Subtidal circulation over the upper slope to the west of Monterey Bay, California

Abstract

Moored current meters were used to describe currents over the continental slope off Monterey Bay, California, from March 1998 to March 2003. The water depth at this location was 1800 m and current observations included of 16-88 m, 210- 290 m, 305 m and 1200 m although measurements at 16-88 m were not continuous. Poleward currents dominated the flow between 24 and 305 m. At 305 m the mean flow was 3.9 cm/s toward 334ʻ. Surprisingly, at 1200 m the mean flow reversed and was 0.8 cm/s toward 169ʻ. The principal axis for the flow at 305 m (1200 m) was 349ʻ (350ʻ), the semi-major axis was 9.4 cm/s (5.8 cm/s) and the semi-minor axis 3.4 (2.0 cm/s). The direction of the principal axis and the mean flow at 1200 m was aligned with the bathymetry to the east of the mooring site. The seasonal cycle at 305 m was dominated by an acceleration of the poleward flow from a minimum near zero on April 15 to maximum, 25 cm/s on July 15. This flow resulted in an increase of temperature at 305 m of 1.2ʻC due to geostrophic adjustment and a corresponding 10 cm increase in sea level due to steric effects. The acceleration of alongshore flow was out of phase with the alongshore pressure gradient which was greatest in mid- April. At 1200 m, the temperature increase (0.2ʻC) only lasted from April 15 to June 1 after which equatorward flow increased and temperature decreased. Mesoscale variability dominated the velocity measurements with maximum variance at about 60- day periods. At 305 m, the eddy kinetic energy was greatest (smallest) in October (December), 40 cm2/s2 (4 cm2/s2) while at 1200 m the maximum (minimum) occurred in July (February), 5 cm2/s2 (0.5 cm2/s2). Poleward events were stronger at 305 m while equatorward events were stronger at 1200 m. The three first empirical orthogonal functions explained 90% of the temporal variability of the horizontal currents. The first, second, and third Z-scores represented flow along the principal axis, undercurrent vs. Davidson current, and upwelling modes, respectively. While the seasonal patterns for the first two modes agreed with seasonal variability described above, the seasonal variability of the upwelling mode (6% of the variance) indicated that the waters between 16 and 88 m flowed onshore during the spring and summer upwelling period.