Surface currents during anomalous upwelling seasons off central California

Abstract

An extensive spatiotemporal surface currents data set from central California is used to examine in detail 2 extreme years in the California Current. The year 2006 is characterized by an extended period of calm winds during the critical early months of the upwelling season (April and May). This leads to little or no upwelling over this period, essentially delaying the onset of the upwelling season. The year 2007 is characterized by extremely strong alongshore wind stress during the first quarter of the year and during the early part of the upwelling season. This produced monthly mean alongshore current velocities over the midshelf for April 2007 in excess of 0.7 m s −1 , which has not been previously observed in this region for periods of more than a few days since the installation of the HF radar instruments in 2001. Both of these extremes appear to have had important consequences for the coastal ecosystem that have been documented elsewhere, including the collapse of parts of the trophic web (2006) and the presence of large quantities of water of deep origin over the continental shelf (2007). Understanding and describing circulation and transport patterns during these extremes is important in light of predictions from global climate change, which may cause these extreme conditions to occur more frequently.