Low salinity intrusions in the western English Channel

Kelly-Gerreyn, B.A., Hydes, D.J., Jegou, A.M., Lazure, P., Fernand, L.J., Puillat, I. and Garcia-Soto, C. (2006) Low salinity intrusions in the western English Channel. Continental Shelf Research, 26 (11), 1241-1257. (doi:10.1016/j.csr.2006.03.007).

Abstract

Low salinity (<35) surface waters (LSSW) at the southern entrance to the western English Channel (48.5°N, 5.1°W, near Ushant) were observed in late winter (March–April) in three successive years (2002–04) during near continuous ship of opportunity operations. The source of the LSSW is the northward spreading plumes from the Loire (47.5°N, 2.5°W) and Gironde (45.6°N, 1.2°W) along the French Atlantic coast. Fastest plume travel times were associated with northeasterly winds, consistent with Ekman theory. Differences between years in the mean winter (January–March) combined river discharges (D) was consistent with the minimum salinities (Smin) of the LSSW (2004: D=4211 m3 s?1, Smin=33.68; 2003: D=3630 m3 s?1, Smin=33.90; 2002: D=1579 m3 s?1, Smin=34.53). Winter mean (1905–74) salinity is otherwise 35.33 near Ushant.
The LSSW intruded into the western English Channel in each year, suggesting a common phenomenon. The low salinity intrusion was freshest (mean=35.11±0.21) and most penetrative (reaching 50.7°N, 1.0°W by the end of the year) in 2003 on account of (1) entering on a spring tide giving greater tidal excursion into the western English Channel and (2) intrusion favourable winds (southwesterly/southeasterly) acting on the longer term residual flow. Less penetration occurred in 2004 when the arrival of the LSSW coincided with a neap tide followed by intrusion-resistant northwesterly winds, resulting in a less saline (mean=35.20±0.23) intrusion. In 2004, transport tended to be offshore to at least 100 km from the French Atlantic coast (47°N, 4.8°W–48°N, 4.7°W). In 2002, the lower volume of plume water relative to the other years produced a more saline intrusion (mean=35.25±0.12). Prevailing westerly winds may have pushed this intrusion northwards beyond the route of the ferry, making it difficult to assess the true extent of the intrusion in 2002. A link of the LSSW to phases of the winter North Atlantic Oscillation index from a literature search of the last 84 years was inconclusive.

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