39 research outputs found
Dynamics of wind-driven upwelling and relaxation between Monterey Bay and Point Arena: Local-, regional-, and gyre-scale controls
In north and central California, equatorward winds drive equatorward flows and the upwelling of cold dense water over the shelf during the midspring and summer upwelling season. When the winds temporarily weaken, the upwelling flows between Point Reyes and Point Arena relax,\u27\u27 becoming strongly poleward over the shelf. Analytical and numerical models are used to describe the effect of alongshore variability of winds, bathymetry, and basin-scale pressure gradients on the strength of upwelling and its relaxation. Alongshore winds weaken to the south of Point Reyes, and the shelf becomes narrower from Point Reyes to Monterey Bay. Both of these lead to reduced upwelling at and to the north of Point Reyes, causing an alongshore gradient of temperature and density on the shelf. These alongshore gradients lead to an along-isobath pressure gradient over the shelf that drive the relaxation flows. A simple analytical model is used to explain the dynamics, magnitude, and structure of the relaxation flows. The modeling also suggests that the depth of origin of the upwelled waters, and thus their temperature, is controlled by the along-isobath pressure gradient that exists over the continental slope. This along-slope pressure gradient is also responsible for the California undercurrent in this region. This pressure gradient is not generated in a model of the Californian coast extending from 32 degrees N to 42 degrees N and integrated for several months, suggesting it is caused by dynamics whose spatial or temporal scales are larger than the Californian coast and/or longer than several months
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Subtidal velocity correlation scales on the northern California shelf
Along- and cross-shelf correlation scales of subtidal cross-shelf (u) and alongshelf
(Μ) velocities are estimated using moored records from several field programs over
the northern California shelf. Over record lengths of 4-6 months, along-shelf correlation
scales of Μ are greater than maximum mooring separations (60 km). In the cross-shelf
direction Μ, is generally correlated between the 60 and 130 m isobaths (10-15 km
separation). Along-shelf correlation scales of u are much smaller than those of Μ and are
often not resolved by minimum mooring separations. Time series between November 1988
and May 1989 do resolve along-shelf correlation scales of near-surface u and indicate that
they are 15-20 km. During this time the along-shelf correlation scale of near-surface u
shows variability on a monthly scale. It is generally long (30 km or more) when correlation
of u with wind stress is high and short (15 km or less) when correlation with wind stress is
low. On at least one occasion, short along-shelf correlation scales coincide with the
intrusion of an offshore mesoscale feature onto the shelf. Cross-shelf correlation scales of
u are resolved for typical mooring separations. In general, u is correlated between the 90
and 130 m isobaths (7-13 km separation) and between the 60 and 90 m isobaths (~5 km)
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Heat and salt balances over the northern California shelf in winter and spring
Heat and salt balances are estimated over the northern California shelf from
early December 1988 through late February 1989 (winter) and from early March
through early May 1989 (spring) from moored meteorological and oceanographic time
series taken in 93 m of water 6.3 km from the coast. We find a winter mean offshore
heat flux of 8.7 x 10â” W mÂŻÂč, about a factor of 5 smaller than earlier estimates of the
mean summer (upwelling season) offshore heat flux on the northern California shelf.
The mean offshore heat flux is predominantly in the surface boundary layer and is
balanced by an along-shelf heat flux divergence (as represented by an eddy along-shelf
temperature gradient flux) and a cooling trend making the mean winter heat balance
fundamentally three dimensional. In contrast to winter, the spring mean offshore heat
flux of 6.4 x l0â” W mÂŻÂč is balanced by a positive air-sea heat flux of 8.3 x 10â” W
mÂŻÂč which is about 80% of the mean air-sea heat flux in summer. This makes the
spring mean heat budget primarily two dimensional, like the summer mean heat budget
off northern California. On timescales of days the dominant terms in the fluctuating
heat budget in both winter and spring are the cross-shelf heat flux and local changes in
heat content. These are well correlated with each other and with the local along-shelf
wind stress. The along-shelf temperature gradient flux, uncorrelated with the along-shelf
wind stress, is usually weak on timescales of days. Occurrences when it is strong
are interpreted as effects of mesoscale features. Mean and fluctuating cross-shelf salt
fluxes provide essentially the same information as cross-shelf heat fluxes. This is not
surprising in light of the strong temperature-salinity relationship on the northern
California shelf
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Dynamics of wind-driven upwelling and relaxation between Monterry Bay and Point Arena : local-, regional-, and gyre-scale controls
In north and central California, equatorward winds drive equatorward flows
and the upwelling of cold dense water over the shelf during the midspring and summer
upwelling season. When the winds temporarily weaken, the upwelling flows between
Point Reyes and Point Arena âârelax,ââ becoming strongly poleward over the shelf.
Analytical and numerical models are used to describe the effect of alongshore variability
of winds, bathymetry, and basin-scale pressure gradients on the strength of upwelling
and its relaxation. Alongshore winds weaken to the south of Point Reyes, and the shelf
becomes narrower from Point Reyes to Monterey Bay. Both of these lead to reduced
upwelling at and to the north of Point Reyes, causing an alongshore gradient of
temperature and density on the shelf. These alongshore gradients lead to an along-isobath
pressure gradient over the shelf that drive the relaxation flows. A simple analytical
model is used to explain the dynamics, magnitude, and structure of the relaxation flows.
The modeling also suggests that the depth of origin of the upwelled waters, and thus
their temperature, is controlled by the along-isobath pressure gradient that exists over the
continental slope. This along-slope pressure gradient is also responsible for the California
undercurrent in this region. This pressure gradient is not generated in a model of the
Californian coast extending from 32°N to 42°N and integrated for several months,
suggesting it is caused by dynamics whose spatial or temporal scales are larger
than the Californian coast and/or longer than several months
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Coastal Perturbations of Marine-Layer Winds, Wind Stress, and Wind Stress Curl along California and Baja California in June 1999
Month-long simulations using the fifth-generation Pennsylvania State UniversityâNational Center for Atmospheric
Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate
perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along
the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind
stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes
near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther
offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the
Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even
if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception
and the Santa Barbara Channel. The Baja California wind stress is upwelling favorable. Although the winds
and wind stress exhibit great spatial variability in response to synoptic forcing, the wind stress curl has relatively
small variation. The narrow band of positive wind stress curl along the coast adds about 5% to the coastal
upwelling generated by adjustment to the coastal boundary condition. The larger area of positive wind stress
curl in the lee of Point Conception may be of first-order importance to circulation in the Santa Barbara Channel
and the Southern California Bight
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Subtidal inner-shelf circulation near Point Conception, California
We discuss connections between innerâshelf and midâshelf circulation near Point Conception, California, as well as the wind forcing of innerâshelf circulation. Point Conception marks the southern edge of a major upwelling zone that extends from Oregon to central California. The coastline makes a sharp eastward turn at Point Conception, and the Santa Barbara Channel to the east is generally assumed to be an upwelling shadow. Consistent with this regional division, innerâshelf currents are strongly correlated with wind north of Point Conception, but not in the Santa Barbara Channel. One exception to this generalization is a location in the Santa Barbara Channel, near a pass that cuts through the coastal mountains, where local winds have a dominant crossâshore component and directly drive crossâshore currents over the inner shelf. Innerâshelf currents in the Santa Barbara Channel, when compared with midâshelf currents in that area, are weaker, but strongly correlated. By contrast, innerâshelf currents north of Point Conception show a far greater incidence of poleward flow than is seen over the midâshelf in that area. Poleward flow events, lasting 1â5 days, transport warm water from the Santa Barbara Channel around Point Conception to the central California coast. These events are associated with relaxation of the generally equatorward wind, but not always with midâshelf flow reversals
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Statistical aspects of surface drifter observations of circulation in the Santa Barbara Channel
Argos-tracked drifters are used to study the near-surface circulation in
the Santa Barbara Channel. The mean consists of a cyclonic cell in the western
Santa Barbara Channel with weaker flow in the eastern Channel. Drifter mean
velocities agree well with record means from near-surface current meters. At
the eastern entrance to the channel, drifter velocities are biased toward outflow
(eastward velocity) conditions. Drifter variability at synoptic and seasonal scales
shows a tendency for upwelling and eastward flow in spring, a strong cyclonic
circulation in summer, poleward relaxation in fall, and weak, variable circulation
in winter. Drifter estimates of eddy stress divergence indicate advective terms play
a secondary role in the mean surface momentum balance. Lagrangian time and
space scales are about 1 day and under 10 km, respectively. The mismatch between
Lagrangian and Eulerian timescales indicates advective terms are important to the
fluctuating circulation.Copyrighted by American Geophysical Union
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Three interacting freshwater plumes in the northern California Current System
The northern California Current System is impacted by two primary freshwater
sources: the Strait of Juan de Fuca and the Columbia River. The Columbia is frequently
bidirectional in summer, with branches both north and south of the river mouth
simultaneously. We describe the interaction of these two warm Columbia plumes with
each other and with the colder plume originating from the strait. The interactions occurred
when a period of strong downwelling-favorable winds and high Columbia River
discharge was followed by persistent and strong upwelling-favorable winds. The
northward plume that developed under the downwelling winds extended over 200 km
along the coast to the Strait of Juan de Fuca and into the strait. The plume subsequently
wrapped around Juan de Fuca Strait water in the counterclockwise seasonal eddy just
offshore of the strait. Inspection for similar wind and outflow conditions (>0.15 N mÂŻÂČ
and 10⎠mÂł sÂŻÂč, respectively) suggest that these events might have occurred in roughly
half the years since 1994. Surface drifters deployed in the Columbia plume near its origin
tracked this plume water northward along the coast, then reversed direction at the onset
of upwelling-favorable winds, tracking plume water southward past the river mouth once
again. ââRecentââ (~1â2 day old) and ââAgedââ (>14 day old) plume water folded around
the newly emerging southwest tending Columbia plume, forming a distinctive ââsockââ
shaped plume. This plume was a mixture of ~10% ââNewââ (<1 day old) water
and ~90% Recent and Aged water from prior north tending plumes
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Evaluation of a coastal ocean circulation model for the Columbia River plume in summer 2004
Realistic hindcast of the Columbia River estuarine-plume-shelf circulation in summer
2004 using the Regional Ocean Modeling System nested within the Navy Coastal Ocean
Model (NCOM) is quantitatively evaluated with an extensive set of observations. The
model has about equal skill at tidal and subtidal properties. Tidal circulation and water
properties are best simulated in the estuary, which is strongly forced and damped, but
worst on the shelf. Subtidal currents are again best in the estuary. However, subtidal
temperature and salinity are best simulated in the surface waters on the shelf, even inside
the river plume. A comprehensive skill assessment method is proposed to evaluate the
cross-scale modeling system with a focus on the plume. The model domain is divided into
five dynamical regions: estuary, near- and far-field plume, near surface and deep layers. A
skill score is obtained for each region by averaging the skills of different physical variables,
and an overall skill is obtained by averaging the skills across the five regions. This
weighting metric results in more skill weight per unit volume in the near surface layer where
the plume is trapped and in the estuary. It is also demonstrated, through model/data
comparison and skill assessment, that by nesting within NCOM, some important remote
forcing, e.g., coastal trapped waves, are added to our model; on the other hand, some biases
are also received. With a finer grid and more realistic forcing, our regional model improves
skill over a larger-scale model in modeling the shelf-plume circulation
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Near-surface trajectories off central and southern California
The near-surface circulation in the Santa Barbara Channel and off the coast of central and southern California is described based on 20 releases of drifters
drogued 1 m beneath the surface from 12 sites within the channel at bimonthly
intervals. This description includes small-scale features of the circulation which are
not part of descriptions based on moored observations or of the statistics of the
drifter releases. The eventual fate of drifters at long time intervals compared to the
residence time in the channel (about 7 days) is also included. In the channel the
trajectories document a persistent cyclonic circulation with a typical recirculation
period between 3 and 5 days. In the spring, currents near the mainland are
weaker than near the Channel Islands, and the overall flow is toward the southeast.
Trajectories document the possibility for water parcels to leave the channel through
the interisland passes. In the late fall and winter a poleward flow with velocities
often exceeding 0.5 m sÂŻÂč is confined within 20 km of the mainland. Between
these two seasons the cyclonic tendency is enhanced, although most of the drifters
eventually migrate westward. The trajectories of drifters released at the same
time from sites only 20 km apart can be remarkably different. Once the drifters
migrate out of the channel, their trajectories can be grouped into a few patterns.
In spring and summer, drifters tend to remain in the Southern California Bight.
Their trajectories often remain close over extended periods, as if they were caught
in convergence zones. In fall the drifters often are caught in a poleward current