Last week we talked about the North Atlantic Oscillation, this ocean's distant, and weaker, cousin to El Nino. The NAO, in technical terms, is the difference in atmospheric pressure between the long term Icelandic Low and the Azores High. When the NAO is high, strong southwesterly winds over the North Atlantic translate to warmer than normal temperatures over Europe and Russia. When the NAO is low, that same region encounters below normal temperatures. As we'll see next week, record low values of the NAO index can result in bitterly cold conditions over large areas, with historic consequences.
The impact of NAO is not confined to Europe and Asia. The strong southwesterlies over the eastern waters of the North Atlantic during high NAO values coincide with strong northwesterlies over western waters, including the Grand Banks, bank waters northeast of the island of Newfoundland (including the Belle Isle Bank and Funk Island Bank) and over the Labrador Sea. These conditions have been linked to variations in the biomass of northern cod.
Utilizing statistics on cod recruitment over the past fifty years and values of the NAO index during the same period, Drs. Mann and Drinkwater of the Bedford Institute of Oceanography have recently established a connection between NAO values and cod stocks. Using oceanographic measurements over the Grand Banks, known properties of pack ice extending from the coast of Labrador to the Grand Banks during later winter and early spring, and drawing upon previous results from other scientists, they have established the cause for this relationship.
The strong cold northwesterly winds that result from high NAO values generate colder than normal conditions over the Labrador Sea, producing more pack ice and colder waters at lower depths. The Labrador Current carries these colder waters southeastward into northern cod regimes northeast of Newfoundland and within the Grand Banks. As the pack ice melts further south, a relatively salt-free layer of water at the surface results in a stable layer that is not conducive to vertical mixing within the ocean. Vertical mixing is necessary for the production of plankton, a critical stage in the food chain of northern cod, since it provides food for cod larvae. Lower than normal temperatures at lower depths are also inhibiting factors in the growth of cod stocks.
During periods of low NAO values, the reverse happens. Winds over the Labrador Sea are relatively lighter and air temperatures are generally above normal. Less pack ice forms and temperatures at lower depths are also warmer. Vertical mixing within the ocean is enhanced, plankton growth is strong, and northern cod stocks thrive in a warm, nutrient-rich environment.
It is well known that low values of the NAO index prevailed throughout the late 1950's, 1960's and early 1970's - a time period coincident with relatively strong periods of cod harvesting. During the 1970's and continuing through the 1980's and 1990's, the NAO index has, with only some exceptions, been high. Cod stocks diminished, of course, and eventually resulted in the closure of the fishery a few years ago.
Mann and Drinkwater are careful not to imply that the failure of the northern cod fishery has been caused by the NAO index. In their own words, "Our emphasis on environmental influences is not meant to deny the harmful effects of overfishing". Nonetheless, given the relative importance of the northern cod stocks to this region, any potential or established contributing factors must be considered within the overall context of gaining an understanding of the habits and trends of this species.
Copyright, Nov. 2, 1998.
Bruce Whiffen.