Prepared by the

Working Group on Oceanic Hydrography

Editor: Bill Turrell

International Council for the Exploration of the Sea

__________________________________________

Conseil International pour líExploration de la Mer

Palægade 2-4 DK-1261 Copenhagen K Denmark

• Summary
• Objective of the Second ICES Ocean Climate Summary
• The North Atlantic Oscillation (NAO (Box 1)) Index
• Useful Marine Climate related Links
• NAO-related Links

  Regional Descriptions:-
  

• Area 1 - West Greenland
• Area 2 - North West Atlantic
• Area 3 - Icelandic Waters
• Area 4 - Bay of Biscay and Eastern Atlantic
• Area 5 - Rockall Trough
• Area 6 - Faroe Bank Channel
• Area 7 - North West European Shelf
• Area 8 - Northern North Sea
• Area 9 - Southern North Sea
• Area 10 - Norwegian Sea
• Area 11 - Barents Sea

- In general 1998 was a year in which ocean temperatures around the North Atlantic were warmer than the long term average, and most areas show a warming trend.

- West Greenland experienced warm sea temperatures and a mild climate for the area.

- In the Northwest Atlantic air temperatures were warmer than average. There was less winter sea ice, and the duration of ice cover was less than normal.

- Off Newfoundland, the warm period which commenced in 1996 following the NAO (Box 1) reversal, continued into 1998.

- Biologically favourable conditions were experienced in the waters around Iceland.

- In the Bay of Biscay the temperature was high in the winter of 1997/1998, followed by low temperatures in summer. The ongoing trend of increasing salinities in the upper layers since 1995 continued.

- Waters in the Northeast Atlantic, along the Northwest European shelf edge, in the North Sea and the Norwegian Sea were warmer and more saline than on average.

- Temperatures in the Barents Sea were close to average in the west and cold in the east. The temperature increased over the whole of the Barents Sea towards the end of 1998.

- The North Atlantic Oscillation (NAO-(Box 1)) index continues to recover, becoming positive in the winter preceding 1998, from a remarkably low negative value in the winter preceding 1996. The 1995/1996 reversal of the NAO (Box 1) index interrupted a trend of increasingly positive values which commenced in 1988.Areas which responded to the 1996 reversal of the NAO (Box 1)index showed a return to high-NAO (Box 1) conditions in 1998/99.

This, the second annual ICES Ocean Climate Summary, continues as a pilot study to develop further uses and dissemination of results from the ICES set of standard sections and stations - the ICES Ocean Observing System (I-OOS). The objective of this yearís summary is to present a simplified version of national reports, in a non-technical manner, in order to set the oceanic climate context for the period 1998/1999. It is intended to be of use to managers of any aspect of the marine environment in the ICES area. In addition to the presentation of data, some expert interpretation has been added by the ICES Working Group on Oceanic Hydrography.

Since the NAO(Box 1) is known to control or modify three of the main parameters which drive the circulation in the ocean area covered by this climate summary (ie wind speed, air/sea heat exchange and evaporation/precipitation) a knowledge of its past and present behaviour forms an essential context for the interpretation of observed ocean climate change in 1998.

The NAO (Box 1)alternates between a "high index" pattern, characterised by strong mid-latitude westerly winds, and a "low index" pattern in which the westerly winds over the Atlantic are weakened. High index years are associated with warming in the southern North Atlantic, NW European shelf seas and cooling in the Labrador and Nordic Seas. Low index years generally show the reverse.

When we consider the NAO (Box 1)index for the present decade, and the present decade in the context of this century, (Figure 1) the 1960s were generally low-index years while the 1990s are high index years. There was a major exception to this pattern occurring between the winter preceding 1995 and the winter preceding 1996, when the index flipped from being one of its most positive values to its most negative value this century. The index is now slowly rising from the extreme low, and the recovery continues during the winter preceding 1998. Thus 1998 has a relatively average, although positive, NAO index (Box 2).

Figure 1. The winter NAO index in terms of the present decade (upper figure) and the present century (lower figure - a 2 year running mean has been applied) (Click on Figure to get data).

In the remainder of the ocean climate summary the descriptions will proceed in a clockwise manner around the North Atlantic, commencing in the waters west of Greenland. This follows the main circulation pattern of the North Atlantic (Figure 3).

Figure 3. Schematic of the general circulation of the North Atlantic in relation to the numbered areas presented in the 1998/99 Annual ICES Ocean Climate Summary

Greenland lies within the area which experiences cool conditions when the NAO index is high. Air temperatures during the period 1989 to 1994 were particularly cold. However, the reversal of the NAO in 1996 brought relatively mild conditions to the area, which persisted into 1998.

Figure 4. Upper Figure - Annual mean air temperature observed at NUUK, Greenland, for the period 1900 to 1997. Lower Figure - Mean summer (mid June) temperature on the top of the Fylla Bank (0-40m), west Greenland

Changes in the ocean climate in the waters to the west of Greenland generally followed those of air temperature, except that in 1997 an inflow of cool, fresh Polar water resulted in cold temperatures. Oceanographic measurements performed in the summer of 1998 in the waters west of Greenland revealed some of the highest temperatures in the surface layer ever recorded since regular oceanographic observations began in the area in 1950 (Figure 4).

Sea ice on the Newfoundland and Labrador Shelves appeared late and left early resulting in a shorter duration of ice than normal during 1998. The areal extent of pack ice during 1998 was less than in 1997 and lower than average.

Off Newfoundland, the depth averaged water temperature ranged from a record low during 1991 (high NAO index in preceding winter), a near record high in 1996 (following the reversal in the preceding winter to the record low NAO index) and near the long-term mean in both 1997 and 1998 (following the recovery of the NAO index). The depth averaged summer salinity, which was below normal during most of the early 1990s returned to near normal values during 1997, and continued near normal during 1998.

A very useful index of the general oceanic environmental conditions off the eastern Canadian continental shelf is the size of the cold intermediate layer (CIL) of sub-zero water trapped between the seasonally heated upper layer and the warmer shelf slope water. During the 1960s, when the NAO was well below normal and had the lowest value ever in this century, the area of CIL water was at a minimum, and during the high NAO years of the early 1990s, CIL areas reached high values. During 1998 the CIL areas off eastern Canada increased compared to 1997 values but were still below normal, continuing a trend established in 1995. Across the Grand Bank the CIL was normal during 1998, a decrease from 1997 (Figure 5).

Figure 5. The area of the cold intermediate layer (CIL) of sub-zero EC water along standard transects across the Newfoundland Shelf during the summer. (Upper figure - Seal Island (Hamilton Bank), Centre - Bonavista, Lower - Flemish Cap (Grand Bank))

Iceland is situated at the meeting point of warm and cold currents on the Greenland-Scotland Ridge. To the south is the warm Irminger current and to the north the cold East Greenland and East Icelandic Currents. The ocean climate influences the air temperature to a great extent , and both impact biological conditions in the waters around Iceland.

In general warmer than normal conditions were experienced in Icelandic waters during 1998. The salinity of the Atlantic water arriving from the south was also greater than normal, and salinities were generally at their greatest value since the ice-years of the 1960s. Warm, saline Atlantic conditions were also experienced in the Irminger Sea towards Greenland during 1998.

These mild conditions in the south and west follow extremely cold conditions in 1995. Conditions began to improve in 1996-97, and continued to do so in 1998. However, to the north of Iceland, a cool, low saline layer was evident in 1998 which has not been seen there since 1992 (Figure 6). Salinities in the East Icelandic Current were also relatively low (<34.7).

Figure 6. Temperature and salinity at station S3 on the Siglunes section north of Iceland

The Bay of Biscay is located between the eastern part of the sub-polar gyre and the subtropical gyre. This region may be affected by both gyres depending on the latitude and the general circulation in the North Atlantic.

Figure 7. Annual mean air temperatures at Santander, northern Spain

The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500m) Wyville-Thompson ridge. It is one pathway by which warm surface North Atlantic upper water reaches the Norwegian Sea.

In May 1998 the surface Atlantic water was unusually warm and saline. The upper ocean salinity reached a maximum for the 24 year record, exceeding values observed during the high salinity years of the 1980s, which themselves followed on from the low salinity period in the mid-1970s (Figure 8).

Figure 8. Temperature and salinity anomalies of surface Atlantic water in the Rockall Trough

The Faroe Bank Channel (FBC) receives an influx of Atlantic water which passes over the Iceland - Scotland Ridge into the Nordic Seas.

Since the mid-1990s, the temperature and salinity of the inflowing Atlantic water have increased, and this trend continued in 1998 (Figure 9), although there may have been a reversal of the trend in salinity during the latter half of the year.

Figure 9. Temperature and salinity in the surface Atlantic waters entering the Faroe Bank Channel

Figure 10. Temperature and salinity anomalies in the surface Atlantic waters lying along the northwest European shelf edge.

The warm, salty conditions seen at the northwest European shelf edge were also experienced within the northern North Sea, continuing the trend of increasing annual mean temperatures and salinities during 1998. In the northwestern North Sea conditions were particularly mild during the first half of the year. The warm and mild conditions are illustrated by the near-bed temperatures and salinities extracted from the January-March ICES International Bottom Trawl Survey(Box 3) (Figure 11).

Figure 11. Temperature and salinity of near-bed water lying within the northern North Sea at 57 30'N 0'E. The observations were performed during the ICES International Bottom Trawl Survey (See Box 3).

From new data derived from a ferry run between Felixstowe and Rotterdam, it would appear that conditions in this area are quite closely linked to the NAO, although forcing may be local rather than due to oceanic forcing. Temperatures rose during 1998, continuing the recovery from the cool event during 1996, following the reversal of the NAO during the preceding winter.

Figure 12. Temperature and salinity in the surface Atlantic waters in the southern Norwegian Sea (Svin¯y Section).

Although there has been a general increase in salinity in the south eastern Norwegian Sea since 1992, there was a decrease from 1997 to 1998. In most of the remainder of the Norwegian Sea salinities have been declining since the early 1980s. This is probably a result of persistent wind driven supply of relatively fresh water from the East Icelandic Current.

The Barents Sea is a shelf sea, receiving an inflow of Atlantic water from the west. The inflow demonstrates considerable seasonal and interannual fluctuations in strength and water mass properties, particularly in heat content and consequently ice coverage.

Figure 13. Temperature anomalies in the section between Norway and Bear Island

After a period with high temperature in the first half of this decade, the temperatures in the Barents Sea dropped to values slightly below the long term average over the whole area in 1996 and 1997 (except March 1997). From March 1998, the temperature in the western area increased to just above the average (Figure 13), while the temperature in the eastern areas stayed below the average during 1998 (Figure 14)

Figure 14. Temperature and salinity anomalies observed in the main currents along the historic Kola section (see Box 4) - The Kola Section Centenary) in the eastern Barents Sea.

The high temperatures observed upstream in the Atlantic current arrived at the western entrance of the Barents Sea between October 1998 and January 1999 when the temperature rose by more than half a degree.

This increase was to a lesser extent observed in the central and eastern Barents Sea. The increased temperatures were also followed by increased salinity. The present temperature conditions is reflected in the ice conditions with little ice in the western area and severe ice conditions in the eastern areas of the Barents Sea.