Deep waters warming in the Nordic seas from 1972 to 2013

The warming of deep waters in the Nordic seas is identified based on observations during Chinese 5th Arctic Expedition in 2012 and historical hydrographic data. The most obvious and earliest warming occurrs in the Greenland Basin (GB) and shows a coincident accelerated trend between depths 2 000 and 3 500 m. The observations at a depth of 3 000 m in the GB reveal that the potential temperature had increased from −1.30°C in the early 1970s to −0.93°C in 2013, with an increase of about 0.37°C (the maximum spatial deviation is 0.06°C) in the past more than 40 years. This remarkable change results in that deep waters in the center of the Lofton Basin (LB) has been colder than that in the GB since the year 2007. As for the Norwegian Basin (NB), only a slight trend of warming have been shown at a depth around 2 000 m since the early 1980s, and the warming amplitude at deeper waters is just slightly above the maximum spatial deviation, implying no obvious trend of warming near the bottom. The water exchange rate of the Greenland Basin is estimated to be 86% for the period from 1982 to 2013, meaning that the residence time of the Greenland Sea deep water (GSDW) is about 35 years. As the weakening of deep-reaching convection is going on, the abyssal Nordic seas are playing a role of heat reservoir in the subarctic region and this may cause a positive feedback on the deep-sea warming in both the Arctic Ocean and the Nordic seas.