Variations of mixed layer properties in the Norwegian Sea for the period 1948–1999

Abstract

The mixed layer of the ocean and the processes therein affect the ocean's biological production, the exchanges with the atmosphere, and the water modification processes important in a climate change perspective. To provide a better understanding of the variability in this system, this paper presents time series of the mixed layer properties depth, temperature, salinity, and oxygen from Ocean Weather Station M (OWSM; 66• N,2 • E) as well as spatial climatologies for the Norwegian Sea. The importance of underlying mechanisms such as atmospheric fluxes, advective signals, and dynamic control of isopycnal surfaces are addressed. In the region around OWSM in the Norwegian Atlantic Current (NwAC) the mixed layer depth varies between ∼20 m in summer and ∼300 m in winter. The depth of the wintertime mixing here is ultimately restrained by the interface between the Atlantic Water (AW) and the underlying water mass, and in general, the whole column of AW is found to be mixed during winter. In the Lofoten Basin the mean wintertime mixed layer reaches a depth of ∼600 m, while the AW fills the basin to a mean depth of ∼800 m. The temperature of the mixed layer at OWSM in general varies between 12• C in summer and 6• C in winter. Atmospheric heating controls the summer temperatures while the winter temperatures are governed by the advection of heat in the NwAC. Episodic lateral Ekman transports of coastal water facilitated by the shallow summer mixed layeris found important for the seasonal salinity cycle and freshening of the northward flowing AW. Atmospheric freshwater fluxes have no significant influence on the salinity of the AW in the area. Oxygen shows a clear annual cycle with highest values in May-June and lowest in August-September. Interannual variability of mixed layer oxygen does not appear to be linked to variations in any of the physical properties of the mixed layer.

Carbon and nutrient mixed layer dynamics in the Norwegian Sea

“…1) is an important high latitude region for processes including uptake and sequestration of CO 2 , primary production and large-scale ocean mixing. As with many other high latitude locations the mixed layer depth undergoes large seasonal fluctuations forcing seasonal dynamics of biology and chemistry in the upper ocean (Nilsen and Falck, 2006). High concentrations of nutrients in winter (nitrate >12 µmol L −1 , phosphate >0.9 µmol L −1 and silicate >5.5 µmol L −1 , Dale et al, 1999) …”

Observations of Irminger Sea Anticyclonic Eddies

“…The anticyclones form near the coast of Norway where there is a rapid slope change (Rossby et al 2009b), and then drift west and eventually coalesce with other anticyclones in the center of the basin (K€ ohl 2007). These anticyclones contain enough heat required to maintain the annually averaged heat loss in the Lofoten Basin (Rossby et al 2009a), and serve to maintain the deep pycnocline in the center of the basin (Nilsen and Falck 2006;K€ ohl 2007;Rossby et al 2009a).…”

“…In the Norwegian Sea, anticyclones observed in the Lofoten Basin (see Fig. 1) have been shown to be essential in maintaining the heat balance there (Nilsen and Falck 2006;K€ ohl 2007;Rossby et al 2009a). In both the Labrador Sea and Lofoten Basin, eddies have been extensively observed and documented, but an equivalent analysis of eddies in the Irminger Sea does not yet exist, and the potential for eddies in the Irminger Sea to affect budgets has not been explored.…”

Impact of Roundup on the marine microbial community, as shown by an in situ microcosm experiment

“…The mixed layer of ocean surface waters is estimated to fluctuate from several meters depth in very calm conditions, to 100 meters or more in strong wind and wave conditions (Bendtsen et al, 2006;Nilsen and Falck, 2006). Therefore, at the beginning of experiment, the water content of such microcosms can be considered as representative of coastal surface waters.…”