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 and Figure 7b ; 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 inclined
flow 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).
Next - Section IV. El Niño
Physical Oceanography Table of Contents