PT5 background

There is an increasing awareness of how oceanic teleconnections link phenomena in distant regions to each other. This was first described for the Pacific Ocean, where the strong events related to El Niņo/Southern Oscillation (ENSO) have been shown to generate secondary anomalies that propagate over thousands of kilometers. Using results from the Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) in conjuncture with observations, such signal propagation has been detected across the central Pacific [Jacobs et al. 1994] and along the American Pacific coastline of the North East Pacific Ocean and the Gulf of Alaska [Melsom et al., 1999, 2000].

Although the Atlantic Ocean lacks a momentous signal such as El Niņo a considerable variability in the equatorial Atlantic Ocean has been detected. This has been attributed to the Gulf of Guinea upwelling (the Benguela Niņo, or the Atlantic El Niņo) [Adamec and O'Brien, 1978; Zebiak, 1993] and the tropical Atlantic dipole [Chang et al., 1997]. The existence of the dipole has, however, been shown to be the exception rather than the rule [Enfield et al., 1999]. Penland and Matrosova [1998] have shown that the influence of the Pacific ENSO enhances the short-term predictability of the north tropical Atlantic SST for lead-time up to 18 months. Sutton and Allen [1997] speculate that the variability in the equatorial Atlantic Ocean is related to anomalous events in the Gulf Stream system and the North Atlantic current.

A large number of studies of the North Atlantic Ocean and the Nordic Seas has been based on the North Atlantic Oscillation (NAO) index [Hurrell, 1995]. Kushnir [1994] demonstrated that winter SSTs in the subtropical and subpolar gyres were warmer during a 15 year period of low NAO index (1950-1964) than during a subsequent 15 year period of high index values (1970-84). In relation to this, there is increasing evidence for the presence of of teleconnections by an oceanic pathway in the Atlantic Ocean: Hansen and Bezdek [1996] and Sutton and Allen [1997] have shown that Kushnir's warm and cold periods involved oceanic teleconnections by propagating warm and cold SST anomaly patches. These anomalies migrate across the North Atlantic Ocean from North America to the northwest of Scotland, following the strong SST gradients that are associated with the Gulf Stream and the North Atlantic Current.

During the last decades, much attention has been focused on the northward transport of Atlantic Water (AW) into the Nordic Seas. This is because the heat transport associated with the AW inflow to the Norwegian Sea and its extension northward is an important factor for climate and biological production in Northern Europe. Most of the calculations of the AW inflow are based on indirect methods where currents are inferred from hydrographic observations, sea surface height from satellite altimetry, numerical models and box models of energy and mass exchanges.

Hydrographic observations in the Faroe-Shetland Channel and the Norwegian Sea reveal the AW inflow as a warm and saline, wedge-shaped current. Based on year-long direct current measurements on the continental slope north-west of Shetland, Gould et al. [1985] calculated the mean inflow to be 7.5 Sv (1 Sv=106 m3s-1). They indicated a seasonality with a maximum in winter. Investigations using satellite altimetry to obtain estimates of the inflow by Pistek and Johnsen [1992] and Samuel et al. [1994] showed annual mean transports of 2.9 Sv and 2.7 Sv. They both showed a strong annual cycle with summer to winter variations of about 100 percent. Monthly geostrophical calculations by Blindheim [1993] for 1990 ranged from a winter maximum of 7.9 Sv to a summer minimum of 2.9 Sv, with an annual mean of 5.5 Sv.

From year-long direct current measurements in the Svinøy section, Orvik and Mork [1996] estimated the annual mean transport to be 5.3 Sv, but this study did not cover the whole width of the AW inflow. The estimates showed no systematic seasonal signal and disputed the widely held view of an annual cycle. However, the lack of significant seasonality corresponded well with Dickson et al. [1990], who from direct current measurements, found little seasonal and inter-annual variation in the overflow from the Nordic Seas. In a recent analysis of observations from a variety of instruments, Orvik et al. [2000] has found that the AW inflow can be separated into an easterly and a westerly branch, and that the transport exhibits a high degree of variability on virtually all time scales.

The main features of the circulation system within the Nordic Seas are a northward flow of warm water on the eastern side and a cold current flowing southward on the western side. North of Jan Mayen the front between these water masses is topographically controlled with small fluctuations in its position. Between Iceland and Jan Mayen variation in the volume of Arctic waters carried by the East Icelandic Current (EIC) may result in relatively large shifts of this front. The East Greenland Current (EGC) forms the cold flow to the west. In its mixed layer, the EGC carries water of low salinity, and ice, from the Arctic Ocean.

Waters of an EGC origin may affect the distribution of water masses in the Norwegian Sea in two ways. This may either take place by transport of Arctic Water (mainly in the EIC) directly into the Norwegian Basin, or indirectly through the Subpolar Gyre in the North Atlantic by Arctic influence of the Atlantic water which later flows into the Nordic Seas. The "Great Salinity Anomaly" (GSA) is an example of the latter case [Dickson et al., 1988]. From the 1950s to the 1970s the temperature and salinity decreased in the upper layers in the western and central Norwegian Sea until the minimum values that define the GSA were reached. This development was mainly due to increased fresh water supply from the EIC [Blindheim et al., 2000]. Since the time of the GSA, the salinity has rebounded. Similar fluctuations have occured in the Kola section, and also in the subpolar gyre of the North Atlantic. Temperature trends that show cooling during the 1970s and 1980s are documented by several authors [e.g. Kushnir, 1994; Reverdin et al. 1997]. These works also show a relationship between interannual fluctuations of SST and surface wind conditions. The Nordic Seas have open connections with the subpolar gyre and, indeed, exhibit similar fluctuations.

The wind forcing plays an important role for the variability of the water mass properties. Blindheim et al. [2000] found a strong correlation between the winter NAO index [Hurrell, 1995] and the lateral extent of AW in the Norwegian Sea. In association with the increased NAO index during the last 30 years, the western extent of AW has retreated eastwards in the Nordic Seas. This has resulted in a reduction of the AW re-circulation within the Nordic Seas, while re-circulation of Arctic Water and Polar Water has increased [Blindheim et al., 2000]. This affects the distribution of water masses in a much wider sense, by a reduction of deep water formation and an increased flux of AW into the Arctic Ocean. Also, Mork and Blindheim [2000] showed that there are large interannual variations in depth of the 3oC isotherm in the central Norwegian Sea and that this variability is influenced by the winter NAO index.

Recent analysis of the ice cover in the Arctic Ocean has established that significant changes have occurred in the latter part of the last century. Based on analysis of upward looking sonar observations from US nuclear submarines it has been shown that the average ice thickness has decreased by 1.3 m, from 3.1 m in the 1958-1976 period to 1,8 m in the 1990īs, in average 4 cm per year, or 40% of the total ice volume [Rothrock et al., 1999]. Furthermore, analysis of microwave satellite observations has established that the total area has decreased by 6% over the last two decades (1978-1998) [Johannessen et al., 1995; Bjørgo et al., 1997; Cavalieri et al., 1997], while the multi-year ice area has decreased 14% over the same period [Johannessen et al., 1999].

Comparison of observations (in situ and satellites) since 1900 with trends seen in two coarse resolution global climate models, forced by observed greenhouse gases and tropospheric sulphate aerosols correlates very well. This is "suggesting strongly" [Vinnikov et al., 1999] that the observed decrease in sea ice extent since 1950 is related to the antrophogenic global warming. Prediction by these two coarse resolution global climate models suggests furthermore a substantial decrease of the ice extent in this century [Vinnikov et al., 1999]. However, the strengthened indices of the Arctic Oscillation (AO) and the NAO indices [Thompson and Wallace, 1998; Hurrell, 1995] (the correlation between these two indices are very high suggesting that they are parts of the same system) has pumped warm air and water masses into the Arctic Ocean from the North Atlantic Ocean through the Norwegian Sea, causing ice melting as well as a mechanism for exporting multi-year ice through the Fram Strait [Rothrock et al., 1999]. Even some effects on the variability of the sea ice from El Ninõ has been reported [Gloersen et al., 1995].

Extrapolating the ice thickness decrease of 4 cm/year [Rothrock et al., 1999] indicates that the Arctic Ocean could be ice free 50 years from now, causing a dramatic change in the albedo, with significant effects on the global climate system. However, we should also be aware that Russian ice thickness estimates based on dispersion relationships between the damping of swell propagation into the Arctic Ocean measured from the North Pole Stations during the period 1972-1991 [Johannessen et al., 1999; Nagurnyi et al., 1994, 1999] indicates an average of 0.5- 1.0 cm decrease per year. This is 4-8 times less than the results form the nuclear submarine data.

Arne Melsom
Last modified: Fri Mar 30 13:13:47 GMT 2001