The space-time scales of variability in oceanic thermal structure off the Central California coast

Abstract

The space-time scales of variability in ocean thermal structure are examined off the Central California coast. In the active coastal upwelling regime off Pr. Sur, there was an equatorward surface jet 15 to 40 km offshore, and a weaker poleward undercurrent near the continental slope. Based on tendency analyses of a set of quasi-synoptic hydrographic surveys, both upwelling and mixing were important in lowering sea-surface temperature (SST). The coastal bathymetry influenced the in situ property distributions around Pt. Sur, and the satellite-derived SST patterns. Based on the satellite data, the upwelling frontal boundary meandered with space and time scale of about 80km (alongshore) and 30 to 40 days, respectively. Over several months, the major upwelling frontal boundary gradually moves the order of 50km offshore. This movement often commences in May, a month or so following the spring transition to coastal upwelling; it may be caused by both cumulative offshore Ekman transport and Rossby was dispersion. Consequently the offshore region influenced by the coastal upwelling regime exceeds the Rossby radius of deformation. The major abrupt decreases in coastal SST (of order 3C) in certain years were attributed to the spring transition to coastal upwelling. The four El Nino episodes of the past 12 years were evident in coastal SST. El Nino events may initiate, and apparently strengthen, the spring transition. Based on daily surface and subsurface temperature, correlation time scales ranged from several days (subsurface) to almost ten days (surface). The vertical temperature field was statistically homogenous with correlation scales of at least 60m. Based on empirical profiles acquired on a mesoscale grid, the limiting horizontal and vertical scales were of the order of 20km and 40m, respectively. These scale estimates provide guidance to the design of observational studies and numerical models.