Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player


Search in the Encyclopedia
Climate fluctuations for the months Oct.-March, Ecomare

Climate fluctuations

A climate fluctuation is a temporary change in the climate which regularly reoccurs. The best known example is El Niño: the climate disturbance which disrupts the weather and sea currents in the Pacific Ocean every 3 to 8 years. A similar phenomenon occurs in the North Atlantic region: the North Atlantic Oscillation (NAO), in which the air pressure above the Azores and Iceland plays a decisive role.

  • NAO in the winter

    The NAO plays an important role particularly in the winter. Normally during the winter months, a stable low-pressure area lies above Iceland and a stable high-pressure area above the Azores. This provides a weather pattern in northwestern Europe of dominant westerly winds and mild winters. However from time to time, a stable high-pressure area develops above Iceland. When that happens, the dominant winds in the North Sea region move to the north and east, producing much harsher winters.

    The graph of the NAO-index shows a fairly irregular pattern, with spectacular dips around 1878, 1895, 1915, 1946, 1963, 1988 and 1995/96. The famous 11-city skating tour took place in many of these winters.

  • Influence of the NAO on marine life

    During severe winters, the average temperature of the seawater in the North Sea is several degrees lower than average. It is known that this lower water temperature leads in the following spawning season to a production of stronger year classes of plaice, sole and cod. Even many shellfish in the tidal areas, such as cockles, produce a lot of descendants after a severe winter; however on the other hand, many adult shellfish freeze to death during such winters.

    A second effect occurs via a change in the sea currents. For example, the herring normally spend the winter at a depth of 200 meters on the western edge of the Norwegian Trench, off the coast of Norway. In this channel, there is a stable current in the direction of Skagerrak. The wintering herring slowly swim against the current, thereby remaining in the same place. However, during winters with dominant winds from the east, the currents in the direction of Skagerrak increase. The schools of herring then move to a spot located further to the southeast. Herring return the following year to their winter home from the previous year. When a number of winters with dominant easterly winds occur in a row, the winter area will continue to shift towards the Skagerrak.

    Between 1860 and 1881, the NAO-index was practically constantly low. This resulted in the wintering area for the herring shifting to the Swedish coast of the Skagerrak. The effect was apparently so strong that the herring remained there in the following winters through 1905. This is called the Bohuslän period, named after the Swedish coastal region there. In the 10 years following with relatively high NAO-indexes, the wintering region shifted back again via the Skagerrak to the Norwegian Trench.

    Between 1945 and 1955, the wintering area shifted again to within the Skagerrak due to an NAO-dip in the middle of the 1940s. And in 1963, the herring even reached the Swedish coast: a second Bohuslän period, however much shorter than the one at the beginning of the century. This period was indeed almost fatal for the North Sea herring: within a few winters, the largest part of the herring in the eastern Skagerrak were fished. Since the spectacular dip in the NAO-index in 1995 and 1996, one expects a shift in the wintering herring.

    The link between the NAO-index and the wintering herring is the best documented phenomenon concerning the influence of climate change on life in the North Sea. However, research institutes have greatly increased their efforts in this subject. One is looking for all kinds of relations, partially in view of a better management of the fish stock and the nature of the North Sea.