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Description of the Affected Environment: Physical Oceanography

(Equatorial) water entering the bight from the south,

principally by way of the California Undercurrent (at

300 m depth). The distribution of these waters in

the bight is such that the top 200 m is typically low in

salinity and high in oxygen content, which identifies

this water mass as principally sub-arctic, even though

the temperatures range between 9° and 18° C. The

next 300 m are consistently high in salinity and low

in dissolved oxygen, identifying it as equatorial Pacific water. The temperature range for this water mass

is 9°– 5° C (Jackson 1986).

The circulation of the Southern California Bight

is dominated by the Eastern Boundary Current of the

North Pacific Gyre system, specifically the California

Current, rather than by local wind forcing. The California Current carries subarctic water equatorward

throughout the year, extends offshore a distance of

about 400 km, and to a depth of 300 meters. The average speed of the California Current is approximately

0.25 m/s. Maximum speeds of the California Current

are found at the surface with the strongest

equatorward flow occurring during the spring and

summer.

Nearer to the California coast, within 150 km,

the surface current periodically reverses to the

poleward direction, which, when it occurs is called the

Inshore Countercurrent. The Inshore Countercurrent

is strongest during the fall and winter, with its

poleward flow reaching its maximum speeds (exceeding 0.04 m/s) typically within 50 km offshore of the

coast.

In Figure 4.4-2, we see that the California Current, flowing in a southerly direction 200 – 500 km

offshore Point Arguello, brings in cold, low-salinity,

highly oxygenated water from the Subarctic region.

The California Current continues in its southerly direction, mixing along the way with the warm, saline,

north-central Pacific water coming in from the west.

South of San Diego, part of the California Current

spins eastward into the Southern California Bight and

then poleward forming the California Counter-Current. It joins the poleward California Undercurrent

which is deeper and inshore of the California CounterCurrent.

Figure 4.4-2. Characteristic oceanic circulation in, and sources of water of, the Southern California Bight

(Browne 1994).
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Delineation Drilling Activities Offshore Santa Barbara County

Below 200 m depth, the poleward California Undercurrent exists throughout the year and is generally confined to within 100 km of the coast along the

continental slope. The California Undercurrent originates in the eastern equatorial Pacific and brings these

warm, saline, low dissolved oxygen waters poleward

into the Southern California Bight. At either end of

the Santa Barbara Channel, the California Undercurrent shows considerable seasonal variability. In winter and early spring, the California Undercurrent is

its weakest with its maximum poleward core flow

found below 200 m depth. During this time the surface flow is typically equatorward. In late summer,

fall, and early winter its poleward core flow increases

in strength and ascends to shallower depths, at times

reaching the surface where it becomes indistinguishable from the poleward Inshore Countercurrent.

Currents at the top 200 m within the boundaries

of the Santa Monica-San Pedro Basins are poleward

year round (Hickey 1992), with their speed ranging

from 15 cm/sec to 20 cm/sec from late spring to winter and approximately 5 cm/sec from late winter to early

spring. Coastally-trapped waves traveling up the west

coast of Mexico are thought to be a primary reason for

surface current fluctuations in the Bight shelf region.

The surface currents are driven geostrophically by these

long period waves and not by local winds occurring in

the Bight.

Previously, biologists have thought that benthic

(sea bottom) sediments were depleting the oxygen content from the waters in the deep basins (700 – 900 m

depth). This may only be partially true, for they reasoned that this apparent consistent lack of oxygen was

due to a very slow overturning of basin water (6 – 18

months). They were looking at water properties only

and had no idea of the nature of the bottom currents

in those basins. Hickey’s measurements indicated that

complete overturning in the basins occurred in only

1-3 months. The lack of oxygen content in the basins

results from a good part of the basin waters coming

from the California Undercurrent, bringing in highly

saline, low-oxygenated, equatorial Pacific water.

During the fall, a relatively large water mass located between the depths of 300 and 600 m and within

the Santa Monica-San Pedro boundaries flows

equatorward against the current direction of the rest

of the water column.

Upon reaching the northern region of the Bight,

the poleward flow typically bifurcates and enters both

Santa Barbara and Santa Cruz Basins at their eastern sills. However, during strong, continuous, northwest winds (upwelling favorable) along the central

California coast, the flow sometimes reverses and

Channel water flows out of its eastern entrance. In

this instance, the poleward moving current in the

Bight is completely diverted to the west flowing along

the southern coasts of the Channel Islands.

4.4.4 SANTA BARBARA CHANNEL - SANTA

MARIA BASIN SURFACE

CIRCULATION

The description of the physical oceanography of

the Santa Barbara Channel – Santa Maria Basin as it

relates to the surface circulation in this area is a composite summary of Harms 1996, Hendershott and

Winant 1996, Harms and Winant 1998, Dorman and

Winant 1995, Winant and Dorman 1997, Dorman and

Winant 2000, Dever 2000, Browne 1994, and Browne

2001. Heaviest emphasis in writing this description

was placed on Harms 1996, Harms and Winant 1998,

and Dever 2000.

4.4.4.1 DESCRIPTION OF THE SANTA

BARBARA CHANNEL AND RECENT

OBSERVATIONAL FIELD ARRAYS

The Santa Barbara Channel basin (fig. 4.4-1) is

located at the northern edge of the Southern California Bight with an east to west orientation. It is

bounded to the north by the California mainland, from

Port Hueneme to Pt. Conception, and to the south by

a string of four islands running from east to west:

Anacapa, Santa Cruz, Santa Rosa, and San Miguel.

The Channel is approximately 100 km long and 40

km wide with a maximum depth of 500 m in its central basin. The shelf width on both sides ranges from

3 to 10 km, the sill depths at the eastern and western

entrances are 220 m and 430 m respectively, and the

island passages are approximately 40 m deep.

The Minerals Management Service entered into

a series of cooperative agreements with the Scripps

Institution of Oceanography, University of California

to conduct research regarding the oceanic circulation

of the Santa Barbara Channel and the southern central California coastal area called the Santa Maria

Basin. The Santa Barbara Channel – Santa Maria

Figure 4.4-3. Santa Barbara Channel-Santa Maria

Basin circulation study moored and meteorological

instrument locations (Harms 1996).
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Description of the Affected Environment: Physical Oceanography

Basin Circulation Study (1991 to 2002) consisted of

two consecutive major field programs: one focusing

on the circulation in the Channel and one focusing on

circulation in the Santa Maria Basin. The information provided below will reflect the analysis and peerreviewed research coming out of the observations of

the first field program (fig. 4.4-3). Analysis and research based on the observations taken in the second

field program is underway at the writing of this document.

4.4.4.2 LARGE-SCALE FORCING

The seasonal variation in the wind field in the

SBC-SMB region is determined by two large-scale atmospheric pressure patterns: the North Pacific anticyclone (high) and the thermal low located over the

southwestern United States, resulting in persistent

winds from the northwest. In the spring the North

Pacific high strengthens and moves northward and

the Southwestern U. S. Low intensifies. The resulting increase in the atmospheric pressure gradient results in persistently strong southeastward winds off

the central California coast, which separate from the

coastline in the vicinity of Point Conception, leaving

the winds in the Southern California Bight more weak

and variable. In the summer the gradient between

the two pressure systems reaches a maximum with

the North Pacific high being displaced slightly further

to the north, causing the winds off the central California coast to be directed more to the south. The

gradients in the wind off Point Conception intensify

(are stronger and occur over a shorter distance) in

the summer. During the fall months, the North Pacific high gradually weakens and eventually moves

south to its wintertime position. Fall wind fluctuations off Point Conception are comparable in magnitude to those in the summer but are typically of shorter

duration. They last no longer than a few days and are

interrupted by equal periods of calm. In the winter

the most energetic atmospheric fluctuations along the

south and central California coast result from propagating storm tracks. These storm systems pass over

the SBC in 2-4 days and are large in size compared to

the size of the Channel.

Currents in the Channel are a superposition of

large-scale flow (scales larger than the length of the

Channel) and a cyclonic circulation characteristic to

the Channel’s interior. Surface pressure observations

off Pt. Sal (PAIN) and in the Southern California Bight

(BARB) and current observations at PAIN and at the

eastern entrance to the Channel (ANMI) indicate close

agreement between direct measurements and geostrophic calculations of the flow in these areas. The

large scale surface flow is equatorward in the spring

due to strong equatorward wind stress. This increase

in equatorward oceanic flow in the spring is accompanied by a decrease in the ocean’s surface temperature

and pressure. In late spring and early summer the

equatorward wind stress off Pt. Conception increases

to its seasonal maximum, but the current flow reverses

to the poleward direction which is maintained throughout the summer, fall, and early winter. This flow reversal in the Southern California Bight, in the northern part of the Channel, and at times in the inner

Santa Maria Basin is due to the setup and increase in

the poleward alongshore pressure gradient. The setup

of the alongshore pressure gradient is due to the markedly lower wind stress and the accompanying increase

in the surface pressure (due to rising temperatures)

in the Southern California Bight compared to waters

offshore Pt. Conception. The alongshore pressure

gradient is fully established by early summer and remains strong until early fall, when it declines as both

the equatorward wind stress off Pt. Conception and

gradients in the wind stress field between Pt. Conception and the lower Southern California Bight

weaken. The reversal of flow in the eastern Channel

entrance from equatorward in the spring to poleward

the rest of the year as well as the poleward flow in the

northern Channel shelf appears to be related to the

strengthening of the alongshore surface pressure gradient. Fluctuations (reversals) in these seasonal trends

can last for periods up to a week.

The currents measured at SMOF in the southwestern corner of the Channel respond to the upwelling-favorable wind stress fluctuations year round,

whereas the correlation between wind stress and the

currents in the northern segment of the western Channel entrance (SMOF) lasts only during the late winter and spring. During the late winter and spring the

flow is typically equatorward all along the western

entrance in a manner consistent of other observations

subject to upwelling. In late May and throughout the

remainder of the year there is very little correlation

of the currents in the northwestern corner of the channel and the windstress as measured at NDBC Buoy

54. Alongshelf currents in the northern Channel shelf

are typically westbound and in opposition to (anti-correlated with) the currents on the southern shelf. The

persistently year-round eastbound flow along the

northern coastline of the islands is primarily due to

wind stress, and except for late winter to early spring,

is weaker than the westbound jet flowing on the northern shelf. The wind stress at GOOF is 1/2 to 1/3 that

at NDBC 54 at the western Channel entrance. This

cross-basin shear between alongshelf currents sets up

a cyclonic circulation in the western half of the Channel. It intensifies to its maximum in the summer and

early fall, weakens in late fall and early winter, and is

weakest (to near non-existence at times) in the winter and early spring. Measurements at SMIN-SMOF,
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Delineation Drilling Activities Offshore Santa Barbara County

ROIN-ROOF, and GOIN-GOOF indicate recurring anticorrelated fluctuations in the alongshelf flow on the

opposing shelves that last on the order of weeks during the summer and early fall, becoming shorter in

duration as the cross basin shear decreases as the

winter season approaches, to non-existence in the late

winter and early spring.

4.4.4.3 SEASONAL VARIATION OF

MONTHLY MEANS

4.4.4.3.1 WINDS

The seasonal and spatial variability of wind

stress in the Santa Barbara Channel is depicted in

figure 4.4-4. Wind stress amplitudes decrease in magnitude from the western SBC entrance toward the east

all along the Channel. They are smallest along the

northern SBC coastline and its adjacent shelf and in

the Southern California Bight due to sheltering from

the coastal mountain range. The maximum wind

stress occurs in the summer at the western SBC entrance, south of Pt. Conception at NDBC Buoy 54. In

the eastern SBC and in the Southern California Bight,

wind stress is strongest in the spring. Minimum

monthly means occur everywhere in February. The

slight displacement to the north of the North Pacific

high during the summer months causes a tremendous

difference in the wind stress gradient near Pt. Conception. They become stronger over a shorter distance.

In spring the wind speeds are twice as strong at the

western entrance than they are in the central Channel, whereas in the summer this difference is doubled

(Dorman 2000, Harms and Winant 1998, Dorman and

Winant 1995).

4.4.4.3.2 CURRENTS

Seasonal near-surface currents (5m) in the SBC

are depicted in figure 4.4-5. As stated before the currents in the Santa Barbara Channel are a superposition of cyclonic flow in its interior superimposed on

the larger-scale flow (central California coast to the

Southern California Bight). Monthly averaged currents on the southern Channel shelf are eastward yearround: they reach a maximum in the spring when the

large-scale flow is equatorward, and a minimum in

the late fall and winter when the large-scale flow is

poleward. Currents on the northern shelf of the Channel are westward throughout the year with maximum

westward velocities occurring during the summer and

early fall. Currents on the southern and northern

Figure 4.4-4. Seasonal cycle of wind stress in the

Santa Barbara Channel region. The monthly mean

time series are rotated for convenience. The

direction of each arrow is the actual direction of

the mean wind stress for that particular month.

Arrows are proportional to the wind stress

magnitude during individual months in pascals.

Solid circles next to the time series show the actual

location of the measurement sites. The beginning of

the year is indicated by “J” (January) (Harms and

Winant 1998).

Figure 4.4-5. Seasonal cycle of 5-m currents in the

Santa Barbara Channel region. The monthly mean

time series are rotated for convenience. The

direction of each arrow is the actual direction of

the mean currents for that particular month.

Arrows are proportional to the current magnitude

during individual months in unit m s-1. The

locations of the time series corresponds to the buoy

locations. The beginning of the year is indicated by

“J”. Monthly averaged alongshelf geostrophic

velocities at the surface relative to 500 dbar

computed from CalCOFI hydrographic

observations obtained between 1949 and 1995 are

shown at two locations to illustrate seasonal

variations in large scale flow in this region.

(Harms).