Internal Waves and Upwelling of Oceanic Water from Mid-depths on to a Continental Shelf

Abstract

IN January 1947 at a time when the winter maximum for inorganic phosphate dissolved in the sea is usually reached, a series of observations was made in the western English Channel. In an area north of the coast of Brittany in water of 35.40–35.45%^o salinity, temperature about 1^o C. higher than in the surrounding water and a zooplankton community typicalof 'south-western' water¹, phosphate around 0.40 mgm.-atom/m.³ P was found. If, as appears probable, this is typical of Atlantic surface water at midwinter, the 'Channel' (setosa)¹ water, characterized by the presence of the chætognath, Sagitta setosa, which has occupied the English Channel in the neighbourhood of Plymouth since 1930–31 and has similar maximum phosphate content (0.42–0.48 mgm.-atom/m.³ P), may well have been derived from it. We should, however, have to look elsewhere for the origin of the higher phosphate content (0.6–0.7 mgm.-atom) of the 'elegans' or 'western' water¹ of the Celtic Sea, which in the nineteen twenties also filled the western end of the English Channel. The most probable source is upwelling of deeper ocean water from the continental slope. Such upwelling may be attributed to off-shore winds, which would need to be from the north and east. South-westerlies are the prevailing winds of the area both in direction and strength, and the winds from the northern and eastern sectors would, in general, seem not to blow hard enough or long enough to produce upwelling on the scale required. Periods of prolonged fresh easterly winds such as February 1947 are rather exceptional.