By Svein Østerhus, Bjerknes Centre, also published in Klima 6/2008
There is a continuous flow of cold and dense water from the Norwegian Sea to the North Atlantic over the submarine ridge between Greenland and Scotland (Greenland–Scotland Ridge, GSR, Figure 1). Together with deep water south of the ridge and water that has been mixed along the way, this forms the lower limb of the Atlantic Thermohaline Circulation (ATHC) or the Atlantic Meridional Overturning Circulation (AMOC). Most climate models (IPCC 2007) indicate that this circulation will be weakened in the future as a result of climate change. An observed weakening has also been reported in several articles (Hansen, et al. 2001; Bryde, et al. 2005), but in an article published recently in the journal Nature, we show, using measurements and models, that the strength of the overflow across the GSR has remained unchanged since 1948 (Olsen, et al. 2008). When the transport of deep water southwards is unchanged, the transport of warm water northwards will also remain unchanged.
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| Figure 1 a). Map of the GSR where the deep currents flowing over the ridge are marked with arrows. Figure 1 b). The deepest trench between the Norwegian Sea and the North Atlantic is the Faero Bank Channel (FBC). Here, we have good measurements of the transport of the cold deep water that flows southwards. |
Measurements over ten years
Since 1995, we have measured the strength of all limbs of the Atlantic Ocean water that flow into the Norwegian Sea and parts of the cold water that return to the Atlantic. At present, we have more than ten years’ worth of good measurements of all the northward flowing currents and very good measurements of the part of the deep water that flows over the GSR through the Faroe Bank Channel. These measurements, along with hydrographic ones such as temperature and salinity going back to 1948, have been combined in a so-called hindcast experiment using a global numerical ocean circulation model. For the overlapping period (1995 to 2005), we find a convincing agreement between model simulations and observations on monthly to inter-annual timescales (Figure 2a). Neither observations nor model simulations show any significant trend in the volume transport of deep water through the Faroe Bank Channel. Nor does the model show a significant trend for the entire period from 1948 to 2005 (Figure 2b). These model simulations also concur with the few available historical observations (Figure 2b).
Strength of overflow is unchanged
Even though volume transport has shown no trend, there is a significant change in temperature and salinity in deep water in the Norwegian Sea (see Gammelsrød, et al. 2008). This has resulted in a decrease in the pressure gradient over the ridge, which in turn led to a decrease in the part of the overflow driven by this pressure difference (Hansen, et al. 2001). This decrease is compensated for by changes in sea level, and thus the total volume transport remains unchanged.
From these studies, we may draw the conclusion that the strength of the deep cold current through the Faero Bank Channel and the total overflow across the submarine Greenland–Scotland Ridge (GSR) remain unchanged, contrary to what we and others have reported previously. The decreased meridional overturning circulation as shown by the climate models may be a result of changes in the North Atlantic south of the GSR. Another reason may be that, so far, the changes have been too small to be demonstrated by direct measurements of the ocean currents.
The heat transport and processes that influence the strength of the northern limb of the ATHC/AMOC will be studied in the projects POCAHONTAS and IPY-BIAC.
References
- Hansen, Turrell and Østerhus, 2001. Decrease overflow from the Nordic Seas into the Atlantic through the Faroe Bank Channel since 1950. Nature, 411, 927-930.
- Bryden, Longworth & Cunningham, 2005. Slowing of the Atlantic meridional overturning circulation at 25°N. Nature, Vol. 438, 655-657
- Olsen, S. M., B. Hansen, D. Quadfasel and S. Østerhus, 2008. Observed and modelled stability of overflow across the Greenland–Scotland ridge. Nature, doi:10.1038/nature07302
- Gammelsrød, Skjelvan, Østerhus & Østrem, 2008. Værskipet Polarfront: Klimaforskningens flaggskip. (The Weather Ship Polarfront: Flagship for Climate Research). Klima 2-2008.
This article has been translated into English by Cicero .