Diurnal-period, wind-forced ocean variability on the inner shelf off Concepciَn, Chile

We characterize the response of diurnal-period ocean current variability to the sea breeze using measurements of current velocity taken off the mouth of the Itata River and wind stress collected at Hualpen Point (central Chile) in spring of 2007 and summer of 2006 and 2008. During these three periods, the winds are predominately towards the northeast, following the coastal topography, with the highest variability found in the near-diurnal and synoptic frequency bands. The sea breeze amplitude is intermittent in time and is associated with synoptic-scale variability on the order of three to 15 days, so that the diurnal-period winds (and currents) are enhanced when the alongshore wind (i.e. upwelling-favorable) is strong. The water current variability in the near-diurnal band is significant, explaining up to 40% (spring 2007) of the total current variance in the first 15 m depth. sponse of the near-diurnal band to the sea breeze is surface-intensified and elliptically polarized with anticlockwise rotation. As is the case with the wind, the ellipses of the daily-period surface current velocity are oriented alongshore, although the currents are significantly less rectilinear. In the along-shore direction, the diurnal-period wind-stress is directed towards the northeast and the shore after mid-day and reverse their direction at night. The currents also show counterclockwise rotation during the day, with nearly in-phase along-shore motion in the surface layer and shoreward (oceanward) motion during the morning (late afternoon). The deep layer also show significant counterclockwise motion but its magnitude is roughly one third smaller than in the surface layer, and as much as 180° out of phase. A two-layer analytical model with no friction but that includes a cross-shelf pressure gradient and wind-stress forcing is used to diagnose key properties of the observed response, suggesting that the observed diurnal-period currents may be wind driven. However, data from different years shows that inertial motions and probably other dynamics can be important when the winds are relatively weak or highly variable.