MESOSCALE OCEANIC RESPONSE TO WIND EVENTS OFF CENTRAL CALIFORNIA IN SPRING 1989: CTD SURVEYS AND AVHRR IMAGERY

Abstract

Analysis of hydrographic data obtained during juvenile groundfish surveys, in relation to local wind forcing and AVHRR sea-surface temperature imagery, reveals that the oceanic region off central California between Point Reyes and Point Sur in 1989 was characterized by complex circulation patterns and considerable temporal and mesoscale variability. The spring transition to upwelling-favorable winds is most clearly evidenced by rapid, large decreases in SST (up to 4-5C) measured at four meteorological buoys. Daily-averaged winds are spatially coherent and oscillate between upwelling-favorable and relaxation conditions at 310-day intervals. Persistent upwelling centers near Point Reyes and Point Aiio Nuevo were characterized by relatively cool, salty (8-10C, 33.6-34.0 psu) water in the upper 50 m, which is derived from offshore water at depths of 50-100 m. Water-mass analysis reveals that upwelled water is advected equatorward from its source. Some upwelled water is transported into shallow coastal areas and warmed. Alongshelffronts between relatively warm, low-salinity (>13C, <33.5 psu) offshore water and cool, higher-salinity upwelled water are advected onshore in response to wind relaxation or reversal events; frontal gradients intensify at these times. AVHRR imagery verifies the spatial patterns and complex mesoscale variability of the near-surface patterns observed in the CTD survey data. Eddylike hydrographic features are noted with horizontal scales on the order of the station spacing (10 km). How the complex circulation patterns and intense mesoscale spatial and temporal variability affect the survival and subsequent recruitment ofjuvenile groundfish is discussed.