing analysis of the GSP data set assimilated into regional circulation models. n addition air-sea interaction controls on convection like fluxes of heat etc. will have to be considered. Answeringthe initial question of how representative the well-observed cycle is for convective conditions in 4t the Green 1and Sea we may cons i der Fi gure 8. t shows a warmi ng and a (marginal) freshening trend for the Greenland Sea Deep Water and supported by tracer-observations (Rhein 1990) one can conclude that (a) there has been no renewal of Deep Water since at least 1981 and (b) there seems to be a slight shift in TS-poperties. Therefore the cycle is representative for an intermediate depth-convection status of the Greenland Sea and falls into aperiod where a change of the Arctic OceanNordic Seas thermohaline circulation system seems to be evident. References: Aagaard K. J.H. Swift and LC. Carmack: Thermohaline circulation in the Arctic Mediterranean seas. J. Geophys. Res GSP-Group: The Greenland Sea Project - A venture towards understanding the role of the Greenland Sea in ocean climate. EOS Trans. AGU

5 J Legutke S.: Modell-Untersuchungen zur Vari abil ität im Strömungssystem des Europäischen Nordmeeres. Berichte aus dem Zentrum für Meeresund Klimaforschung University of Hamburg No MlZEX '87-Group: MlZEX East 1987 Winter Marginal lee Zone Experiment in the Fram Strait and Greenland Sea. EOS Quadfasel O. and J. Meincke: On the thermal structure of the Greenland Sea gyre. Oeep-Sea Res Rhein M.: Ventilation rates of the Greenland and Norwegian seas derived from distributions of the chlorofluoromethanes Fll and F12 (submitted to Oeep-Sea Res.) Rudels B.: Haline convection in the Greenland Sea. Oeep-Sea Res. (in press) Rudels B. O. Quadfasel H. Friedrich and M.-N. Houssais: Greenland Sea convection in the winter of J. of Geophys. Res. 94 (C3) Leaend to Fiaures Fiqure 1: Figure 2: Bathymetry of the European Polar Seas with the four major deep basins. : eeland Basin L: Lofoten basin G: Greenland basin B: Boreas basin. Oepth eontours in 1000 meters. The shading indicates the region of the repeated hydrographie surveys for the watermass census of the Greenland Sea Project Oots give positions of long term moorings and triangles those of the tomographie array in the central Greenland basin. Profiles of potential temperature e and salinity S from GSPstation 0-6 (75 N 4 W) in the central Greenland basin in summer 1988 fall 1988 winter 1989 and summer The seasonal variability of the stratification is limited to the uper 1800 m of the water eolumn.

6 -6 - Fiqure 3: Repeated section of potential temperature and salinity along 75 N from Bear sland (right) due west to the ice edge. Upper panel: August 186. Lower panel: March '87. Winter convection did not exceed 200 m. Figure 4: Section of potential temperature along 75 N from Beer sland (right) due west to the slope off East Greenland in March A convective event was observed to a depth of 1350 m. Fi gure 5: Time seri es of temperature at 1250 m depth obtai ned from an instrument moored over the Greenland slope at N 11 10'W for the period June 1987 to June Figure 6: Profiles of potential temperature 8 and salinity S showing the deepening of the conveetive layer down to 1600 m during Febr. March Figure 7: Mean loeation of the East Greenland sea ice edge and its varianee for winter months 1987 to The data were read from the Norwegian ice charts. Varianee is represented by the differenee of maximum at minimum ice extend during the month indieated. Figure 8: Solid line: Time series of average potential temperature measured below 2000 m in the Greenland Basin. Dashed line: Time series of average salinity in the deep Greenland Basin. Open cireles: Estimated assuming eonstant Deep Water density. Full eire 1es: Measurements (Courtesy J. Swi ft).

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