Propagation of internal tides from the upper slopes of the Bay of Biscay

Abstract

The shelf-break region of the Celtic Sea (north of the Bay of Biscay) is characterized by a reduced sea-surface temperature (∼1 °C) throughout the summer months¹. The cool region stretches for ∼300 km along the slopes and exhibits enhanced levels of surface phytoplankton abundance. It has been postulated² that physical mixing creates this favourable environment for phytoplankton growth by mixing inorganic nutrients from below the seasonal thermocline, allowing the plant cells to grow more favourably in the well-illuminated surface waters. One possibility is that the mixing is due to the production and propagation of internal tides^3–6. The present study was undertaken to detect and observe the nature of the internal tide at the Celtic Sea shelf break. We show here that internal tides, generated at spring tides close to the 200-m contour^7–9, propagate simultaneously both on shelf and into the ocean as distorted progressive waves, and when this happens, nutrients are mixed towards the sea surface.