Coastal circulation under weak upwelling-favorable wind conditions in the northern Baja California region

Coastal upwelling in the California Current System depends on both planetary-and synoptic-scale winds. In this system, the effects of the latter scale on coastal ocean circulation are particularly important at south of Point Conception. Upwelling and relaxation in the northern Baja California coastal waters are examined using shipboard measurements as well as satellite-derived variables. In this region, synoptic-scale wind (16-3.5 days) is year-round. An upwelling event and a relaxation event that occurred in fall 2009, both spanning approximately 5-day period, are reported here. The quasigeostrophic theory is used to elucidate differences in upwelling and relaxation behavior. There are noticeable differences in the potential vorticity budget on two typical isopycnals of the study area, namely the 24.1 σθ and the 26.0-σθ surfaces. 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 surface, relative vorticity is larger during upwelling than during relaxation, while stretching is of the same order in both processes; on the other hand, in the lower isopycnal surface, relative vorticity is of the same order in both processes, while stretching is larger during upwelling than during relaxation. As consequence of conservation of potential vorticity, two ageostrophic circulation cells are established during upwelling; they are missing during relaxation