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

Subtidal velocity correlation scales on the northern California shelf

Along- and cross-shelf correlation scales of subtidal cross-shelf (u) and alongshelf (ν) 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 ν are greater than maximum mooring separations (60 km). In the cross-shelf direction ν, 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 ν 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)

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 10⁵ W m¯¹, 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 l0⁵ W m¯¹ is balanced by a positive air-sea heat flux of 8.3 x 10⁵ W m¯¹ 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

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 ‘‘relax,’’ 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

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 University–National 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

Subtidal inner-shelf circulation near Point Conception, California

We discuss connections between inner‐shelf and mid‐shelf circulation near Point Conception, California, as well as the wind forcing of inner‐shelf 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, inner‐shelf 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 cross‐shore component and directly drive cross‐shore currents over the inner shelf. Inner‐shelf currents in the Santa Barbara Channel, when compared with mid‐shelf currents in that area, are weaker, but strongly correlated. By contrast, inner‐shelf currents north of Point Conception show a far greater incidence of poleward flow than is seen over the mid‐shelf in that area. Poleward flow events, lasting 1–5 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 mid‐shelf flow reversals

An evaluation of the thermal properties and albedo of a macrotidal flat

The thermal properties of sediment and the albedo are critical in calculating the heat flux of a tidal flat. However, they are not well known because of the difficulties of sampling and observing tidal flats. We use extensive field observations of a macrotidal flat on the western coast of Korea to determine its sediment heat capacity and albedo. The estimated heat capacity of the upper 0.1 m is 3.65 x 106 J m¯³ K¯¹ with a water content of 70%. Heat capacity decreases with depth to 2.96 x 106 J m¯³ K¯¹ at 0.4 m depth. Estimated thermal diffusivities are 0.47–0.63 x 10¯⁶ m² s¯¹ and 0.38–0.64 x 10¯⁶ m² s¯¹ in spring and summer, respectively. The calculated albedo is a strong function of the solar altitude and the atmospheric transmittance. Atmospheric transmittance is especially important to the albedo when the solar altitude is low. Seasonal mean albedos are 0.13 and 0.15 in spring and summer, respectively. The heat capacity and albedo values obtained above were verified by using them to make independent heat flux estimates at other stations. Estimates based on heat capacity were correlated to albedo-based heat flux estimates with an r² greater than 0.7

Characteristic patterns of shelf circulation at the boundary between central and southern California

The coastal circulation in the Santa Barbara Channel (SBC) and the southern central California shelf is described in terms of three characteristic flow patterns. The upwelling pattern consists of a prevailing equatorward flow at the surface and at 45 m depth, except in the area immediately adjacent to the mainland coast in the SBC where the prevailing cyclonic circulation is strong enough to reverse the equatorward tendency and the flow is toward the west. In the surface convergent pattern, north of Point Conception, the surface flow is equatorward while the flow at 45 m depth is poleward. East of Point Conception, along the mainland coast, the flow is westward at all depths and there results a convergence at the surface between Point Conception and Point Arguello, with offshore transport over a distance on the order of 100 km. Beneath the surface layer the direction of the flow is consistently poleward. The relaxation pattern is almost the reverse of the upwelling pattern, with the exception that in the SBC the cyclonic circulation is such that the flow north of the Channel Islands remains eastward, although weak. The upwelling pattern is more likely to occur in March and April, after the spring transition, when the winds first become upwelling favorable and while the surface pressure is uniform. The surface convergent pattern tends to occur in summer, when the wind is still strong and persistently upwelling favorable, and the alongshore variable upwelling has build up alongshore surface pressure gradients. The relaxation pattern occurs in late fall and early winter, after the end of the period of persistent upwelling favorable winds

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

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 m¯² and 10⁴ m³ s¯¹, 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. ‘‘Recent’’ (~1–2 day old) and ‘‘Aged’’ (>14 day old) plume water folded around the newly emerging southwest tending Columbia plume, forming a distinctive ‘‘sock’’ shaped plume. This plume was a mixture of ~10% ‘‘New’’ (<1 day old) water and ~90% Recent and Aged water from prior north tending plumes