III. Wind, Upwelling and Oceanographic Seasons
Within the coastal regime, sea surface flow undergoes a seasonal reversal. During the late fall and winter the direction is primarily poleward while equatorward flow dominates during the spring and summer. The equatorward flow is coupled with the intensification of northwesterly winds that generally parallel the central California coastline. Wind intensity is proportional to the barometric pressure difference between the North Pacific High and the thermal low pressure centered in southern Nevada and California. This pressure gradient begins to form and strengthen in the spring. The sudden strengthening of the northwesterly winds, usually in March- May, may result in the "spring transition" in which upwelling commences and local sea surface temperatures fall by as much as 5°C within a few days (Figure 5). Surface waters are advected offshore, and equatorward geostrophic flow is established after baroclinic adjustment. During late fall, the North Pacific High weakens and migrates southward and the thermal low disappears. The surface flow reverses to poleward and can be regarded as the surface signature of the California Undercurrent, although some investigators refer to this poleward current as the Davidson Current. The timing and phasing of these coupled oceanographic and meteorological processes has been extensively studied along the California coast north of Pt Reyes (Brink and Cowles 1991).
Locally the alongshore wind stress is persistently from the north and does not reverse direction, while along the Mendocino coast and further north, the direction of the wind stress changes seasonally (Strub et al. 1987). During late fall and winter, winds become more variable as storms periodically reverse the wind direction. Maximum seasonal wind stress at 35°N occurs in May-June where at 39°N the maximum wind stress occurs in July. This seasonal variation in wind patterns has several effects. When winds are strongly from the northwest (between March and September along the central California coast, (Figure 6; Strub et al. 1987), the wind-driven (Ekman) transport of the waters between the surface and about 50 m has an offshore component. The sea surface is lowest along the coast, and tilts upward by about 20 cm across the width of the California Current (1000 km). Surface waters moved seaward are replaced by deeper upwelled waters which flow shoreward and upward beneath the Ekman layer. The isopleths of density, temperature, salinity and other tracers tilt upward by approximately 50 m in 100 km (Figure 7a; Lynn and Simpson 1987) and locally by as much as 100 m in 20 km (Rosenfeld et al. 1994). Upwelling is the combined process of the vertical movement of the pycnocline and inclinedflow along it. Upwelling speeds may reach 1 m/day or greater (Breaker and Mooers 1986) under favorable wind conditions and from depths as great as 200 m (Smith 1968). The seasonal rise and fall of temperature isopleths is observed to 500 m (Breaker and Broenkow 1994).
The Bakun (1973) upwelling index provides an estimate of the offshore Ekman transport and is computed from large scale barometric pressure distributions. The upwelling indices may yield different strength and phasing of upwelling than that inferred from winds measured from coastal buoys or shore stations (Breaker and Broenkow 1994), and neither is a perfect predictor of local upwelling strength, which also depends on the local wind stress curl. Two areas of coastal upwelling are present in the MBNMS: one near Point Año Nuevo (Rosenfeld et al. 1994), and a stronger upwelling locus south of Point Sur (Traganza et al. 1981). These upwelling areas are readily observed in AVHRR satellite images as cool areas (Figure 3). Surface temperature differences between the upwelling areas and 100 km offshore are typically 3 to 5°C (Lynn and Simpson 1987).
Rosenfeld et al. (1994) investigated the upwelling center off Point Año Nuevo north of the Bay. They suggest that surface waters enter the Monterey Bay principally from the north, while Broenkow and Smethie (1978) suggest flow into the Bay is often from the south. AVHRR satellite images often reveal a tongue of cool water extending across the mouth of the Bay. Rosenfeld et al.'s (1994) and other images (Figure 3) generally show that the upwelling locus to the south of Monterey Bay is cooler and more extensive than that near Point Año Nuevo. An AVHRR image in April 1993 (Figure 8) shows two symmetrical, apparently anti-cyclonic eddies southwest of San Francisco and Monterey Bays with cool coastal water near Point Año Nuevo and Point Sur.
In early oceanographic studies inside Monterey Bay, Skogsberg (1936) and Skogsberg and Phelps (1946) described three periods during which upwelling, wind relaxation and winter storm conditions prevailed. They used the terms "cold water phase" or "upwelling period" for the months between mid-February and November when cool surface waters were found in Monterey Bay; the "warm water phase" or "oceanic period" between mid-August to mid-October; and the "low thermal gradient phase" or "Davidson Current period" between December and mid-February. Those descriptions may be useful to describe the changing hydrographic conditions along the central California coast, but in reality these periods overlap extensively and do not recur with clockwork punctuality. The timing reflects changes in local winds and external effects such as El Niño (Norton and McLain 1994). Skogberg's "cold water phase" that he originally (1936) described as occurring between March and November emphasizes the problem of trying to generalize the timing of upwelling. Because of the irregular timing of changes in the wind field and associated oceanographic effects, contemporary oceanographers may be reluctant to simply state that upwelling in the MBNMS occurs between March and August, that the "oceanic period" happens between September and November and that the "Davidson Current period" occurs between December and February. Some oceanographers may avoid using these terms at all. For example, one intense upwelling event in Monterey Bay occurred in mid-December 1972 during a period of very strong northwesterly winds (Broenkow and Smethie 1978).
When upwelling ceases (sometimes abruptly) at the end of summer (typically August or September), sea level along the coast and inside Monterey Bay rises and the California Current slows. Sea surface temperatures along the coast may rise markedly. Later in the year (typically November) when winter storms bring occasional strong southerly winds, Ekman transport is shoreward, and in places the surface current becomes northerly. Some authors refer to this northward-flowing current as the Davidson Current, and others recognize it as the surfacing of the California Undercurrent. This flow (Wickham 1975) is a deep coastal boundary current with a core depth of about 250 m during spring and summer. Speeds in the core of the California Undercurrent are as strong as in the surface California Current. In some locations near points and capes, the surface California Undercurrent geostrophic speeds can be considerable: for example this writer measured northward surface geostrophic speeds of about 50 cm/sec near Point Lobos during the winter of 1973. Tisch et al. (1992) observed flow in the California Undercurrent to be at a core depth between 70 and 460 m with speeds of 5 to 35 cm/sec. Rischmiller (1993) observed anticyclonic eddies from AVHRR images, and his current meter records near Point Sur show generally northward surface and deep currents of about 20 cm/sec. Recent current meter measurements by oceanographers at the Navy Postgraduate School (Ramp in press) commonly found currents of 40 cm/sec in the core of the California Undercurrent.
Rosenfeld et al. (1994) emphasized that wind-driven upwelling does not occur within Monterey Bay. That view agrees with earlier work in the Bay (Broenkow and Smethie 1978), and is worth repeating here. Winds within Monterey Bay are almost directly from the west in the central Bay at Moss Landing due to the topographic break of the coastal mountains afforded by the Salinas Valley (and see Geology section).
Section II. Water Masses and Hydrography