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Description of the Affected Environment: Physical Oceanography

shelves at the western entrance behave differently than

the rest of the SBC due to wind stress. Just north of

San Miguel the currents are directed southeast yearround rather than east like the rest of the southern

SBC in response to windstress at that location. In

spring, just south of Pt. Conception on the northern

SBC shelf, the flow turns southwest to south in response to upwelling-favorable wind stress at that location. In the summer, and throughout the rest of the

year, the mean flow at that location is westward in

response to the alongshore pressure gradient set up

along the coastal shelf. The flow at the eastern SBC

entrance is southeast out of the SBC during the spring,

and is northwest (poleward) the remainder of the year.

Anticorrelated events between the SBC shelves occur

only when the flow at the eastern SBC entrance is

poleward into the SBC.

Currents at depth in the western and eastern

portions of the SBC along its central axis were measured at NDBC Buoys 54 and 53 respectively at downward looking APCD bins between 24 m and 328 m

(fig. 4.4-6). In the early spring, vertical profiles directed eastward at both locations are strongly sheered

in the top 100 m with maximum velocities at the surface. Minimum velocities occur at mid-depth. From

late spring to winter similarities between the two profiles disappear. In the western central SBC the maximum currents remain at the surface, but the vertical

shear is sharply reduced. The major portion of the

profile, including the surface currents, rotate clockwise towards the west as the year progresses. In the

eastern central channel, the flow is stronger than in

the west with its vertical profile directed westward

with maximum currents at approximately 140 m in

late spring to early fall, shifting to the surface in mid

to late fall. Unlike the flows at depth in the western

and eastern central basin, the 100m flow at the SBC

eastern entrance does not change direction seasonally but remains poleward throughout the year.

4.4.4.3.3 SURFACE WATER TEMPERATURE

Figure 4.4-7 depicts monthly averaged contour

maps of water temperatures in the central California

coast to the Southern California Bight. The coldest

temperatures are found along the central California

coastline near Pt. Arguello in the spring due to the

persistent upwelling of cold nutrient rich water to the

surface caused by strong equatorward winds in that

region. The warmest waters are found in the Southern California Bight in the summer due to the wind

sheltering by the coastline mountain ranges and local

solar heating cycles.

Inside the SBC the temperatures typically increase from the southwest corner to the northeast.

The temperature gradient between these two regions

varies due to the seasonal variation of the balance

between the two oceanic circulation forcing mechanisms: the upwelling favorable winds along the central California coastline and the poleward alongshore

pressure gradient caused by temperature driven surface pressure increases in the Southern California

Bight. In the spring when the upwelling favorable

wind forcing is dominant, cold water is upwelled at

Pts. Arguello and Conception and cold water spreads

eastward into the SBC with the coldest water appearing along the southern SBC shelf. In the late spring

and summer the alongshore pressure gradient builds

up, the currents at the eastern SBC entrance (ANMI)

reverse to the poleward direction and warmer Southern California Bight water is introduced along the

northern shelf of the SBC continuing to Pt. Conception. Temperature gradients between the colder waters at the southwest region of the SBC and the

warmer waters in the SBC’s northeast region are at a

maximum during the summer (June to September).

In mid-fall temperature gradients decrease as warmer

Bight water replaces the cold waters offshore Pts. Conception and Arguello and the southern central California coast. Temperature gradients decrease further as

SBC surface water temperatures decline to their winter

values.

 4.4.4.3.4 SURFACE PRESSURE FIELD

Surface pressure (SSP) at various stations is directly related to bottom and baroclinic pressures, the

latter of which is derived from temperature measurements (Harms and Winant; 1994). Computed surface

pressure values are compared to monthly fluctuations

in measured bottom pressure, computed baroclinic

pressure, and measured wind stress for southern and

northernmost current stations, and the monthly wind

stress differences between NDBC Buoys 53 and 54 in

figure 4.4-8. Figure 4.4-9 illustrates monthly differences between the northernmost and southernmost

stations in surface, bottom, and baroclinic pressure.

Figure 4.4-6. Seasonal fluctuations in the vertical

structure of currents at NDBC buoys 53 and 54.

Monthly averaged ADCP velocities between 24-m

and 328-m depths are shown for every other month.

Vertical resolution is 16 m. Arrows are

proportional to the current magnitude in units m s1 (Harms 1996).
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Delineation Drilling Activities Offshore Santa Barbara County

Figure 4.4-7. Contour maps of monthly average 1-m temperatures. Temperature unit are °C. Contour interval

is 0.5 °C (Harms 1996).

Minimum surface pressure occurs everywhere in

April, while maximum surface pressures occur in the

late summer and early fall. Surface pressure decreases

to its minimum in the spring with the onset of

equatorward wind stress. In late spring the surface

pressure begins to steadily increase until fall despite

the fact that the upwelling favorable winds continue

to blow strong off Pt. Conception throughout the summer. Part of this increase is due to bottom pressure

which increases uniformly to approximately the same

values everywhere. Baroclinic pressure, on the other

hand, is always at higher values in the Southern California Bight than offshore the central California coast

and increases at a faster rate in the Southern California Bight than in the Santa Maria Basin. The increase in baroclinic pressure begins and reaches its

annual maximum at the southernmost station, BARB,

one month before the northernmost station, PAIN. Its

these differences between these two regions in the rate

and degree of increase in baroclinic pressure, which is

temperature driven, that causes the poleward along-
4-41

Description of the Affected Environment: Physical Oceanography

shore pressure gradient in the region. As we see in

Figure 4.4-9, the maximum poleward surface pressure

gradient occurs in August during a period of strong

wind stress gradient between the western and eastern

SBC regions (fig. 4.4-8).

4.4.4.4 BALANCE OF ALONGSHORE WIND

STRESS AND PRESSURE

GRADIENTS IN EFFECTING THE

CIRCULATION

The alongshore momentum balance between upwelling-favorable wind stress and the alongshore pressure gradient at any particular location in the SBC

largely determines the surface currents at that location. The relative importance of these two major forcing mechanisms can be determined at any particular

location in the SBC by evaluating the momentum

equation which balances the alongshelf acceleration

term with the alongshelf pressure gradient term and

the local wind stress term. Using the right-handed

Cartesian coordinate system (x,y,z) where “+ y” is

alongshelf in the poleward direction and (u,v,w) are

the corresponding velocity components:

Mv/Mt = - 1/D (Mp/My) + 1/D (MJ/Mz)

acceleration term = - (alongshore pressure gradient)

+ (surface wind stress & linear drag @ surface layer

bottom)

where p is the surface pressure, D is the reference

density, J is the surface wind stress, and z is the affected layer depth.

Figure 4.4-8. (a)-(c) Seasonal variability of

synthetic subsurface pressure (SSP) bottom

pressure (P100) at selected stations. Mean values are

relative to the 3-year mean. Units are kilopascals.

(d) Seasonal fluctuations in alongshelf wind stress

at NDBC buoys 54 and 53 in pascals. The difference

between both time series is shaded (Harms and

Winant 1998).

Figure 4.4-9. (a) SSP , bottom pressure (P100) and

baroclinic pressure (SSP-P100) differences between

BARB and PAIN. A positive pressure difference

corresponds to a poleward pressure gradient. Units

are kilopascals. The shaded time series represents

seasonal variations in the alongshelf (304°N) wind

stress at NDBC buoy 54 in units Pa. Note that the

along-channel SSP difference opposes the wind

stress year round. (b) Baroclinic pressure (SSPP100) at BARB and PAIN. (c) Bottom pressure (P100)

at BARB and PAIN. (Harms and Winant 1998).
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Delineation Drilling Activities Offshore Santa Barbara County

The otherwise typical coriolis term is not included with the acceleration term on the left hand side

of the equation. This is because observations indicate that the coriolis term represents motions that

are on shorter length scales than the other terms, and

therefore, exhibits no correlation with them. The resultant current patterns for various values of wind

stress and alongshore pressure gradient is illustrated

in figure 4.4-10. Analysis of time series of both forcing mechanisms indicate that upwelling-favorable wind

stress and the alongshore pressure gradient:

• oppose each other year-round going

equatorward and poleward respectively,

• are seasonal, and that they are both strongest

in the summer and fall and weakest in winter,

• in summer and fall are dominated by fluctuations of time scales longer than 10 days,

Figure 4.4-10. Average 5-m current velocity (mS-1) as a function of equartorward wind stress (in pascals) at

NDBC 54 along 124° N and the along-channel SSP difference (102Pa) between GOIN and PAIN. The analysis

period is January 1, 1994, to December 31, 1995. The ranges for the wind stress and the SSP difference,

displayed along the top for wind stress and along the left for the SSP difference, span all values observed

during the years 1994 and 1995. The number of realizations (i.e., hours) of each combination of forcing is

shown in the upper right corner of each panel (Harms and Winant 1998).
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Description of the Affected Environment: Physical Oceanography

• are strongly anticorrelated with each other

with the alongshore pressure gradient lagging

the upwelling favorable wind stress by one day.

This last bullet strongly suggests that the alongshore pressure gradient is a response to this mesoscale upwelling favorable wind stress forcing. Observations strongly indicate that for both larger and

smaller than SBC length scales, spatial variation in

the upwelling favorable wind stress is responsible for

establishing the poleward alongshore pressure gradient.

The simple momentum balance equation above

can be used to determine the degree of dominance of

one forcing agent over another in eliciting alongshore

flow in three major regions of the SBC: the southern

shelf, the northern shelf, and the eastern entrance.

The southern shelf of the SBC is exposed to the

upwelling favorable winds off the central California

coast where the surface wind stress is 2 to 3 times

smaller just north of Santa Cruz island (GOOF) than

that at the western entrance (NDBC Buoy 54). This

amount of wind stress however, is much greater than

anywhere along the northern SBC shelf. Not surprisingly, the acceleration term is strongly correlated with

the wind stress term, and has negligible correlation

with the opposing (to wind stress), but much smaller,

alongshore pressure gradient term calculated for this

area. Fluctuations of correlated wind stress and acceleration terms are on the order of days.

In the vicinity of GOIN on the northern shelf of

the SBC the wind stress is weak to negligible and the

currents flow westward year round. The largest calculated term in the right hand side in the momentum

equation is the alongshore pressure gradient which is

characterized by low frequency fluctuations on the

order of a month. The second largest calculated term

is the local acceleration term which is characterized

by higher frequency fluctuations on the order of days.

These two forcing terms are highly correlated with

the respective low and high fluctuations in current

flow at this location. There is no correlation between

the surface stress (the smallest term in the equation)

and the currents. Together the alongshore pressure

gradient and acceleration terms exclusively predict the

current flow at GOIN.

The current flow at the eastern SBC entrance

(ANMI) is polarized in the alongshelf direction. In

the spring the current flow is equatorward out of the

SBC, and the rest of the year the flow is poleward into

the SBC. Superimposed on this seasonal pattern are

fluctuations lasting on the order of days to weeks that

are strong enough at times to reverse the flow. These

higher frequency fluctuations occur many times

throughout the year. The annual mean surface flow,

however, is poleward into the SBC.

The surface wind stress at the eastern entrance

of the SBC is weaker than it is at GOOF on the southern shelf, but stronger than what occurs on the northern shelf. All terms on the right hand side of the

momentum equation are similar in magnitude. The

surface stress and the alongshore pressure gradient

are significantly anticorrelated when both exhibit low

frequency fluctuations on the order of weeks to

months. Surface stress also exhibits higher frequency

fluctuations on the order of days as dose the local acceleration term. The correlation between calculated

and observed currents at ANMI is strong when both

the surface stress and the alongshore pressure gradient is taken into account. This suggests that, unlike

the northern and southern SBC shelves, both alongshore pressure gradient and wind stress play an important role in determining the flow at the eastern

SBC entrance.

 4.4.4.5 CHARACTERISTIC SYNOPTIC

PATTERNS OF THE CIRCULATION

The different patterns of current flow in the

SBC-SMB are largely determined by the upwelling

favorable wind stress (and its gradients) and the opposing alongshore surface pressure gradient.

Hendershott and Winant, 1996, Harms 1996, and

Harms and Winant 1998 through much collaborative

effort, subjectively deduced six flow regimes characteristic of the SBC-SMB area from inspecting synoptic displays of daily averages of near-surface current,

wind, temperature, and pressure observations. The

flow regimes are called Upwelling, Cyclonic, Relaxation, Propagating Cyclones, Flood East, and Flood

West and are illustrated in figure 4.4-11. These regimes were later objectively verified by subjecting the

5 and 45 meter current observations to empirical orthogonal function (EOF) analysis. Three (two) EOF

modes described 50% (53%) of the 5m (45m) current

fluctuations, which when combined with their respective mean current fields depict spatial current patterns similar to the six flow regimes determined by

more subjective means. The EOF analyses of the 5 m

current observations also indicated that from late

spring through the fall there is a repeating sequence

of four flow states: Upwelling-Cyclonic-Relaxationquiescent period, that cycles approximately every 16

days. A brief summary of the six flow regimes are

given below.

When the equatorward upwelling-favorable

winds are strong off the central California coast and

the poleward alongshore pressure gradient is weak,

the flow everywhere except possibly in the northeast

corner of the SBC is south to southeastward (including the flow at the eastern SBC entrance). This flow

regime is called Upwelling, and is characteristic of

what we see in the early spring.
4-44

Delineation Drilling Activities Offshore Santa Barbara County

When the equatorward upwelling-favorable winds

are strong, but we now have a strong poleward alongshore pressure gradient established, we have poleward

flow into the eastern entrance of the SBC, strong westbound currents over the northern SBC shelf, strong

eastbound flow over the southern SBC shelf. There is

strong cross basin shear in this case and this flow

condition is called Cyclonic. The Cyclonic flow regime is found to occur most frequently in the late

spring through the summer.

When the poleward alongshore pressure gradient is still strong, but the upwelling favorable winds

have significantly weakened (relaxed) we have strong

poleward flow into the eastern SBC entrance, strong

westerly flow along the northern SBC shelf, strong

westerly flow at the northwest SBC corner (SMIN),

and poleward flow along the central California coast

(usually as far offshore as PAIN). The flow along the

southern SBC shelf is weak, but continues to be eastward. The Relaxation flow regime occurs most prominently in the early fall to early winter.

There are times when the alongshore pressure

gradient and the wind stress are acting in the same

direction, or one of these forcing agents simply does

not exist, and we get either a Flood East or Flood West

where the flow everywhere in the SBC flows in either

one of these directions. These two flow regimes do

not last very long, are not particularly strong flows,

and typically occur in the winter.

During a time when the shear between the

alongshelf flows on the northern and southern shelves

is strong and we have

Figure 4.4-11. Synoptic views of the circulation in the Santa Barbara Channel: (a) Upwelling, (b) Relaxation,

(c) Cyclonic, (d) Propagationg Cyclones, (e) Flood East, (f) Flood West (Harms and Winant 1998).

• significant anti-correlation between the currents at stations along the same longitudinal

transect within the SBC,

• the signal at current stations at both shelves

lead their western neighbor by a lag time of 4

to 5 days, and

• the current fluctuations on the southern shelf

lead current fluctuations on the northern shelf,

we get smaller-than SBC scale cyclones that

originate in the eastern SBC and propagate

westward.

When this occurs we have what is known as a

Propagating Cyclone flow regime. Table 4.4-1 summarizes these flow regimes along with a description

of the relative strengths of their forcing agents.

 4.4.4.6 AVHRR SATELLITE IMAGERY AND

FREE FLOATING SURFACE DRIFTER

DATA

AVHRR satellite imagery and free floating surface drifter deployments give information about the

synoptic surface currents, synoptic surface temperatures, and general movement of water masses for not

only the SBC-SMB area and the SCB, but the entire

California coastal area. Daily AVHRR satellite images of the SBC-SMB area and surrounding area can

be found on the MMS/Scripps website: wwwccs.ucsd.edu/oilspill/. Black and white and color im-
4-45

Description of the Affected Environment: Physical Oceanography

ages depicting the meso-scale, and some finer scale,

oceanographic processes occurring on a particular day

of interest can be found. Water masses from different

sources have different salinity and temperatures, and

they leave their temperature signatures on the ocean

surface. Consequently, their location and general

movement can be tracked by AVHRR satellite imagery. This information supplied a visual ground-truth

to what other observations were indicating to Scripps

scientists. “A typical image of the SBC-SMB area includes upwelling of water along the southern central

California coastline and the southwestern corner of

the SBC (deep blue), warmer water entering the eastern SBC entrance and moving westward along the

northern SBC coast to Pt. Conception (yellow to deep

orange), and a temperature gradient between these

two water masses in the central portion of the SBC

(yellowish-green)” (Browne 2001).

Free-floating drifters designed to follow the top

meter in the water column were constructed and deployed in support of the SBC-SMB Circulation Study.

Twenty-nine drifter deployments either from 12 or

24 locations in the SBC and SMB were conducted from

1993 to 1999 in a manner that would allow a reasonable sampling over the four seasons. Three major flow

regimes are easily defined by these drifter data: Upwelling, Convergent, and Relaxation. Dever (2000)

renames the Cyclonic flow regime as “Convergent.”

This was done because remnants of the western SBC

cyclone exist during many of the flows and the word

Convergence more aptly defines the resulting condition of an equally strong poleward alongshore pressure gradient and an equatorward upwelling-favorable

wind stress existing simultaneously in the SBC. Figures 4.4-12 through 4.4-14 illustrate surface drifter

tracks representing these three flow conditions. All

drifter tracks depicted in these figures represent

drifter travel over 40 days. Drifters have traveled as

far north as San Francisco and as far south as the

Baja over a 40 day period. Figures 4.4-12 and 4.4-14

depict drifters striking the coastline of northern San

Table 4.4-1. Relation between the current patterns and the forcing terms. *Due to the absence of a pressure

gauge in the center of the SBC, the measurements do not resolve the cross-shelf distribution of the surface

pressure when the Cyclonic flow regime prevails. During Cyclonic flow events we expect suface pressure to be

low in the center of the SBC and higher over the shelves.

Figure 4.4-12 (a). Free-Floating drifter tracks from

March 1995 deployment in the Santa Barbara

Channel primarily depicting the upwelling flow

regime (http://www.ccs.ucsd.edu/oilspill/ - click

“Interactive Drifter Track Plotting”).

Current Pattern Upwelling Relaxation Cyclonic No Flow Flood East Flood West

Wind Stress strong

upwelling favorable

strong gradients

weak

upwelling favorable

weak gradients

strong

upwelling favorable

strong gradients

weak

upwelling favorable

weak gradients

strong

upwelling favorable

weak gradients

strong

downwelling

favorable

weak gradients

Surface Pressure weak poleward

alongshelf gradient

onshore

cross-shelf gradient

strong poleward

alongshelf gradient

offshore

cross-shelf gradient

strong poleward

alongshelf gradient

*no

cross-shelf gradient

weak poleward

alongshelf gradient

no

cross-shelf gradient

equatorward

alongshelf gradient

onshore

cross-shelf gradient

poleward

alongshelf gradient

offshore

cross-shelf gradient

Diego county and inside Monterey Bay respectively. A

description of the surface drifter’s construction, the

entire drifter data set for the study, and graphical interactive displays of surface drifter tracks can be found

at the website: www-ccs.ucsd.edu/oilspill/ under “Surface Drifter Tracks.”
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Delineation Drilling Activities Offshore Santa Barbara County

4.4.4.7 APPLICATIONS OF SBC-SMB

OCEANOGRAPHIC RESEARCH:

DETERMINING THE SYNOPTIC

FLOW STATE FOR THE SBC-SMB

FROM REAL-TIME DATA PRESENTED

ON THE MMS/SCRIPPS WEBSITE

The SBC-SMB Circulation study, with its comprehensive field program and rigorous analysis and

modeling effort, adequately fulfills the research requirements necessary to allow a small array of moored

instruments, strategically located, to effectively monitor the oceanographic conditions in the SBC-SMB area

in near real-time. Through a new cooperative agreement between the MMS and Scripps and an interagency agreement with NOAA, a monitoring array for

the SBC-SMB area was deployed in the fall of 1999

and will be in place until September 2004. It consists

of four current and temperature observation moorings reporting surface currents in near real-time, three

NDBC Buoy stations reporting near surface winds in

near real-time, daily satellite imagery, and a cache of

drifters ready for deployment to observe special

oceanographic phenomena upon short notice of their

occurrence. The near real-time observations can be

found both in data stream and in graphical format at

the MMS/Scripps website: www-ccs.ucsd.edu/oilspill/

Figure 4.4-13 (a). Free-Floating drifter tracks from

September 1994 deployment primarily in the Santa

Barbara Channel depicting the “cyclonic,” also

called “convergent,” flow regime (http://

www.ccs.ucsd.edu/oilspill/ - click “Interactive Drifter

Track Plotting”).

Figure 4.4-12 (b). Free-Floating drifter tracks from

April 1998 deployment in the Santa Maria Basin

depicting the “upwelling” flow regime. Note that

many drifters traveled south-east through the

Santa Barbara Channel to get into the southern

portion of the Southern California Bight (http:/

www.ccs.ucsd.edu/oilspill/ - click “Interactive

Drifter Track Plotting”).

Figure 4.4-13 (b). Enlargement of Santa Barbara

Channel portion of Figure 4.4-13 (a): “cyclonic,”

also called “convergent,” flow regime (http:/

www.ccs.ucsd.

edu/oilspill/ - click “Interactive Drifter Track

Plotting”).
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Description of the Affected Environment: Physical Oceanography

.

At any time, a trained user can determine the

particular flow regime, and get a fair idea of its intensity, by looking at the near real-time wind and current data reported on the MMS/Scripps website. This

information can be used for marine biological research

as well as studying the physics of the ocean. It can

also be used in time of crisis, such as an oil spill event,

whether it be from a tanker in the area or from an

offshore platform or pipeline. In the particular event

of an oil spill, this information can be used to determine oil spill trajectory by subjective calculation or

by inputting the near real-time current and wind data

directly into an oil spill trajectory model. However

we do this, there is certain information that a person

attempting to predict oil spill trajectory in the SBCSMB area must know to be successful. They must

know the fundamental causes and spatial variation of

the Upwelling, Convergent, and Relaxation flow regimes that are characteristic to the SMB-SBC area,

and how to read the near real-time current and wind

data to determine what particular flow regime is occurring at the advent of an oil spill crisis.

“When faced with an actual oil spill, or oil spill

alert, the trained oceanographic forecaster will then

use (1) the basic knowledge learned from the Santa

Barbara Channel-Santa Maria Basin Circulation

Study, (2) real-time oceanographic and meteorological data obtained from monitoring stations, (3) the

latest satellite imagery of the area, (4) knowledge of

drifter trajectory statistics, (5) results from the latest

numerical circulation and oil-spill trajectory model

runs, and (6) personal ability to synthesize the results of these analyses and background knowledge into

accurate estimates of surface trajectories of water/

pollutant particles” (Browne 1994).

4.4.4.7.1 THREE PRIMARY SYNOPTIC FLOW

REGIMES USED IN REAL-TIME

APPLICATIONS

Dr. Ed Dever of Scripps (Dever 2000) describes

these three flow regimes in a bit more detail than what

was provided in the description of the characteristic

synoptic flow regimes above. He also provides rules

of thumb, based on strong statistics, on how to read

the near real-time current and wind data provided on

the MMS/Scripps website.

Figure 4.4-14 (a). Free-Floating drifter tracks from

December 1996 deployment in the Santa Barbara

Channel depicting the “relaxation” flow regime

(http://www.ccs.ucsd.edu/oilspill/ - click

“Interactive Drifter Track Plotting”).

Figure 4.4-14 (b). Free-Floating drifter tracks from

November 1997 deployment in both the Santa

Barbara Channel and the Santa Maria Basin

depicting the “relaxation” flow regime (http://

www.ccs.ucsd.edu/oilspill/ - click “Interactive

Drifter Track Plotting”).
4-48

Delineation Drilling Activities Offshore Santa Barbara County

“The synoptic flow states described below are a

compact way of describing certain commonly-observed

features of the large-scale circulation in the Santa Barbara Channel-Santa Maria Basin (SBC-SMB) region.

Though they are subjectively defined, statistical descriptions of the near-surface circulation demonstrate

similar spatial structures.

These descriptions are intended to be used with

information available from the MMS oil spill response

page(http://www-ccs.ucsd.edu/oilspill/) maintained by

the Center for Coastal Studies, Scripps Institution of

Oceanography. Each synoptic flow state is described

in terms of its diagnostic features in the observed surface currents off Purisima Pt., Pt. Conception, San

Miguel Island, and the eastern entrance to the Santa

Barbara Channel. Ancillary information such as regional winds and satellite sea surface temperature

imagery are also described as a function of synoptic

flow state.

It is important to remember the synoptic flow

states are merely a conceptual model. They can be

unambiguously identified about 60% of the time.

Small-scale features, transitions between different

synoptic states and uncommon patterns can make it

difficult to identify the observed flow with a single

synoptic state. Therefore, variations on the basic synoptic states will also be described.

Upwelling: The upwelling state gets its name

from upwelling of cold (approximately 11° C) subsurface waters near Pt. Conception which often occur

during it. The upwelling state occurs primarily in

spring, though it has also been observed in other seasons. In terms of the conceptual models of the momentum balance, it occurs when strong (10 m/s or

more) persistent (several days or more) upwelling favorable (equatorward) winds overwhelm any poleward

along-shelf pressure gradient.

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 broad 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.

Very weak (<10 cm/s) velocities 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.

Convergent: The convergent state gets its

name from the convergence of southward flow west

of Pt. Arguello with westward flow south of Pt. Conception. The convergent state occurs primarily in

summer, though it has also been observed in other

seasons. In terms of the conceptual models of the momentum balance, it tends to occur when upwelling favorable winds and a strong poleward along-shelf pressure gradient exist.

The most characteristic feature of the resulting

flow field is a strong (with velocities often 40 cm/s or

more — velocities of 70 cm/s are not unheard of) cyclonic recirculation in the western SBC with about

equal strength in the northern and southern limbs of

the recirculation. While northwestward flow at the

eastern entrance to the SBC often occurs during the

convergent state, northeastward flow 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 offshelf 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.

Relaxation: The relaxation state gets its name

from the fact that it generally occurs when winds off

Pt. Conception “relax” from their usual equatorward

direction. The relaxation state occurs primarily in

fall and early winter. In terms of the conceptual models of the momentum balance, it occurs when poleward

along-shelf pressure gradients overwhelm upwelling

favorable or weak winds.

The most characteristic feature of the resulting

flow field is a strong westward flow (50 cm/s or more)

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
4-49

Description of the Affected Environment: Physical Oceanography

sometimes accompany the convergent state. The average winds at NDBC 46054 during convergence are

nearly equal to those observed in upwelling, above 7

m/s to the southeast.

Satellite Imagery: In the convergent state, satellite sea surface temperature images often show warm

water (17°-20° C) extending from the eastern Santa

Barbara Channel north and westwards 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 westwards or southwestwards.

4.4.4.7.2.3 RELAXATION CRITERIA

Surface Currents: Relaxation occurs when

(subtidal) flow in the Santa Maria Basin is northward,

flow at the eastern entrance to the Santa Barbara

Channel is northwestward, and flow at Pt. Conception (SMIN) is westward (fig. 4.4-15c).

Winds: Winds during a relaxation event tend to

be weak equatorward or poleward at NDBC 46054 at

the western entrance to the Santa Barbara Channel.

The average winds at NDBC 46054 during relaxation

are under 4 m/s (to the southeast).

Satellite Imagery: Satellite sea surface temperature images during relaxation will often show

warm water (17°-20° C) extending from Pt. Conception northwestwards into the Santa Maria Basin”

(Dever 2000).

 4.4.4.7.3 MONTHLY, SEASONAL, AND

ANNUAL FREQUENCY OF

OCCURRENCE OF SYNOPTIC FLOW

REGIMES

Using the criteria above, the SBC-SMB moored

current data is characterized into “Upwelling,” “Convergence,” and “Relaxation” flow regimes or “other”

for December 1993 to November 1999 in table 4.4-2.

“Other” occurs when flow conditions did not satisfy

any of the criteria for the three flow regime states.

Only days with good velocity data at ANMI, SMIN,

and PAIN are considered.

 “From table 4.4-2 we can easily see a seasonal

preference of occurrence for the synoptic flow regimes:

upwelling occurs primarily in Feb-June, convergence

throughout the year (except April), and relaxation

from September through January” (Dever 2000). “By

looking at the number of days of occurrence for each

flow regime in each month, as is detailed in the last

four columns of the table, we can determine the annual percentage of occurrence of each flow regime by

dividing the annual totals of days for each flow rean offshore in addition to a poleward component of

flow. Closer to shore in the SMB averages are weaker

and in some nearshore locations, southward.

The highest velocity fluctuations occur west of

Pt. Conception in the region where the westward flow

from the SBC is turning poleward into the SMB.

There is a secondary maximum in the western SBC

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” (Dever 2000).

4.4.4.7.2 REAL-TIME DATA CRITERIA USED

TO DETERMINE SYNOPTIC FLOW

STATE

A trained user can easily obtain surface current

data from three to four moorings, winds from NDBC

Buoy 46054 and others in the area, and at times

AVHRR satellite imagery, which, when combined with

the knowledge from the greater field program, will

indicate which of the three major synoptic flow regime states is occurring that very hour (Browne 1994).

The criteria that indicate which flow regime is actually occurring are summarized below by Dever 2000.

4.4.4.7.2.1 “UPWELLING CRITERIA

Surface Currents: Upwelling occurs when

(subtidal) flow in the Santa Maria Basin (PAIN) is

southward and flow at the eastern entrance to the

Santa Barbara Channel (ANMI) is southeastward (fig.

4.4-15a).

Winds: During upwelling, the wind field tends

to show strong velocities (averaging above 8 m/s) to

the southeast, south of Pt. Conception at NDBC 46054.

Within the SMB winds are generally onshore and

equatorward. Within the eastern SBC winds can be

relatively weak.

Satellite Imagery: 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 southwards from

Pt. Conception past San Miguel Island and eastwards

from San Miguel towards the eastern entrance to the

Santa Barbara Channel.

4.4.4.7.2.2 CONVERGENCE CRITERIA

Surface Currents: Convergence occurs when

(subtidal) flow in the Santa Maria Basin is southward,

flow at the eastern entrance to the Santa Barbara

Channel is northwestward, and flow at Pt. Conception (SMIN) is westward (fig. 4.4-15b).

Winds: In the convergent state, the wind field

can resemble the upwelling wind field although this

is not diagnostic in the sense that weak winds can