Anomalously warm July 2005 in the northern California Current: Historical context and the significance of cumulative wind stress

Abstract

In the northern California Current, the onset of the 2005 upwelling season was five weeks later than usual, and well‐established upwelling with a cold surface signature did not occur until about seven weeks after this. As part of the joint US‐Canada Pacific hake survey, from 14–16 July 2005 we occupied the Newport Hydrographic line at 44.65°N, from the Oregon coast to 83 km offshore. Instead of the cold surface layer expected in July, we observed anomalously warm water. For example, 10‐m temperature at the shelf station NH‐5 was the warmest ever recorded in July at this location: 6.2°C above average, with observations back to 1961. We explore the pivotal role played by cumulative (time‐integrated) wind forcing in the development of upwelling, in both 2005 and previous years. We find that 80% of July surface layer (0–30 m) interannual temperature variance can be explained by cumulative upwelling index from the spring transition.