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Variability in the Nordic Seas Exchange

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Variability in the Nordic Seas Exchange 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 Arctic Ocean, 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. 1 ICES CM 19991L:18 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 Iceland 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 Atlantic Ocean down to 20° S Arctic Ocean and the Northcape Current moving (Koberle et al., 1999) is prescribed constant in onto the Barents Sea Shelf. Parts of the WSC time. The hydrography at the open boundary is branch off south of Fram Strait and recirculate taken from Levitus and Boyer (1994). southward in the western Greenland 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 East Greenland Current 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 Norwegian Sea. 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 --------------- 2 ICES CM 1999!L:18 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 Norway 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.
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