Oceanic Variability Off the Central California Coast

Abstract

Mesoscale variability off the Central California coast is strongly influenced by coastal upwelling and related processes. Off Point Sur, there is significant space-time variability in oceanic properties over periods of days and distances of several tens of km. However, the internal density field, averaged over space (ca. 120km alongshore) and time (ca. 18 days), reveals the expected characteristics of a coastal upwelling regime, including an equatorward surface jet 25km to 40km offshore and weaker poleward flow below 150m and within 20km of the coast. Upper ocean circulation is strongly influenced by the bathymetry offshore to water depths of at least 1000m.

Based on repeated horizontal grids of oceanographic stations, horizontal correlation scales for temperature are of the order of 30km in the upper 100m; hence, a horizontal sampling rate of ca. 10km is desirable. Vertical correlation scales for temperature are of the order of 100m; hence, a vertical sampling rate of ca. 10m is desirable. Similarly, there are indications of a temporal correlation scale of a few weeks; hence, a temporal sampling rate of ca. a few days is desirable. Empirical Orthogonal Functions (EOFs) in the cross-shore vertical plane suggest the influence of coastally-trapped motions within 40km of the coast, with an intense alongshore, near-surface, transient coastal jet located ca. 20km offshore.

Sudden spring transitions to coastal upwelling conditions are often pronounced in coastal temperature records, and they may precede the offshore migration of the major upwelling front by at least a month or more. The spring transitions occur over a period of about a week. In 1980, the spring transition propagated poleward along the California coast at a speed of ca. 64km d−1.

During spring and summer, movement of a coastal upwelling front, often located between 15km and 50km offshore, is a major contributor to cross-shore variability. The upwelling front meanders with an alongshore scale of about 80km, a time scale of 30 to 40 days, and an amplitude scale of 10km RMS, based on satellite infrared (IR) imagery. The along-shore mean position of the major upwelling front migrates offshore during spring and summer. Offshore displacement of the front may be caused by Ekman transport over periods of days-to-weeks, and by Rossby wave dispersion over longer periods. During summer, a time-dependent offshore scale based on Rossby wave dynamics was more appropriate than the baroclinic Rossby radius.

Every three to four years, during 1971 through 1984, El Nino episodes produced interannual variability in coastal sea surface temperature (SST) off Central California. Most of the warming occured during fall and winter. El Nino episodes were often followed by major spring transitions. The recent 1982/1983 episode was 2 to 3°C warmer than the three previous episodes over the past 14 years, and its influence lasted almost twice as long.