On the subduction of upwelled waters in the California Current

We use 20 years of hydrographic data from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program off southern California to search for water mass signatures of subducted upwelled waters.-A search for secondary oxygen maxima in more than 6000 water column profiles identified 629 cases of deep oxygen inversions. The additional criteria of a minimum magnitude of the deep oxygen inversion and an accompanying reduction in dissolved nutrients identified 106 (similar to 17%) of these as subduction signatures. The physical and chemical characteristics of these water masses suggest that they had been photosynthetically modified in the euphotic zone, and perhaps ventilated at the surface, before being subducted and eventually advected downstream within the California Current. Surface waters with similar physical characteristics as the subduction signatures (theta similar to 8.5 degrees C, S similar to 34.0 psu, sigma(theta) similar to 26.4kg m(-3)) were observed during the spring-summer upwelling period at a shore station upstream from the CalCOFI grid. Even with conservative search criteria and coarse spatial-temporal sampling, subduction signatures were observed in all seasons and in each year of the record. These results imply that the processes leading to the subduction, cross-shore transport, and downstream advection of upwelled water masses are common and persistent in the California Current. This has important implications for the transport of coastal waters, and resident planktonic organisms and organic carbon, out of the coastal euphotic zone, and provides a mechanism for frequent ventilation of the upper thermocline of the California Current.