A method for a rapid measurement of geostrophic currents in coastal waters: A bay case study

Abstract

Geostrophic currents were computed using the method of fast thermohaline measurements from data obtained with an undulating CTD along two parallel transects in coastal waters. The method allows for the calculation of geostrophic currents in every quadrilateral set formed by each consecutive cast of the parallel hydrographic lines. With this methodology, it was possible to obtain very rapid data; for example, if the length of a section was 20 km, the distance between soundings was 1 km and the measuring depth was up to 300 meters, it would only take ~5 hours to complete the survey. Validation of the proposed method was carried out in Banderas Bay, México, where a very dense survey was taken; almost 120 casts were sampled over 2 days. With this sampling it was possible to construct a regular mesh where hydrographic data were objectively mapped and then the geostrophic currents were calculated in the traditional way using the GSW_TEOS10 toolbox. The root-mean-square deviation between both calculations was less than 3.5 %. In the case of the present study, the methodology was tested in a bay, where currents showed relatively stable patters that were persistent and well structured, suggesting this methodology may be applied to other sites, but caution is suggested in sites with large variability.