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Article Dans Une Revue Continental Shelf Research Année : 2006

Low salinity intrusions in the western English Channel

Résumé

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 m 3 s -1, Smin=33.68; 2003: D=3630 m 3 s -1, Smin=33.90; 2002: D=1579 m 3 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.

Dates et versions

hal-04115094 , version 1 (02-06-2023)

Identifiants

Citer

B. A. Kelly-Gerreyn, D. J. Hydes, A. M. Jégou, P. Lazure, L. J. Fernand, et al.. Low salinity intrusions in the western English Channel. Continental Shelf Research, 2006, 26, pp.1241-1257. ⟨10.1016/j.csr.2006.03.007⟩. ⟨hal-04115094⟩
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