A ten-year time series from Monterey Bay, California: Seasonal, interannual and long-term patterns

S E A S O N A L   C Y C L E

Fig. 2.   Seasonal cycles of upwelling index at 36°N, 122°W, alongshore winds (rotated to 330°/150°) measured at NODC buoy 46012, alongshore currents measured at M1 with an ADCP at 8 meter bins centered at 9.5, 65.5, 105.5 and 201.5 meters depth, SST, temperature at 60 m, sea surface salinity, sigma-t, surface nitrate, nitrate at 60 m, surface chlorophyll, integrated primary production, centric diatoms, pennate diatoms, dinoflagellates, and small phytoplankton (less than 5 µm). Notice the coherence between winds and surface currents and the presence of a poleward undercurrent. Notice also the coherence between subsurface temperatures, nitrate, chlorophyll, primary production and centric diatoms. A species assemblage change occurs in the late summer and fall. (The seasonal cycles were derived from the data shown in Fig. 5).

Winds and Currents

Wind data from NDBC buoy 46012 (Fig. 1) show that daily winds offshore of MB are predominantly from the northwest at 5-10 m/s, with interruptions primarily in winter (Fig. 2). Alongshore daily wind stresses are thus predominantly equatorward with reversals in winter (Fig. 2). The average year of alongshore wind stresses (Fig. 2) shows intermittent positive (poleward) stresses December-March, minimum negative (equatorward) stresses in April and June, and moderate negative stresses July-November. There are interesting differences between the winds and the upwelling indices calculated at PFEL. These indices are calculated from operational pressure fields that integrate model and observations and as such are likely indicative of larger-scale phenomena. The upwelling indices show a broad maximum centered in June (Fig 2).

Fig. 3. Contour of the vertical distribution of the alongshore currents for the composite year. The poleward undercurrent shoals from March onwards and surfaces as the Davidson Current October.   The lined area represents a southward flow, while the unlined area represents a northward flow.

The near-surface currents respond to the wind forcing with maximum equatorward flow during April. The equatorward flow is restricted to the upper 50m and to spring and early summer (Figs 2 and 3). The poleward undercurrent, a ubiquitous feature of eastern boundary currents, is deep and weak during the early part of the year, shoals and intensifies to a maximum in the fall and surfaces in the winter as the so-called Davidson current (Figs 2 and 3).

Temperature and Salinity

The average annual cycle of sea surface temperature (SST) shows spring as the cooler time of the year in phase with the local wind field (Fig 2). The deeper temperature (100 m) reaches a minimum in June, slightly later than SST and more in phase with the upwelling indices (Fig 2). Sea surface salinity and density are more in phase with the deeper temperatures. High salinity being representative of upwelling and southerly waters and low salinity being representative of the California Current and runoff. Salinity stays high late into the year. During November-January the shallow water column (<50 m) is unstratified and moderately warm (12.5-14 ^oC) and fresh (33.2-33.5 pss) relative to other parts of the year.

Nutrients

Sea surface nitrate values are low from September to February and begin increasing in March with the onset of upwelling. Concentrations reach their maximum values in June (Fig 2). Subsurface the maximum is broader and a mirror image of the subsurface temperature field.

Chlorophyll and primary production

The chlorophyll and primary production seasonal cycles are well correlated with each other and with the subsurface fields of nitrate and temperature. They display a broad maximum centered around May and June (Fig 2).

Phytoplankton taxonomic groups

The centric diatoms show the same broad seasonal maximum as chlorophyll and primary production. The other taxonomic groups, the pennate diatoms, dinoflagellates and the cyanobacterium Synechococcus, show maximum in the late summer and fall during the oceanic period (Fig 2).

Next: Interannual Variability