Coastal circulation driven by short-period upwelling-favorable winds in the northern Baja California region

Abstract

Coastal upwelling in the California Current System depends on the synoptic variability of wind stress fields. In this system, the effects of short-period (days) wind stress fluctuations on coastal ocean circulation are particularly important south of Point Conception. Upwelling and relaxation events in northern Baja California’s coastal waters are examined using shipboard measurements of water properties and currents during October 2009. Satellite-derived variables are also included in the analysis. An upwelling event and a relaxation event, both with short periods (4–3 days), are reported here. The quasigeostrophic theory is used to elucidate differences in upwelling and relaxation ocean behavior. There are noticeable differences in the potential vorticity budget on two typical isopycnals from the study area, namely, the 24.1σθ and the 26.0σθ. Planetary vorticity is the leading term (10−9) on the two isopycnals not only during upwelling but also during relaxation. However, in the upper isopycnal, relative vorticity is larger during upwelling than during relaxation, while the stretching term is of the same order in both events; conversely, in the lower isopycnal, relative vorticity is of the same order in both events, while the stretching term is larger during upwelling than during relaxation. As a consequence of the conservation of potential vorticity, two ageostrophic circulation cells are established during upwelling, one in the upper isopycnal and the other in the lower isopycnal. The upwelling front dynamics generates the upper ageostrophic circulation cell, and the symmetric instability generates the lower ageostrophic circulation cell. The ageostrophic circulation cells are missing during relaxation.