Upper mixed layer temperature and salinity variability in the tropical boundary of the California Current, 1997-2007

Abstract

Spatial and temporal interannual variability of mixed layer (ML) temperature and ML salinity off Baja California are examined using empirical orthogonal functions analysis. Conductivity-temperature-depth data collected from October 1997 through January 2007 over a grid based on Mexican Research of the California Current quarterly survey cruises are analyzed. Net heat flux (NHF) and sea surface height anomaly (SSH) from satellite products are also analyzed. The first leading mode of both ML temperature and ML salinity show a single-signed loading pattern, in which the variability increases southward. Those patterns have been reported before, but they are lacking a quantitative explanation. ML temperature variability is mainly associated with NHF variability, while ML salinity variability is mainly associated with large-scale SSH variability. The principal component (PC) of ML salinity is correlated with North Pacific Gyre Oscillation and Warm Water Volume climate indices, while the PC of ML temperature is only correlated with the latter index. Those results indicate that the principal mode of ML salinity variability is a diagnostic variable of basin-scale process. An abrupt freshening (similar to-0.7) and cooling (similar to-4 degrees C) event from January 1998 to January 1999 and an abrupt freshening (similar to-0.5) event from January 2002 to January 2003 are conspicuous features in the mixed layer. The 1998-1999 events are associated with the major El Nino-La Nina cycle in the 10-year period. The 2002-2003 freshening is related to an enhancement of subarctic water into the equatorward flow that started during the summer of 2002 off Oregon (49 degrees N).