ICES CM 19991L:18

Variability in the Nordic Seas Exchange

- Model results 1979-1993

by

Michael Karcher, J. Brauch, B. Fritzsch, R. Gerdes, F. Kauker, C. Kiiberle, and M. Prange

Alfred Wegener Institute for Polar and Marine Research Bremerhaven, Germany

Abstract

As a part of the EC MAST III VEINS programme a high resolution coupled seaice-ocean model has been set up for the , the Nordic Sea and the subpolar North Atlantic. It is driven with daily atmospheric data from the ECMWF reanalysis to cover the period from 1979 to 1993. The run is analysed and compared to available oceanic and ice-cover data.

Propagating signals constitute a large part of the variability in the area. The winter temperature of the Atlantic Water along the Norwegian coast shows warm periods between winters 82/83-83/84 and 88/89-92193 and a cold period from 85/86-87/88. The cause for these anomalies is discussed. In the area northeast of Island the variability of the East Icelandic Current is responsible for anomalous cold and fresh conditions during high NAO states. The analysis also addresses the question, whether a link between the variability in the Nordic Seas and the subpolar gyre across the ridges is established in the model.

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1 Introduction Nordic Seas and the northern North Atlantic down to approximately 50° N where the only The Nordic Seas receive wann and saline water open boundary is implemented. from the North Atlantic. The Atlantic water The horizontal resolution is 114° x 114° on a enters through the gaps between and rotated grid, the vertical resolution is 60 levels Faroe and between Faroe and Scotland, feeding where the level thickness increases with depth. the two branches of the Norwegian Atlantic At the open boundary the longtenn - mean Current (N wAC). In the northern Norwegian streamfunction extracted from a coarse - Sea the NwAC splits into the West Spitsbergen resolution version of the same model enclosing Current (WSC) flowing northward towards the the entire down to 20° S Arctic Ocean and the Northcape Current moving (Koberle et al., 1999) is prescribed constant in onto the Shelf. Parts of the WSC time. The hydrography at the open boundary is branch off south of and recirculate taken from Levitus and Boyer (1994). southward in the western Sea. The model is forced with daily mean 2-metre Between this recirculating Atlantic Water and air temperature and dew point temperature, the Greenland coast, the cloudiness, precipitation, absolute wind speed (EGC) transports cold and fresh polar water and surface windstresses from the ECMWF southward towards the Denmark Strait. The reanalYSIS dataset from the period 1979 to 1993 East Icelandic Current (EIC) branches off from (Gibson et al., 1997). the EGC and carries cold and fresh water into The sea surface salinity is restored to observed the area north of Iceland. data from the EWG-atlas (EWG, 1997) for the The entire current system of the Nordic Seas Arctic Ocean and the Nordic Seas and Levitus and the hydrographic properties of its (1994) for the rest of the domain with a watennasses are subject to intense variability on restoring timescale of 50 days. An FCT seasonal to interanual tirnescales (e.g. Dickson algorithm according to Gerdes et al. (1991) is et aI., 1988; Furevik. 1999; Blindheim, 1999). implemented for the advection of tracers. The intensity of the North Atlantic Oscillation The model run is initialized from a 20 year (NAO). a dominant pattern in the winter sea integration without coupling to the ice model, level pressure fields over the North Atlantic windforcing from Hellerman and Rosenstein (Hurrel, 1995). has been disucussed as an index (1983) and restoring of SST· and SSS to which is highly correlated with the variability of observed data (EWG. 1997; Levitus and Boyer, numerous atmospheric and oceanographic 1994). properties in the Nordic Seas area, like the temperature of the NwAC, the ice export For the following analysis we use means of the through Fram Strait or the precipitation over the winter halfyears (ONDJFM) as a basis for . calculating anomalies from the longtenn The following model study aims at a better wintertime mean of the total period (winter understanding of the variability in this system 79/80 to winter 92/93). and its interaction with the atmospheric conditions. The present paper pre,ems first results from an analysis of the upper ocean 3 Results hydrographic variability for the period 1979 - 1993. Upper ocean anomalies in temperature and salinity

2 Model Description The hydrographic properties in the Nordic Sea as resulting from the model run forced with The numerical model used for the present study atmospheric data from the period 1979 - 1993 is a coupled ice-ocean model. based on the exhibit strong interanual variability. MOM-2 code (Pacanowski, 1995) for the oceanic part and a viscous-plastic ,ea-ice Fig. I a,b shows the temperature anomalies in model (Hibler, 1979; Harder, I 998). 100 m depth for the winters 1982/83 and The model domain covers the Arctic Ocean, the 1983/84, Fig. 2 a,b the SST for the winters ------

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1989/90 and 1990/91. The figures only present for this anomaly being advected from south of a part of the total model domain. the Iceland-Scotland gap. In winter 82/83 (Fig. I a) a large anomalously warm structure dominates the southern part of A prominent feature in the temperature anomaly the Nordic Seas stretching from the Greenwich patterns shown is a very intense, elongated meridian in northeastern direction to the Barents anomaly strechting eastward from the northern Sea, following the baseline of the Norwegian tip of Iceland. [t is strongly negative between continental slope. It marks the front between the 1982 and \-985 and positive between 1987 and Atlantic Water and the interior water of the 1991. This feature is connected to changes in Nordic Seas gyre. This front is marked by the position of the frontal zone between Atlantic large-scale meandering and at the present state Water entering the Iceland-Faroe gap and cold, of analysis it is difficult to separate signals fresh water branching off the EGC north of propagating along the front and the large-scale Iceland. The dynamics of these changes still lateral shifts of the front. here indicating an have to be analysed in detail. It is unclear for anomalously far western distribution of warm example wether the frontal changes are a Atlantic Water. consequence of or a precondition for the In the Barents Sea remnants of a previous cold intensified branching of the EGC. However, anomaly advected with the NwAC are still they are connected to the intensity of the filling large areas. A warm anomaly in the inner inflowing Atlantic Water, which at times is able branch of the NwAC is visible close to the to bend westward into the area north of Iceland Norwegian coastline from southern to before flowing eastward to feed the offshore the Lofoten Islands. One year later (Fig.lb) this branch of the N w AC. The surface salinity waJm anomaly of the inner Atlantic Water pattern is also affected by this phenomenon, branch has moved into the southern Barents Sea visible as zonal fluctuations in the position of and· slightly intensified in amplitude. the 35 PSU isoline between Iceland and Amplitudes are up to I K. Now a next cold Norway. In hydrographic data from this area (on anomaly is approaching through the Faroe­ a zonal section at 65 ON) Blindheim el al. (1999) Scotland gap along the path of the NwAC. Its find a strong positive correlation of the eastward peak will reach the Barents Sea in about 1987. position of the 35 PSU isohaline with the NAO Both cold anomalies and the warm anomaly at a time-lag of 2 years between the mid which occured along the Norwegian coast in the seventies and the mid nienties. They attribute Atlantic watelmasses are depth intensified and the eastward positions to a strong EIC, carrying can be identified already south of the Iceland­ cold and fresh water from the EGC far eastward. Scotland gap before being advected into the Nordic Seas proper. Correlation with NAO Fig. 2 a.b shows the very different developement of a second warm anomaly of the In the following, the NAO index is used for NwAC and the coastal waters in the late 1980s lagged linear regressions with the temperature and early 1990s. Now the temperature in 100 m depth and with the SSS. Only anomalies at the sea surface are shown. This correlations above 0.6 are shown, correlations> anomaly covers large parts of the eastern Nordic 0.66 are significant above the 90% level. Seas and the North Sea at the same time. It Fig. 3a,b shows the associated patterns of the started in 1987/88 and has its maximum temperature at 100 m depth (detrended, 3-year amplitude and areal coverage in 1989/90 (Fig. running mean) with the NAO index (see Kauker 2a). In contrast to the Atlantic Water anomalies el aI., 1999) for time lags of zero and one year. described before. this warm anomaly of the In phase with the NAO index positive early 1990s has its maximum amplitude at the anomalies are visible along the entire surface. In the following. winter it has Norwegian coast, maximum correlations are diminished in the southern areas but intensified above 0.9, maximum amplitudes (n'lt shown) south of Spitsbergen. The widespread occurence are 0.8 K. At a lag of one year the correlation is at the same time, the surface intensification and at maximum along the pathways of Atlantic the strong intensification while moving Water in the northern parts: in the Barents Sea, northward point to anomalous atmospheric the WSC and along its recirculation path heatfluxes as a source .. There are no indications ,­,

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offshore from the EGC (maximum amplitudes the upper ocean temperature during this period are in the range of 0.6-0.8 K). In the frontal is obvious. The timing and amplitudes, as well zone northeast of Iceland the maximum as the dependence on depth, are consistent with negative correlation (>0.9) occurs at lag zero observations (e.g. Furevik, 1998, Blindheim et with very large amplitudes in excess of 2 K. ai., 1999). Furevik (1998) and Haugan (1999) conclude from observations that the warm The correlation of the SSS (detrended, 3-year anomaly of the early 1990s is rather driven by running mean) with the NAO index is given in local heatfluxes, while the earlier cold and Fig. 4. At lag zero a maximum negative warm anomalies are main! y propagated from the correlation exists along the path of the EGC south. This finds confi~ation in the model from Fram Strait to the southern tip of results. Greenland. Amplitudes range from -0.16 in the The advective timescales of the anomalies we below the ice cover to -0.3 can deduce from the model results are about 2 along the marginal ice zone southeast of years from the Faroe-Scotland gap to the Greenland. Also in the frontal zone northeast of Barents Sea. Though difficult to distinguish Iceland we see negative correlations up to 1.0 from lateral displacements of the NwAC there and amplitudes up to -0.24. Again here the are indications for anomalies entering through maximum correlation occurs at lag zero. Strong the Iceland - Faroe gap which move with the positive correlations can be found in the central offshore branch of the NwAC. These seem to be Barents Sea maximal at lag I with amplitudes slower compared to anomalies in the inner of up to 0.06. At lag I also the central branch, possibly due to the strong meandering Greenland Sea exhibits negative correlations of the offshore flow. above 0.8 with amplitudes of up to -0.16. The area northeast of Iceland, where water from the EGC carried eastward by the EIC meets Obviously the central Greenland Sea and the Atlantic Water entering through the Iceland - Norwegian Sea behave very dilTerent in Faroe gap, exhibits huge anomalies due to response to the NAO. While one year after displacements of the front. It remains to clarify NAO positive years the Norwegian Sea is saltier the dynamics and dependencies of the different and warmer, the Greenland Sea is fresher but currents systems. also warmer than mean, the warmer water The lagged correlation patterns of the upper stemming from the Atlantic Water recirculation. ocean temperature and salinities exhibit a strong The reason for this difference is not yet clear. A dependency of several features on the NAO possible mechanism might be an intensified index, namely the temperature and partly the drift/mixing of freshwater from melting and the salinity of the inflowing Atlantic Water along EGC into the inner Greenland Sea in or after the Norwegian coast, as well as its branches into NAO positive years. For example Blindheim et the Barents Sea, the ESC and the recirculation al. (/999) discuss the enhanced northerly wind south of Fram Strait. High correlations of the in NAO negative years as a cause for a NAO index with the salinity of the EGC and the narrowing EGC and a reduced branching of the frontal area northeast of Iceland are obvious, EIC. too. The time period covered by the presented model results, is limited to the 1980s and early I 990s. The strong influence of the NAO 4 Conclusions however. does not necessarily hold for other time periods as well. Kauker et al. (/999) have A a coupled seaice-ocean model of the Arctic shown that the index which gives the maximum Ocean and the Nordic Seas has been forced with correlation to prominent features in the Nordic atmospheric data from 1979 to 1993. This Seas in the 1960s and 1970s is the SST in the results in a large degree of variability in the 'storm formation region', rather than the NAO coupled ice-ocean system. The analysis at which gives the maximum correlations in the present shows shifts in the frontal zones of the 1980s and 1990s. Atlantic Water which give rise to large anomalies in temperature and salinity. The passing of several warm and cold anomalies in 4 ICES CM 19991L:18

Acknowledgements Hellerman, S., and M. Rosenstein, Normal monthly windstress over the world ocean with Part of this work was funded by the EC -MAST error estimates, J. Phys. Oceanogr., J3, 1093- III programme under grant MAS3~CT96-0070 1104,1983. (VEINS) and by the BMBF under. grant 01 LA982317. Hibler, W.D., A dynamic thermodynamic sea ice model, l Geophys. Res., 9, 815-846, 1979.

Hurrel, J. w., Decadal trends in the North Literature Atlantic Oscillation: Regional temperatures and precipitation. Science 269, 676-679, 1995. Blindheim, J., V. Borokov, B. Hansen, S.Aa. Malmberg, W.R. Turrell, and S. Osterhus, Kauker, F., R. Gerdes and C. Koberle, Upper layer cooling and freshening in the Propagation of temperature and salinity Norwegian Sea in relation to atmospheric anomalies in the Nordic seas as derived from a forcing, submitted to Deep Sea Res., 1999. multi-decadal OGCM simulation, ICES CM J999/L:12,1999. Dickson, R., J. Meincke, S. Aa. Malmberg and A. J. Lee, The 'Great Salinity Anomaly· in the Koberle, c., R. Gerdes and F. Kauker, Northem North Atlantic 1968-1982, Prog. Oc., Mechanisms Determining Frarn Strait Ice Exprt 20, 103-151,1988. Variability, ICES CM 1999lL:25, 1999 .

EWG - Environmental Working Group, Joint Levitus, S. And T.P. Boyer, World ocean atlas U.S. Russian Atlas of the Arctic Ocean, 1994, US Dep. Of Commerce, Washington, DC, NSlDC/ClRES, University of Boulder, CO, USA, 1994. . USA, 1997 Pacanowski, R.C., MOM 2 Documentation, Furevik, T., On the Atlantic Water Flow in the user's guide and reference manual, GFDL Nordic Seas: Bifurcation and Variability, Ocean Group Tech. Rep. No.3, Geophysical Dr.ScielZt. Thesis, Geophysical Institute, Fluid Dynamics Laboratory, Princeton University of Bergen. Norway, 1998. University, Princeton, NJ, 1995.

Gerdes, R., C. Koberle and J. Willebrand, The influence of numerical advection schemes on the results of ocean general circulation models, Climafe Dyn., 5, 211-226, 1991.

Gibson, J.K., P. Kallberg, S. Uppala, A. Nomura, E. Serrano and A. Hernande:, ERA description. ECMWF Reanalysis Project Report 1: Project organization. Tech. Rep., European Centre of Medium Range Weather Forecast. Michael Karcher, J. Brauch, B. Fritzsch, R. Retlding, UK, 1997. Gerdes, F. Kauker, C. Koeberle, M. Prange:

Harder, M., P. Lemke, and M. Hilmer, Alfred Wegener Institute for Polar and Marine Simulation of sea ice transport through Fram Research, Strait: Natural variability and· sensitivity to Postfach 120161, forcing, J. Geophys. Res., 103, 5595-5606, D 27515 Bremerhaven, Germany 1998. tel: +494714831826, Haugan, P. M., Exchange of mass and heat fruc +494714831 425, acres the Barnets Sea Opening, Prog. Oc., in email: [email protected] revision, 1999. 1982/83 1983/84

2.0 1.6 1.2 0.8 0.4 0.0 -0.4 - 0.8 - 1.2 - 1.6 - 2.0

Fig. 1 a,b: Temperature anomalies [K] from the long-term mean at 100 m depth in winters 1982/83 and 1983/84.

1989/90 1990/91

2.0 1.6 1.2 0.8 0.4 0.0 -0.4 - 0.8 - 1.2 - 1.6 - 2.0

Fig. 2a,b: Temperature anomalies [K] from the long-term mean at the sea snrface (SST) in winters 1989/90 and 1990/91. ------_.. -._------lag 0 lag 1 1.0 0.9 0.8 0.7 0.6

0.6 0.7 0.8 0.9 1.0

Fig. 3a,b: Associated patterns of the temperature at 100 m depth with the NAO index at lag zero and 1 year. Correlations> 0.66 are significant above the 90% level.

lag 1 1.0 0.9 0.8 0.7 0.6

0.6 0.7 0.8 0.9 1.0

Fig. 4a,b: Associated patterns of the sea surface salinity (SSS) with the NAO index at lag zero and 1 year. Correla­ tions > 0.66 are significant above the 90% level.