Seasonal and synoptic variability of diurnal currents in an upwelling system off northern Chile near 30° S

Abstract. This study documents the seasonal and synoptic variability of diurnal currents in northern-central Chile (~ 30° S), using current measurements from four sites collected over more than one year. The study area includes a coastal upwelling center well exposed to sea wind and a large bay system (~ 100 km long) located just north of the above mentioned upwelling center. This bay system consists of several smaller bays with different orientations and morphologies, which affect the internal hydrodynamics and favor local recirculation patterns. Inertial oscillations in the area have a period of ~ 24 h, which is the same as that of the periodic wind forcing due to the sea breeze, and thus, this coupling may cause system resonance, as has been reported in other regions. The most intense diurnal currents (with amplitudes of ~ 30 cm s⁻¹) were recorded in the surface layer in one of the areas exposed to the wind and farthest from the coastline (to ~ 22 km). In contrast, within the bay system, which is sheltered from the wind, diurnal currents were less intense (~ 10 cm s⁻¹). Diurnal currents had higher seasonal variability in the more exposed areas than in the protected ones and were more intense in spring and summer than in autumn and winter. This was consistent with the Lagrangian measurements of the surface currents, which showed a higher diurnal energy in summer than in winter. The diurnal wind variability was modulated by the synoptic-scale circulation, which directly affected the diurnal current response. Under upwelling-favorable winds, diurnal currents were mainly forced by daily wind variations due to the sea breeze, while a sudden decrease in wind speed generated inertial oscillations that decayed with depth, especially in the area farthest from the coast. In general, the greatest variability in the diurnal currents occurs in the most exposed area to the wind and farthest from the coast, due to resonance between diurnal wind forcing and inertial oscillations, and possibly by near-diurnal internal gravity waves.