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 2000 and 3500 m. The ob-servations at a depth of 3000 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 2000 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.

Heat Insulation and Dissipation Processes in Nordic Seas in the Summer

The Nordic Seas have a significant impact on the climate system.Here 23-day air-sea heat fluxes were analyzed from an in situ air-sea coupled buoy deployed in the Lofoten Basin from 5 August 2012 to 27 August 2012.The buoy captured two stages of strong south and north winds.The observations indicate that warm and wet air transported by the south wind can lead to decreased sensible and latent heat fluxes and net longwave radiation.The total oceanic heat loss was 50-60Wm−2.Thus,this stage was called the heat insulation process.By contrast,the heat dissipation process occurred with the north wind condition dur-ing advection of the cold and dry air.During this process,sensible and latent heat fluxes and net longwave radiation notably in-creased.The total oceanic heat loss during the heat dissipation process reached 240Wm−2,which was four-fold greater than that in the heat insulation process.Given that the heat insulation process is dominant in summertime,the ocean lost minimal heat but absorbed strong solar energy.Thus,a large quantity of energy was stored in the ocean.Heat was transported to the Arctic Ocean and accelerated Arctic warming.The heat dissipation process is dominant in autumn and winter when the ocean releases consid-erably more energy.The two processes revealed in this paper can be applied to warm-water areas in high latitudes.

Comparing the steric height in the Nordic Seas with satellite altimeter sea surface height

In this study the steric height anomaly which is calculated from the hydrological data （EN3） is compared with the sea level anomaly derived from satellite altimetry in the Nordic Seas. The overall pattern of steric height is that it is higher in the margin area and lower in the middle area. The extreme values of steric height linear change from 1993 to 2010 occur in the Lofoten Basin and off the Norwegian coast, respectively. Such a distribution may be partly attributed to the freshening trend of the Nordic Seas. The correlation between SLA （sea level anomaly） and SHA （steric height anomaly） is not uniform over the Nordic Seas. The time series of SLA and SHA agree well in the Lofoten Basin and northern Norwegian Basin, and worse in the northern Norwegian Sea, implying that the baroclinic effect plays a dominant role in most areas in the Norwegian Sea and the barotropic effect plays a dominant role in the northern Norwegian Sea. The weaker correlations between SLA and SHA in the Greenland and Iceland Seas lead a conclusion that the barotropic contribution is significant in these areas. The area-mean SHA over the entire Nordic Seas has similar amplitudes compared with the SLA during 1996-2002, but SHA has become lower than SLA, being less than half of SLA since 2006.

Deep water distribution and transport in the Nordic seas from climatological hydrological data

Deep water in the Nordic seas is the major source of Atlantic deep water and its formation and transport play an important role in the heat and mass exchange between polar and the North Atlantic. A monthly hydrolog-ical climatology—Hydrobase II—is used to estimate the deep ocean circulation pattern and the deep water distribution in the Nordic seas. An improved P-vector method is applied in the geostrophic current calcula-tion which introduces sea surface height gradient to solve the issue that a residual barotropic flow cannot be recognized by traditional method in regions where motionless level does not exist. The volume proportions, spatial distributions and seasonal variations of major water masses are examined and a comparison with other hydrological dataset is carried out. The variations and transports of deep water are investigated based on estimated circulation and water mass distributions. The seasonal variation of deep water volume in the Greenland Basin is around 22×103km3 whereas significantly weaker in the Lofoten and Norwegian Basins. Annual downstream transports of about 1.54×103 and 0.64×103 km3 are reported between the Greenland/Lofoten and Lofoten/Norwegian Basins. The deep water transport among major basins is generally in the Greenland-Lofoten-Norwegian direction.

Slowdown of sea surface height rises in the Nordic seas and related mechanisms

A slowdown of sea surface height （SSH） rise occurred in the Nordic （GIN） seas around 2004. In this study, SSH satellite data and constructed steric height data for the decades before and after 2004 （i.e., May 1994 to April 2014） were used for comparative analysis. The findings indicate that the rate of slowdown of SSH rises in the GIN seas （3.0 mm/a） far exceeded that of the global mean （0.6 mm/a）. In particular, the mean steric height of the GIN seas increased at a rate of 4.5 mm/a and then decreased at a slower pace. This was the main factor responsible for the stagnation of the SSH rises, while the mass factor only increased slightly. The Norwegian Sea particularly experienced the most prominent slowdown in SSH rises, mainly due to decreased warming of the 0-600 m layer. The controlling factors of this decreased warming were cessation in the increase of volume of the Atlantic inflow and stagnation of warming of the inflow. However, variations in air-sea thermal flux were not a major factor. In the recent two decades, mean halosteric components of the GIN seas decreased steadily and remained at a rate of 2 mm/a or more because of increased flow and salinity of the Atlantic inflow during the first decade, and reduction in freshwater inputs from the Arctic Ocean in the second decade.