Johan Blindheim

The Arctic Ocean Response to the North Atlantic Oscillation

The climatically sensitive zone of the Arctic Ocean lies squarely within the domain of the North Atlantic oscillation (NAO), one of the most robust recurrent modes of atmospheric behavior. However, the specific response of the Arctic to annual and longer-period changes in the NAO is not well understood. Here that response is investigated using a wide range of datasets, but concentrating on the winter season when the forcing is maximal and on the postwar period, which includes the most comprehensive instrumental record. This period also contains the largest recorded low-frequency change in NAO activity—from its most persistent and extreme low index phase in the 1960s to its most persistent and extreme high index phase in the late 1980s/early 1990s. This longperiod shift between contrasting NAO extrema was accompanied, among other changes, by an intensifying storm track through the Nordic Seas, a radical increase in the atmospheric moisture flux convergence and winter precipitation in this sector, an increase in the amount and temperature of the Atlantic water inflow to the Arctic Ocean via both inflow branches (Barents Sea Throughflow and West Spitsbergen Current), a decrease in the late-winter extent of sea ice throughout the European subarctic, and (temporarily at least) an increase in the annual volume flux of ice from the Fram Strait.

Arctic intermediate water in the Norwegian Sea

Abstract At least two types of intermediate water propagate into the Norwegian Sea from the Iceland and Greenland seas. North Icelandic Winter Water flows along the slope of the Faroe-Iceland Ridge towards the Faroes. The distribution of this intermediate water is limited to the southern Norwegian Sea. The second type intrudes between the bottom water and the Atlantic Water, and can be traced as a slight salinity minimum of the entire area of the Norwegian Sea. There seems to be along-isopycnal advection of this water type along the Arctic Front from both the Iceland and Greenland Seas. Although the salinity minimum is less distinct along the slope of the continental shelf than in the western Norwegian Sea, this intermediate water separates the deep water and the Atlantic Water, and prohibits direct mixing of these two water masses.

The Nordic Seas, Main Oceanographic Features

This article presents a review of the main oceanographic features of the Nordic Seas (the Greenland, Iceland, and Norwegian Seas). It includes a short description of previous research, the bathymetry of the area, its general circulation, and the exchanges with the North Atlantic and Arctic Oceans. Water mass structure and temporal variations are discussed separately for the various basins. The most prominent variations are a cooling and freshening in the upper layers since the 1960s, particularly in the southwestern Norwegian Sea, and a rise in temperature and salinity in the deeper layers, particularly in the Greenland Sea. Some relations of these variations to ocean-atmosphere interactions are described, and the deep-water warming is ascribed to an increased influence of deep water from the Arctic Ocean and a reduced formation of deep water in the Greenland Sea. Renewal of the deep water in the Greenland Sea occurred in the late 1960s, and atmospheric observations suggest that this may have been the only renewal event in the 20th century.

Variations in the Atlantic inflow to the Nordic Seas, 1955–1996

Abstract Hydrographic observations on the Svinoy section, which runs NW from about 62°N on the Norwegian coast to 64°40′N on the prime meridian, have been used to investigate variations in the Atlantic inflow to the Nordic Seas. The data are from the winter 1955 to 1973 and from the spring and the summer 1978 to 1996. The observations show that in the summer there exist two cores of water with salinity above 35.25 at about 100xa0m depth, but in the spring and the winter the core furthest offshore is less distinct. Geostrophic calculations show that, for all data sets, there are two kernels with northward currents in the Svinoy section: one over the inner part of the continental slope (over 600–800xa0m depths) and one further offshore (over 1400–2500xa0m depth). The mean volume transports of Atlantic water in the section are 4.5, 5.0 and 5.6xa0Sv in spring, summer and winter, respectively. Results from an Empirical Orthogonal Function analysis of the summer data indicate that there is a dual structure in the section when interannual variations are considered. In the western part of the section the temperature and salinity are negatively correlated with the winter North Atlantic Oscillation index, but in the eastern part of the section they are weakly positively correlated. This is not found in the winter data from 1955 to 1973. The geostrophic volume transport in the summer through the Svinoy section is positively correlated with the NAO indices for winter and spring, where the March NAO index gave the best correlation. Also, the summer volume transports west and east in the section seem to be in opposite phase. From the summer data 1978 to 1996 there is a positive linear trend for the eastern transport (+2.4xa0Sv), but there is a negative linear trend for the western transport (−2.0xa0Sv). The transport through the whole section has increased by 1.1xa0Sv during the same similar period. The atmospheric conditions were different in years with relatively low temperatures and salinities in the western part of the section compared to those years with relatively high temperatures and salinities. It is proposed that interannual variations of temperature, salinity and volume transport in the Svinoy section are controlled mostly by a large-scale variable pressure system (i.e. the North Atlantic Oscillation index).

Distribution, migration and abundance of norwegian spring spawning herring in relation to the temperature and zooplankton biomass in the Norwegian sea as recorded by coordinated surveys in spring and summer 1996

Abstract The distribution and migration of Norwegian spring spawning herring (Clupea harengus) in the Norwegian Sea in spring and summer 1996 were mapped during 13 coordinated surveys carried out b...

The mean sea level pressure gradient across the Denmark Strait as an indicator of conditions in the North Icelandic Irminger Current

Winter means of the atmospheric sea level pressure gradient across the Denmark Strait were correlated to a time series of temperatures at a repeat hydrographic station, S3, in the waters north of Iceland. These data show that variations in North Icelandic shelf waters are closely related to the gradient. The correlation between 3-year moving averages of the gradient and the temperature at S3 of the following year was 0.70. This climatic relation clearly reflected the abrupt change from Atlantic to Arctic conditions on the North Icelandic shelf in the 1960s and the variability since then.