Sea Ice Changes Driven By Wind & Oceans – Not CO2
By Paul Homewood
As we know, sea ice extent in the Arctic has reduced in recent years from its level in the 1980s. The area where the greatest reduction has been seen is the Barents Sea and adjoining area.
Dramatic climatic shifts in this part of the world have in fact been commonplace, even during the 20thC, and they are always connected to oceanic changes.
In 2007, Bob Dickson and Svein Osterhus wrote a seminal paper, One hundred years in the Norwegian Sea, which was published in the Norwegian Journal of Geography. They described the major climatic events in that part of the world.
At the turn of the 20th century oceanographic research in Norway became extensive and reached a peak in 1909 when Helland-Hansen & Nansen published The Norwegian Sea, a comprehensive description of the oceanographic features of the Nordic Seas. Since then, oceanographers have contributed to two main aspects that were largely inaccessible to the early pioneers, namely quantifying the exchanges of heat, salt and mass between the Atlantic and Arctic Oceans through the Nordic Seas, and piecing together indication of the longer term variability of the system. During the past 100 years, six events in particular have made an impact on the physical environment of the Norwegian Sea on a timescale of decades, each seeming to involve a different mix of ocean-climate processes, and each therefore providing new insight into the varied nature of physical change and ecosystem response: 1) the Great Chill, 1900–1920; 2) the Warming in the North, 1920–1960; 3) the Great Salinity Anomaly, 1968–1982; 4) the Warming of the Abyss, 1970–present day; 5) the freshening of the subarctic seas; and 6) the warming of the Arctic
The Great Chill, 1900–1920
Today, it is be generally accepted that through its northward transport of warm tropical waters, the Atlantic meridional overturning circulation (AMOC) contributes effectively to the anomalous warmth of northern Europe (Large & Nurser 2001; Rhines & Hakkinen 2003); the oceanic fluxes of volume, heat and salt that pass north across the Greenland-Scotland Ridge from the Atlantic to the Arctic Mediterranean have now been soundly established by direct measurement (8.5 Sv (1 Sv = 106 m3 s−1), 313.1012 W (reference temperature = 0°C) and 303.106 kg s−1 respectively; Østerhus et al. 2005), as have the corresponding fluxes to the Arctic Ocean (Ingvaldsen et al. 2004a; b; Schauer et al. 2004).
It is instructive that around the turn of the 20th century, the northern Norwegian Sea and Barents Sea were undergoing a period of extreme cooling, with lower air and sea temperatures than have been observed since, and with very evident impacts on the local ecosystem. As the yield, weight, liver weight, and roe weight of spawning cod (skrei) descended to what would prove to be a 100-year minimum (ICES 1996), Helland-Hansen & Nansen (1909) were among the first to formulate a clear link to environmental temperature, highlighting the contemporary minima in the extent of open water in the Barents Sea and in the sea temperatures at Kola, Lofoten and Stad. Though the climatic data set for the area was only just becoming established at that time, it seems that their inference was correct. The long-sustained Russian monitoring of the Kola Section of the Barents Sea (33.5°E, 70.5°–72.5°N) (Fig. 1, right-hand panel) shows that the minima in this 100-year record occurred c.1900–1903 and c.1916–1918 (Loeng 1991), while a reconstruction of annual and seasonal means of surface air temperature for the Barents Sea since 1900 (Fig. 3) confirms that these low sea temperature episodes were present as cold extrema in the air temperature record also in spring (March, April, May), autumn (September, October, November) and especially winter (December, January, February).
Fig. 3. Reconstructed annual and seasonal air temperatures for the Barents Sea 1880–2000 (data provided by M. Kelly (personal communication, 2005)). MAM = March–May (spring); JJA = June–August (summer); SON = September–November (autumn); DJF = December–February (winter).
The Warming in the North, 1920–1960
As our hydrographic time series is lengthened into the middle decades of the 20th century, it begins to capture evidence of one of the largest and most widespread regime shifts to affect our waters within the modern instrumental record. These were the decades of ‘the Warming in the North’, when the salinity of North Atlantic water passing through the Faroe-Shetland Channel into the Norwegian Sea reached a century-long high (Dooley et al. 1984), when salinities were so high off Cape Farewell that they were rejected as erroneous (Harvey 1962) and when a precipitous warming by more than 2°C in the 5-year mean pervaded the West Greenland banks (Fig. 6), and also when the northward dislocations of biogeographical boundaries for a wide range of species, from plankton to commercially important fish, terrestrial mammals, and birds, were at their most extreme in the 20th century. The astonishing nature of these radical events is vivid in the contemporary scientific literature, most notably in the classic accounts by Knipowitsch (1931), Sæmundsson (1934), Hansen et al. (1949), Stephen (1938), Jensen (1939), Tåning (1943), Tåning (1949), Fridriksson (1949), and many others summarized in a comprehensive bibliography by Arthur Lee (1949) and reviewed in an ICES special scientific meeting on ‘Climatic Changes in the Arctic in Relation to Plants and Animals’ in Copenhagen in 1948.
Fig. 6. (a) Sea surface temperature (SST) anomalies for West Greenland 1876–1974 (J. Smed’s data from Buch & Hansen 1988); (b) SST anomalies from Grimsey Island, Iceland Sea (S.-A. Malmberg, personal communication 1984) with near-surface (10 m) temperatures from OWS MIKE (2°E, 66°N) in the Norwegian Sea superimposed; (c) the salinity of the North Atlantic water in the Faroe-Shetland Channel 1902–1982 (from Dooley et al. 1984); (d) winter sea surface temperatures in the Faroes, 1975–1969 (from Hansen & Meincke 1984); (e) 3-year running averages of yearly temperature along the Kola Section of the Barents Sea, 1900–1990, (from Loeng 1991), all plotted to a common time base.
The Great Salinity Anomaly, 1968–1982
During winters of the 1960s the leading mode of wintertime atmospheric pressure variability in the sector under discussion – the North Atlantic Oscillation (NAO) – evolved to its extreme low index state in an instrumental atmospheric record of over a century’s duration (Hurrell 1995; Hoerling et al. 2001), and possibly much longer (e.g. Cook et al. 2002; Luterbacher et al. 2002). With anomalously high pressure persistently dominant over Greenland, a record northerly airflow swept the Norwegian Sea–Greenland Sea bringing an increasing proportion of polar water south to the seas north of Iceland in a swollen East Greenland Current. The East Icelandic Current, which had been an ice-free Arctic current in 1948–1963, became a polar current in 1965–1971, transporting drift ice and preserving it (Malmberg 1969). Aided by active ice formation in these polar conditions, the Oceanic Polar Front spread far to the south-east of normal, with sea ice extending to the north and east coasts of Iceland.
This large increase in the southward transport of ice and freshwater by the East Greenland Current, preserved by the suppression of winter convection north of Iceland, passed out to the open Atlantic through Denmark Strait in the late 1960s, and was traceable thereafter as the ‘Great Salinity Anomaly’ (GSA) around the subpolar gyre for over 14 years until its return to the Greenland Sea in 1981–1982………….
However, the GSA is certainly one of the most dramatic events of the century in the Norwegian Sea. Returning northwards to the west of Norway between 1976 and 1979, its most extreme expression was the almost total disappearance of ‘Atlantic Water’ as conventionally defined (salinities >35.00) from the Norwegian Atlantic Current west of Svalbard in 1978–1979 (Fig. 11) (Dickson & Blindheim 1984). Ecologically too, the GSA was a quite exceptional event. Jakobsson (1992) concluded that ‘the “Great Salinity Anomaly” has probably generated more variability in fisheries during the last quarter of a century than any other hydrographic event in recent years’. During its passage, Cushing (1995) found a significant reduction in recruitment in 11 out of 15 deepwater fish stocks examined. Further, as its harsh conditions closed down the ‘warming in the North’, they also established a ‘veritable desert’ for Calanus finmarchicus in waters north of Iceland (Dragesund et al. 1980; Jakobsson 1980), and set in train a change in the migration pattern of Norwegian spring spawning herring in the Nordic Seas that has taken 35 years to unfold (Vilhjálmsson 1997; Holst et al. 2004). In the mid-1960s, the traditional feeding migration to North Icelandic waters stopped completely (Fig. 12A) (Holst et al. 2004), with the older fish foraging in the Norwegian Sea east of the East Icelandic Current in 1965–1966 (Fig. 12B), and along the Ocean Polar Front to Spitsbergen in 1967–1968 and 1969 (Fig. 12C). In 1969, their overwintering grounds also retracted eastwards, shifting from the east of Iceland to the west coast of Norway. Thereafter, reinforced by a stock collapse in the late 1960s through heavy overfishing, the stock conducted its spawning, feeding and overwintering movements close in along the Norwegian coast between 1972 and 1988 (Fig. 12D and 12E), and westward feeding migrations into the Norwegian Sea stopped altogether. Renewed westward foraging by the Norwegian spring spawners has only recently been observed, since 1995 (Fig. 12F) (Holst et al. 2004).
The changes on a decadal scale were clearly huge, and were driven by oceanic processes.
It is therefore interesting to read this recent paper, Variability and impacts of Atlantic Water entering the Barents Sea from the north.
Below is the summary provided by the Bjerknes Centre for Climate Research in Norway :
A recent study by the Institute of Marine Research, the University of Bergen and the Bjerknes Centre for Climate Research in Norway shows that the northwest Barents Sea warmed substantially during the last decades. The temperature of the subsurface Atlantic Water in the northern Barents Sea increased rapidly during the late 1990s.
This was partly caused by the general warming of Atlantic Water in the North Atlantic Ocean. Additionally, a regional wind pattern indirectly strengthened a warm deep current that enters the Barents Sea from the north.
A warm deep current enters the Barents Sea from the north. The current is a branch of the Arctic Ocean Boundary Current. The branch enters the Barents Sea below the ice cover and the colder and fresher upper waters (shown in blue). © 2012 Sigrid Lind
Protects ice cover
The warm deep current enters the Barents Sea from the Arctic Ocean. It flows into the Barents Sea below colder and fresher upper waters. The current is a branch of the Arctic Ocean Boundary Current that carries warm Atlantic Water below the cold surface waters through the Arctic Ocean.
The branch enters the Barents Sea between Svalbard and Franz Josef Land. It carries warm Atlantic Water far into the Barents Sea. On its path through the northern Barents Sea the Atlantic Water gradually mixes with the cold waters above, such that the cold waters are warmed from below.
The cold waters protect the ice cover on the surface from the warm Atlantic Water below. And more importantly, the cold waters protect the ice cover from the massive amount of even warmer Atlantic Water south of the ice edge.
The study argues that easterly winds along the Barents Sea shelf-slope will lead to a lift of the Arctic Ocean Boundary Current. This will increase the inflow of Atlantic Water to the Barents Sea. The study also shows that the regional wind pattern was favourable for easterly winds and increased inflow during the late 1990s and early 2000s.
The easterly wind is part of the dipole pattern of near surface atmospheric pressure in the Arctic that was an unusual feature during this period. This is the first research article that links the regional wind with the cross-shelf exchange of Atlantic Water on the northern Barents Sea shelf.
The cold waters in the northern Barents Sea maintain a front against warmer and heavier water further south in the Barents Sea (the Polar Front). The Polar Front hinders the warm water further south from flowing into the northern Barents Sea. If the warm water could cross the Polar Front, the seasonal ice cover would not develop in winter. Thus the cold waters protect the seasonal ice cover of the Barents Sea.
The warm Atlantic Water coming into the Barents Sea from the north may become a large threat to the ice cover. The Atlantic Water is not only warm, but also very saline. Thus, increased inflow of Atlantic Water from the north will increase the salinity and density of the cold upper waters.
This will reduce the strength of the Polar Front; the warm water further south will start to cross the Polar Front; and the ice extent will be reduced. A substantial reduction of the ice cover will have large impact on the ecosystem, fisheries management, petroleum activity and shipping in the Barents Sea.
Lind, Sigrid and Randi B. Ingvaldsen (2012): Variability and impacts of Atlantic Water entering the Barents Sea from the north. Deep-Sea Research Part I 62 70–88 .
This study reminds us just how dominant the oceans are in our climate.