Origin and development of recurrent dipolar vorticity structures in the outer Ría de Vigo (NW Spain)

Abstract

Two short-range (46MHz) Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde HF were operating in the Ría de Vigo during one year between September 2012 and August 2013 to permit observations of the surface circulation in the outer region of the Ría de Vigo (NW Spain). An analysis of low-pass current and vorticity conducted over one year revealed two opposite vorticity structures in the HF radar coverage area. Simultaneously, wind stress and its curl, calculated at two meteorological stations (Cíes and Borneira) in the HF radar coverage area, were assessed to establish the main mechanisms promoting the formation of these two opposite vorticity structures. Researchers selected three periods of 58, 41.5, and 77 days within the study year in which radar and wind data were simultaneously recorded at two stations.Examining HF radar vorticity at specific grid points representing both vorticity structures reveals a greater correlation between HF radar vorticity and the y-component of wind stress at the outermost meteorological station (Cíes) for all periods (R=0.52–0.80).Only one period (1 Sep–28 Oct 2012) displayed a correlation between the vorticity and the curl wind stress calculated at these two meteorological stations (R=0.50–0.62).Once wind forcing has been reported as the main factor responsible for generating these dipolar vorticity structures at subtidal frequencies, these structures can be shown to develop in two patterns: (1) counter-clockwise and clockwise relative vorticity in the northern and southern outer ría respectively, mainly developing during the transition from upwelling to downwelling, and with a lifetime normally dependent to the duration of the subsequent downwelling/relaxation conditions; and (2) clockwise and counter-clockwise vorticity in the northern and southern outer ría respectively, mainly generated during moderate or intense upwelling events. The life span of these vorticity structures ranges from 2 to 8 days. The largest range between positive and negative vorticity, or the most extreme vorticity, is observed/recorded during downwelling events. This is the first time these double vorticity structures have been observed at low frequencies in the outer Ría de Vigo. Their existence is key to understanding the mechanisms of transport between the Ría de Vigo and the shelf, and they could help explain the retention of plankton or pollutants inside some regions of the ría.