About Santa Barbara Channel¶
The Santa Barbara Channel is located along the southern California coast of the U.S.
Background¶
The Santa Barbara Channel region is oceanographically complex because of the variety of atmospheric and oceanic conditions that may be present in the area at one time, some with distinctly different regimes separated by up to 10 kilometers. Cold, upwelled water meets warm, subtropic Southern California Bight water in Santa Barbara Channel (Chelton 1984; Lynn and Simpson 1987). The large-scale surface flow is southward (alongshore) in spring and northward (alongshore) between summer and winter, but these large-scale flows can reverse for periods up to a week (Harms 1996). The mean flow of Santa Barbara Channel is a cyclonic (counterclockwise) eddy located at the western end of the channel.
The circulation in the Santa Barbara Channel is driven by (Harms 1996):
Wind Stress.
Wind Stress curl (Ekman pumping).
Along-shelf pressure gradients.
The Santa Barbara Channel Location File simulates mean conditions in the six circulation modes defined by Harms (1996). Using the Location File, you may not be able to simulate real conditions on a particular day; however, the Location File is sufficient for building intuition and creating scenarios.
Current Patterns¶
The user’s six circulation choices were created using combinations of three current patterns: two barotropic patterns representing flow along the mainland and island shelves, and a current pattern representing flow due to typical density (temperature) gradients in the Santa Barbara Channel. All current patterns were created by the NOAA Current Analysis for Trajectory Simulation (CATS) hydrodynamic application.
The current pattern representing the density gradients is the minimum barotropic mode (Galt, 1980, and Galt, et al. 1978) derived from a data assimilation model courtesy of Dr. Dong-Ping Wang (State University of New York), Dept. of the Interior Mineral Management Service, and the University of California (San Diego) Center for Coastal Studies. The model is a three-dimensional primitive-equation, coastal ocean general circulation model with a constant z coordinate that uses three-dimensional “blending” for data assimilation (Chen & Wang, submitted). An average of the model spring density fields (days 86 to 156 of year 1994) was used to create the local dynamic height fields. A level of no motion of 250 meters (m) was assumed since the model is homogeneous below 250 m. The NOAA CATS hydrodynamic application was used to calculate the corresponding current pattern.
Upwelling State¶
The upwelling state is named for the upwelling of cold (approximately 11°C) subsurface waters near Pt. Conception that often accompanies this state. The upwelling state occurs primarily in spring, although it has also been observed in other seasons. In terms of the conceptual models of the momentum balance, the upwelling state occurs when strong (>10 m/s), persistent (several days or more), upwelling favorable (equator ward) winds overwhelm any poleward, along-shelf pressure gradient.
Currents: The most characteristic feature of the resulting flow field is southward flow at the western entrance to the SBC, which continues eastward from San Miguel to Santa Cruz and out the eastern SBC entrance. However, even during upwelling, the flow can be weakly (10 cm/s) westward on the mainland coast of the SBC. While there can be a cyclonic (counterclockwise) recirculation in the western channel during upwelling, the southern limb of the circulation is almost always stronger than the northern limb. Weaker velocities tend to occur in the eastern SBC over the road shelf between Port Hueneme and Santa Barbara and in the SMB within 5 km of the coast. Within the SMB, the strongest (20 cm/s) velocities are observed over the 100 m isobath between Purisima Pt. and Pt. Arguello, where strong southward velocities are observed. Very weak velocities (<10 cm/s) are often observed within 5 km of the shore in San Luis Obispo Bay and between Pt. Sal and Purisima Pt. During upwelling, velocity fluctuations (relative to the mean upwelling state) are strongest southwest of Pt. Conception. This may be an expression of the tendency for an upwelling jet to fluctuate in direction and speed during upwelling. The weakest fluctuations are found over the northeast SBC shelf between Santa Barbara and Ventura, as well as the above-mentioned nearshore regions (within 5 km) of the SMB coast.
Winds: During upwelling, the wind field tends to show strong velocities (10 m/s) from the northwest (to the southeast) south of Pt. Conception at NDBC 46054. Within SMB, winds are generally onshore and equator ward. Within the eastern SBC, winds can be relatively weak.
Sea-Surface Temperatures: When available, satellite sea-surface temperature images often show cold water (11-12°C) between Pt. Arguello and Pt. Conception. Cooler water can be seen spreading southward from Pt. Conception past San Miguel Island and eastward from San Miguel towards the eastern entrance to the SBC.
Convergent State¶
The convergent state is named for the convergence of southward flow west of Pt. Arguello with westward flow south of Pt. Conception. The convergent state occurs primarily in summer, although it has also been observed in other seasons. In terms of the conceptual models of the momentum balance, the convergent state tends to occur when upwelling favorable winds and a strong poleward, along-shelf pressure gradient exist.
Currents: The most characteristic feature of the resulting flow field is a strong cyclonic recirculation in the western SBC with about equal strength in the northern and southern limbs of the recirculation. During the convergent state, velocities in the western SBC are often 40 cm/s or more, up to 70 cm/s. While northwestward flow at the eastern entrance to the SBC often occurs during the convergent state, northeastward flow directly across the eastern entrance to the SBC can also occur. The convergent synoptic state averages are accompanied by southward flow in the SMB near the shore and off-shelf flow further away from the coast. The combination of westward flow at the northeast SBC entrance and southward flow along the SMB coast is associated with convergence and offshore flow southwest of Pt. Conception. Relative to the upwelling state, stronger velocities are observed in the western SBC and in most of the SMB. The highest velocity fluctuations are observed at the western entrance to the SBC. The lowest velocity fluctuations are again found between Santa Barbara and Ventura and in San Luis Obispo Bay.
Winds: In the convergent state, the wind field can resemble the upwelling wind field, although this is not diagnostic; weak winds sometimes accompany the convergence state, but not always. The average winds at NDBC 46054 during convergence are nearly equal to those observed in upwelling, above 7 m/s from the northwest (to the southeast).
Sea-Surface Temperatures: In the convergent state, satellite sea-surface temperature images often show warm water (17-20°C) extending from the eastern SBC north and westward along the mainland coast. South of Pt. Conception, this warm water turns south and, in exceptionally clear images, a counterclockwise recirculation of warm water can often be discerned. Cold, upwelled waters are still present between Pt. Conception and Pt. Arguello, often with tongues of cold water reaching westward or southwestward.
Relaxation State¶
The relaxation state is named for the time periods when winds off Pt. Conception “relax” from their usual equator ward direction. The relaxation state occurs primarily in fall and early winter. In terms of the conceptual models of the momentum balance, the relaxation state occurs when poleward, along-shelf pressure gradients overwhelm upwelling favorable or weak winds.
Currents: The most characteristic feature of the resulting flow field is a strong westward flow (>50 cm/s) through the SBC and into the SMB. Flow in the SMB is strongest along the mainland coast. Cyclonic recirculation in the western SBC is often present, but with a northern limb strengthened with respect to the southern limb. Poleward flow continues out the western entrance to the SBC into the SMB. Within the SMB, the strongest poleward averages are found offshore of the 100 m isobath, where there is generally an offshore, in addition to poleward, component of flow. Closer to shore in the SMB, the flow velocity averages are weaker poleward flow and, in some nearshore locations, southward flow.
The highest velocity fluctuations occur west of Pt. Conception in the regions where the westward flow from the SBC is turning poleward into the SMB. A secondary maximum in the western SBC occurs where recirculating cyclonic flow rejoins the westward flow along the mainland coast. The lowest velocity fluctuations are again found between Santa Barbara and Ventura and in San Luis Obispo Bay.
Winds: Winds during relaxation tend to be either weak and variable or poleward. That is, weak or northwestward winds are usually seen at NDBC 46054 at the western entrance to SBC.
Sea-Surface Temperatures: Satellite sea-surface temperature images during relaxation will often show warm water (17-20°C) extending from Pt. Conception northwestward into the SMB.
Decision Tree¶
In the decision tree, we will be referring to different current stations by their abbreviations. This list shows the general location of each current meter shown in the three circulation patterns:
SAMI Purisma Point
SMIN Point Conception
SMOF San Miguel Island
ANMI Eastern Entrance
The surface circulation patterns are simplified representations of a dynamic, changing system. They were developed statistically, although you can only expect to pick a single pattern unambiguously 60% of the time. Small-scale circulation features, transitions between synoptic states, and uncommon patterns can make it difficult to select a single circulation pattern. We have included information on the ranges within the patterns to help you narrow down your choices. If your data does not fit any of the above circulation patterns, you may be in a transitional period, or smaller-scale phenomena (e.g., eddies) may be masking the larger-scale circulation that might otherwise be evident in the current meter reading. You may want to look at the data for the previous few days to improve your sense of what is happening in the channel.
Remember, currents are described by where they are flowing to, and winds are described by where they are blowing from. So winds and currents in the same direction (e.g., both to the south) are described from different directions [e.g. south(ward) current and north wind].
What season of the year?
If spring, then look at UPWELLING.
If summer, then look at CONVERGENT.
If autumn or winter, then look at RELAXATION.
Compare SAMI and ANMI currents
If both currents are flowing equator ward, then look at UPWELLING.
If both currents are flowing poleward (although ANMI could be flowing across the entrance), then look at RELAXATION.
If the currents are pointing toward each other, then look at CONVERGENT.
Compare the two stations at the western entrance to the Santa Barbara Channel: SMIN and SMOF
If both are about the same magnitude (arrows the same size) but pointing in different directions (SMIN westward and SMOF eastward), then look at CONVERGENT.
If SMIN shows currents flowing westward at 50 cm/s or more and SMOF is flowing between as fast westward to more slowly eastward, then look at RELAXATION.
If SMIN shows currents flowing southward or weakly westward (<10 cm/s) and SMOF is flowing faster and toward the east, then look at UPWELLING.
Look at winds at NDBC buoy 46054
If weak or SE winds (<4 m/s), then look at RELAXATION.
If strong (>8 m/s) NW winds, then look at UPWELLING.
If strong (>7 m/s) NW winds, then look at CONVERGENT.
Look at the winds in the area
If winds have been strong (>10 m/s) and equator ward for several days, then look at UPWELLING or, possibly, CONVERGENT; otherwise, consider RELAXATION or CONVERGENT.
If winds in the SMB are onshore or equatorward and winds in the SBC are relatively weak, then look at UPWELLING or, possibly CONVERGENT; otherwise, consider RELAXATION or CONVERGENT.
References¶
Oceanographic
Chelton, D. B. (1984). Seasonal variability of alongshore geostrophic velocity off central California. Journal of Geophysical Research. 89(C3): 3473-3486.
Chen, C.-S. and D.-P. Wang. Data Assimilation Model Study of the Santa Barbara Channel Circulation. Submitted to Journal of Geophysical Research.
Dever, E. P. (1998), M. C. Hendershott, and C. D. Winant (1998). Statistical aspects of surface drifter observations of circulation in the Santa Barbara Channel. Journal of Geophysical Research. 103(C11): 24,781-24,797.
Galt, J. A. (1980). A Finite-Element Solution Procedure for the Interpolation of Current Data in Complex Regions. Journal of Physical Oceanography. 10: 1984-1997.
Galt, J. A., J. E. Overland, C. H. Pease and R. J. Stewart. Numerical Studies–Pacific Marine Environmental Laboratory Report to OCSEAP. September, 1978, 177 pp.
Gunn, J.T., et al (1987). Santa Barbara Channel Circulation Model and Field Study. OCS Study MMS 87-0089, 393 pp.
Harms, Sabine (1996). “Circulation Induced by Winds and Pressure Gradients in the Santa Barbara Channel.” Doctoral dissertation, University of California, San Diego, 161 pp.
Hickey, B. M. (1979). The California Current System—hypotheses and facts. Progress in Oceanography, Vol. 8, pp. 191-279.
Lynn, R. J. and J. J. Simpson (1987). The California Current System: The seasonal variability of its physical characteristics. Journal of Geophysical Research. 92(C3): 12947-12966.
Wind and Weather
Scripps Institution of Oceanography (SIO)
An informative weather-related web site. Offers local and regional weather reports, forecasts, and related links.
Provides recent and archived wind and current data for the California Bight, including Santa Barbara Channel.
NOAA National Weather Service (NWS)
Current weather observations, forecasts, and warnings for the entire U.S.
Oil Spill Response
NOAA's Emergency Response Division (ERD)
Tools and information for emergency responders and planners, and others concerned about the effects of oil and hazardous chemicals in our waters and along our coasts.