Vertical structure and seasonal variability of the shelf currents off Bahía Magdalena, Mexico in 2011–2012: ADCP measurements

Abstract

Currents in the continental shelf off Bahía Magdalena were examined with an Acoustic Doppler Current Profiler (ADCP) anchored at a depth of 100 m from February 2011 to February 2012. Variability of the currents in the entire water column was modulated by tidal movements, which represented approximately 22% (15% diurnal, 7% semidiurnal) of the total kinetic energy. These movements had a steep vertical structure characterised by diurnal currents with average speed up to 60 cm s−1 at the surface layer of 20 m in depth. Below this depth, the tide harmonic amplitudes decreased rapidly to a relatively weak barotropic flux. Movements in the upper layer were dominated by a diurnal radiational S1 constituent, which was coherent with the local sea breeze. Significant sea currents of semidiurnal radiational S2 constituent were also associated with sea breezes. The components of the gravitational tide showed a barotropic characteristic in the entire water column, except for M2 harmonics that had intensification at the surface layer. The analysis of residual currents confirmed an important feature of the shelf circulation in the southern part of the Baja California peninsula, which consists in the formation of a northward barotropic flow in the entire water column in summer (July–beginning of August). This flow provides an intrusion of tropical water masses along the coast at least to the middle part of the peninsula. Similar manifestations are occasionally observed in the autumn months.