Observations of the warm Atlantic water in the Arctic Ocean
The earth radiation deficit at high latitudes necessitates a poleward transport of heat in the atmosphere and ocean, in order to keep up the temperature. The oceanic part is evident in the high latitude Arctic Seas where warm Atlantic water enters from south over the Greenland Scotland ridge and much colder water leaves the area as deep overflows at depressions in the ridge (see Hansen and Østerhus 2000, for a recent review). The deep outflow is one of the main sources of North Atlantic Deep Water. On its way northward, the Atlantic water follows the Norwegian coast up to North Cape where it splits into two branches, one entering the Barents Sea and the other continuing west of Svalbard, where it enters the Arctic Ocean through the Fram Strait. The Fram Strait branch circulates the Arctic Ocean in easily identified boundary currents with a warm core at 200–600 m depth (Rudels et al., 1996). The Barents Sea branch is cooled by air-sea interaction and freshened by ice melting and then enters the Eurasian Basin of the Arctic Ocean as a cool and relatively fresh water mass at the north-eastern part of the Barents Sea (Schauer et al., 2001).
The Arctic region has been the focus of very large scientific efforts during the last decade since it is a key region in the global climate system (see e.g. Arctic Climate System Study, ACSYS, and SHEBA ice camp). One reason is that global scale model simulations for future greenhouse gas scenarios show that the largest temperature increase will occur in the Arctic (Barnett et al., 1999) indicating an enhanced sensitivity in this region. It is also likely that an anthropogenic effect will be seen first in the Arctic which may give an early warning of global climate change.
An important issue for Arctic and global climate research is to monitor and understand fluctuations of the ice cover and ocean circulation. Observational evidence for large changes in the ice cover and ocean state has accumulated during the last decade. The release of submarine draft data has made it possible to compare the ice thickness between different time periods. These data indicate that the thickness has decreased by about 1 m when comparing the 1958–1976 and 1993–1997 time periods (Rothrock et al., 1999). Satellite observations of sea ice extent indicate a negative trend, with ice extent decreasing by 2.8% per decade during the period 1978–1996 (Gloersen et al., 1999). Regarding the oceanic condition Steele and Boyd (1998) showed that the ”cold halocline layer” in the Arctic Ocean has retreated during the 1990s to cover significantly less area. Observations made during 1993 in the southern Canadian basin showed a recent warming of nearly 1oC in the layer of Atlantic origin (Carmack et al., 1995). The temperature of the Atlantic layer (150–400 m) in the Eurasian basin increased by about 1oC between 1991 and 1996 (Schauer et al., 2001).
In order to monitor fluctuations of the Atlantic water circulation in the Arctic Ocean there is a great need for additional oceanographic observations. Before the Arctic Ocean 2001 expedition there existed only two sections with full depth coverage and high precision instrumentation which crossed the entire Eurasian Basin from the Northern Barents Sea slope to the Lomonosov ridge. These were taken during the Arctic 1991 expedition with the Swedish icebreaker Oden (Anderson et al., 1994) and the Arctic ’96 expedition with the German ice breaking research vessel Polarstern (Schauer et al., 2001). As a complement to these sections, there is a shorter section by Polarstern from 1986, and data from several submarine cruises within the US SCICEX programme 1993–1999. The SCICEX data does cover the entire section and consists of data from expendable instruments with lower precision.
During the Arctic Ocean 2001 expedition we collected a total of 63 CTD profiles of temperatures and salinity, of which 16 were collected during leg 1 around Svalbard, and 31 during leg 2 from the slope north of Svalbard and across the Eurasian Basin. The rest of the profiles were collected in a short section at the Lomonosov ridge and during the drift phase of the expedition (leg 5). The equipment used for the oceanographic measurements was a Sea-bird CTD sond together with a rosette with 24 bottles for water collection.
The Atlantic water is clearly seen as a warm layer between 150 and 600 m with the highest temperature at the slope near Svalbard where the inflow occurs. The temperature near surface is close to freeing point as a result of winter cooling and ice production. One interesting finding during this year is that a low salinity surface water (with a large fraction of river water, clearly seen as a blue colored area in the upper right hand corner of the salinity section) was spread almost over the entire Amundsen Basin, in contrast to 1995 when almost no river water was present at the European side of the Lomonosov ridge (Björk et al., 2001). The position of the river water front is highly interesting in connection with the ice growth, since the fresh water shields the ice cover from the heat in the Atlantic layer. One can therefore expect that the ice growth has been larger in the Amundsen Basin in 2001 compared to 1995.
Dates
June–September 2001
Participants
Principal investigator
Göran Björk
Department of Oceanography, University of Gothenburg
Sweden
Anna Nikolopoulos
Department of Meteorology, Stockholm University
Sweden
Michael Steele
Applied Physics Laboratory, University of Washington
Seattle,USA
Johan Söderqvist
Department of Oceanography, University of Gothenburg
Sweden
Peter Winsor
Department of Oceanography, University of Gothenburg
Sweden
References
Anderson, L.G., Björk, G., Holby, O., Jones, E.P., Kattner, G., Koltermann, K.P., Liljebladh, B., Lindergren, R., Rudels, B. and Swift, J. 1994. Water masses and circulation in the Eurasian basin: results from the Oden 91 expedition. J. Geophys. Res. 99, 3273–3283.
Barnett, T.P., Hasselmann, K., Chelliah, M., Delworth, T., Hegerl, G., Jones, P., Rasmusson, E., Roeckner, E., Ropelewski, C., Santer, B. and Tett, S. 1999. Detection and Attribution of Recent Climate Change: A Status Report. Bulletin of the American Meteorological Society, July.
Björk, G., Söderqvist, J., Winsor, P., Nikolopoulos A. and Steele, M. (submitted) Return of the cold halocline to the Amundsen Basin of the Arctic Ocean. Geophys. Res. Lett.
Carmack, E. C., Macdonald, R.W., Perkin, R.G., McLaughlin, F.A. and Pearson, R.J. 1995. Evidence for warming of Atlantic water in the southern Canadian basin of the Arctic Ocean: results from the Larsen 1993 expedition. Geophys. Res. Lett. 22, 1061–1064.
Gloersen, P., Parkinson, C.L., Cavalieri, D.J., Comiso, J.C. and Zwally, H.J. 1999. Spatial distribution of trends and sesonality in the hemispheric sea ice covers: 1978–1966. J. Geophys. Res. 104, 20827–20835.
Hansen, B. and Østerhus, S. 2000. North Atlantic-Nordic Seas exchanges. Prog. in Oceanography 45, 109–208.
Rothrock, D. A., Yu, Y. and Maykut, G.A. 1999. Thinning of the Arctic sea-ice cover. Geophys. Res. Lett. 26, 3469–3473.
Rudels, B., Anderson, L.G. and Jones, E.P. 1996. Formation and evolution of the surface mixed layer and the halocline of the Arctic Ocean. J. Geophys. Res. 101, 8807–8821.
Schauer U., Rudels, B., Jones, P., Anderson, L.G., Muench, R.D., Björk, G., Swift, J.H., Ivanov, V. and Larsson, A-M. (in press) Confluence and redistribution of Atlantic waters in the Eurasian Basin: Results from ACSYS-96. Annales Geophysicae.
Steele, M. and Boyd, T. 1998. Retreat of the cold halocline layer in the Arctic Ocean. J. Geophys. Res. 103, 10419–10435.