Continental Shelf Research 23 (2003) 533–561 Observations of the physical structure and seasonal jet-like circulation of the Celtic Sea and St. George’s Channel of the Irish Sea J. Brown a, * ,1 , L. Carrillo b,c,2 , L. Fernand a , K.J. Horsburgh b , A.E. Hill c , E.F. Young a,c , K.J. Medler a a Centre for Environment Fisheries and Aquaculture Science (CEFAS), Pakefield Rd., Lowestoft, Suffolk, NR33 OHT, UK b School of Ocean Sciences, University of Wales Bangor, Menai Bridge, LL59 5EY, UK c Proudman Oceanographic Laboratory, Bidston Observatory, Bidston Hill, Prenton, CH43 7RA, UK Received 2 April 2002; accepted 8 January 2003 Abstract During July–September 1998 a comprehensive survey was undertaken of the Celtic Sea and St. George’s Channel of the Irish Sea in order to resolve the three-dimensional density field and circulation. Towed undulating CTD data revealed that isolated below the seasonal thermocline was a dense (cold and saline) pool flanked by strong near-bed density gradients. The trajectories of 23 satellite-tracked Argos drifters demonstrated the existence of a cyclonic (anti- clockwise) circulation pattern. This was in accord with persistent and narrow (ca. 20 km) cores of cyclonic flow (dense water to the left) as revealed by geostrophic current estimates and moored acoustic Doppler current profiler data, with velocities >0.1 m s 1 and exceeding 0.3 m s 1 in St. George’s Channel. The results are consistent with a seasonal baroclinic circulation that advects saline Atlantic water through the south and west of the region to St. George’s Channel. Westward flow across St. George’s Channel is directed south into the Celtic Sea and west along the Irish coast. The southward flow, an artefact of the baroclinic circulation, often appears in infrared satellite imagery as a meander in the Celtic Sea front. At the southward extension a secondary clockwise circulation is formed (diameterB40 km) before water joins westward flow. Fresher water of coastal origin, present in the northern Celtic Sea in late spring, is advected westward by the summer circulation. The advent of summer thermal stratification and associated flow fields reduces exchange between the Celtic and Irish Seas. The understanding provided by such datasets such as those described here provides the necessary basis for environmental management and knowledge of the pathways for contaminants, the dynamics of nutrients and an understanding of the movement of larvae and juvenile fish. It also provides a template against which to test the prognostic capabilities of present and emerging baroclinic numerical models. Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. Keywords: Jet; Density driven; Baroclinic circulation; Drifters; Satellite observations; Contaminant advection *Corresponding author. Tel.: +44-151-653-8633; fax: +44-151-652-3950. E-mail address: jbrown@bodc.ac.uk (J. Brown). 1 Now at The British Oceanographic Data Centre (BODC), Bidston Observatory, Bidston Hill, Merseyside, Prenton, CH43 7RA, UK. 2 Now at Scripps Institution of Oceanography, 8602 La Jolla Shores Drive, La Jolla, CA 92037, USA. 0278-4343/03/$ - see front matter Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0278-4343(03)00008-6