REGIONAL OCEANOGRAPHY
Tides
On the Pacific coast of California tides effectively change four times a day. Each 24hour period experiences two high tides and two low tides of different magnitude ("mixed diurnal
tides"). The action of tides is caused by the gravitational pull of the sun and the moon. When
the moon is full or in a new phase it is in a position to combine its gravitational pull with that of
the sun's. During these times high and low tides will be effectively more extreme, having the
effect of exposing or covering more of the beach with each change of tide. When the moon is in
its mid-phase, its gravitational effect pulls against that of the sun. During these times of the
month tides will still change yet the differences between high and low tides will be substantially
less.
Currents
Current patterns within Santa Barbara Channel are complicated by topography/
bathymetry and wind. Summary discussions have been prepared by MMS (2001) and Hickey
(1993). Additional details can be found in Harms and Winant (1998) and Winant, Dever, and
Henderschott (2003). The primary ocean current off the west coast of the U.S. is the "California
Current," a broad, slow current, extending offshore extending from the edge of the continental
shelf to offshore about 250 miles, flowing equatorward at about 0.5 knots. This current brings
cold water, sub-arctic water down from the North at a rate of more than 35 million cubic feet per
second. The California Current continues its southerly flow, even as the shoreline trends
eastward past Pt Arguello and Pt Conception. This creates a large, counterclockwise gyre, the
"Southern California Counter Current," in the Southern California Bight (Figure 1). South of
San Diego, part of the California Current, mixed with warm, saline north-central Pacific water
from the west and warm, saline
equatorial Pacific water from the south,
spins eastward, toward shore, then
poleward along the coast. Beneath
these surface currents is the "California
Undercurrent" that flows poleward
carrying warmer equatorial water.
Figure 1 illustrates a possible
winter circulation pattern in the
Southern California Bight. Seasonal
variability in the Santa Barbara
Channel
is
described
below.
Variability also exists in the San Pedro
Channel, south of the Palos Verdes
Peninsula, where currents are usually
poleward.
Figure 1. Circulation in Southern California Bight
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A Bight is a bend or curve in the coastline. The waters off southern California are
known as the "Southern California Bight" (SBC). The land boundaries of the
SBC stretch 621 miles, from Point Conception north of the Santa Barbara
Channel to Punta Colnett, about 130 miles south of San Diego. This is the main
tectonic plate of where the Pacific Plate collides with the North American Plate
creating our checkerboard pattern of banks and basins along the coastline.
Daily changes in wind, tidal fluctuations, minor eddies, and transitions from one general
state to the other make it difficult to impossible to predict currents at a specific place and time.
An example of Santa Barbara Channel currents at a point in time is provided in Figure 2 (source:
http://www.icess.ucsb.edu/iog/realtime/index.php). Despite this short-term variability, general
patterns are well described There are three primary flow regimes in the Santa Barbara Channel
(Figure 3). They can occur anytime during the year, but are found predominantly as noted.
References re SBC Currents
Harms and Winant. 1998. Characteristic patterns of the circulation in the Santa Barbara
Channel. J. Geophys. Res. 103(C2):3041-3065.
Hickey, B.M. 1993. Chapter 5. Physical Oceanography. In M.D. Dailey, D.J. Reish, & J.W.
Anderson (eds). Ecology of the Southern California Bight. University of California
Press. 925 p.
Minerals Management Service (MMS). 2001. Delineation Drilling Activities in Federal Waters
Offshore Santa Barbara County, California. Draft Environmental Impact Statement.
Winant, Dever, and Henderschott. 2003. Characteristic patterns of shelf circulation at the
boundary between central and southern California. J. Geophys. Res. 108:3021.
Figure 2. Santa Barbara Channel Currents at 7:00 a.m., 6-22-07.
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Upwelling State: Primarily
February-June. Strong north
winds (equatorward) at western
end of Santa Barbara Channel;
weaker to the east (protected by
the mountains). Cyclonic
(counterclockwise) flow in the
Channel; weaker current speed on
mainland side, strong along the
islands. Southerly flow off Pt
Conception. Flow out of eastern
end of Channel.
Convergent State: Primarily all
year (except mid-spring). Similar
wind regime as during upwelling,
but insufficient to overcome the
poleward current. Cyclonic flow
in the Channel; similar speeds on
either side of the Channel.
Western flow off Pt Conception.
Flow into eastern end of Channel.
Relaxation State: Primarily
September-January. Weaker north
winds. Strong western flow on
northern (mainland) side of
channel, including at Pt
Conception, continuing poleward
north of Pt Conception; weak
eastern flow along island side of
Channel. Flow into eastern end of
Channel.
Figure 3. Primary Current Regimes of Santa Barbara Channel
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Biogeographical Description of CINMS
Steep persistent isotherms from sea-surface temperature maps and abrupt shifts in
community assemblages found in field surveys can help define and map biogeographical
boundaries. In the Channel Islands, existing information on oceanography and rocky intertidal
assemblages (Valentine 1966; Seapy and Littler 1980; Kanter 1980; Murray et al. 1980; Littler
1980) should be sufficient to allow gross biogeographical designations. An in-depth treatise of
the area is provided in Dailey et al. 1993 (Ecology of the Southern California Bight. University
of California Press. 925 p.)
Sea surface temperature maps suggest that Santa Rosa and Santa Cruz may represent a
transition between cooler and warmer temperate waters. Prevailing currents in the Santa Barbara
Channel affect the level of exposure, the water temperature, and, consequently, the species
composition along the northern coasts of Santa Rosa and Santa Cruz. There are numerous
biological and physical similarities between Anacapa Island and the eastern tip of Santa Cruz
Island, which are surrounded by warm temperate waters for most of the year. In contrast,
physical and biological characteristics of the south side of Santa Cruz Island are similar to those
around Santa Barbara Island and along the south side of Santa Rosa Island. Three main
biogeographic regions emerge when the area is subdivided according to physical and biological
differences:
Group I. Anacapa and east Santa Cruz Islands,
Group II. Santa Barbara, south Santa Cruz and south Santa Rosa Islands,
Group III. San Miguel, north Santa Rosa, north Santa Cruz Islands.
Upwelling
When physical and topographic conditions are just right the deposits of detritus may be
lifted to the surface by upwelling. Upwelling occurs primarily off the west coast of continents
where nearshore surface water is moved offshore by the interaction of winds and the rotation of
the earth. In the northern hemisphere, surface water will move to the right of the wind's
direction. When the wind is moving equator-ward (north or northwest wind), approximately
parallel to shore, the resulting water movement is away from shore. The displaced nearshore
water is replaced by upwelled deep water. This upwelled water is cold and nutrient-rich
compared to surface water and, along with the sunlight, produces large blooms in phytoplankton
which is translated to food for zooplankton, fish, birds, and marine mammals
Upwelling off the west coast of the U.S. is sometimes predictable during winter and
spring. During the summer months the coast is cooled by northwest winds and a flow of cold
water from the north. Around November the Northwest winds diminish and the cold southward
flowing water begins to sink. A thin layer of northward flowing warmer water, usually found at
depths greater than 600 feet moves to the surface and forms a wedge between the coast and the
southward flowing currents. The warm wedge flows northward. By February and March the
Northwest winds pick up again and upwelling returns. El Niño currents can disrupt this pattern.
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El Niño
Irregular changes in the climate, winds, and water levels in the equatorial Pacific create
conditions known as El Niño. During El Niño years, such as 1976-1977, 1982-1983, and 19971998, the influence of the California Current weakens and the water of the Southern California
Bight moves northward. The movement of warmer equatorial waters farther north than usual,
raises the temperature of the waters in the SBC by several degrees above normal. Some of the
marine mammals subjected to this temperature increase cannot tolerate it and leave, while those
that prefer or require the warmer water come into the area, making it more diverse.
The change of water temperature also changes the storm tracts. During El Niño years,
storms pound on the California coastline one after another in the winter, and slowly disappear
leaving the summer months to be hot and dry.
See Section 6.3 for an illustration of impacts of large El Niño storms on the coast of
Santa Barbara.
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OCEAN WAVES
Waves
Wind, blowing over the surface of water, creates waves. A gentle breeze may result in
ripples, while hurricane force winds result in "mountainous" waves. Regardless of size, all
waves can be described by the following terms (Figure 6.4):
1)
2)
3)
4)
Height (from trough to crest)
Length (from crest to crest)
Steepness (angle between crest and trough)
Period (length of time between crests)
Figure 6.4. Wave nomenclature.
The influence of wind on waves is a function of three factors:
1) Wind speed
2) Length of time the wind has blown
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3) Distance of open water that the wind blows over (called "fetch")
All of these factors have to work together to create waves. The greater each of the variables is in
the process, the greater will be the waves.
Seas
Seas are waves that are still under the direct influence of the wind. Strong winds,
blowing over the water for long periods of time, can create large, steep waves. The crests of
these waves may break up, or the wind my "blow the tops off," causing white caps. Powerful
weather systems in the Pacific can create waves ("seas") that subsequently persist long after the
storm is over ("swells"), and travel thousands of miles beyond the local storm area of influence.
Swells
Swells are the remnants of seas. The seas created by a storm become (after the storm)
large smooth looking waves (swells) that roll along the sea surface. Swells can travel thousands
of miles beyond the local storm area of influence.
Frequently, the waves that one encounters offshore are a combination of swell (from
storms many miles and many days ago) plus sea (generated by the local wind).
Shore Waves
Waves on the shore have special characteristics beyond the generic waves discussed
above. As swells approach the shore and encounter shallower water, they change from deepwater waves to shallow water waves (where water depth ≤ 1/2 wavelength) (Figure 4). The
following then occurs:
• They slow down (celerity decreases)
(celerity = speed of propagation of an ocean surface wave)
• Wavelength decreases
• Period stays the same
• Height increases
• Wave breaks and becomes swash, then backwash
When the wave height increases and the trough flattens out, the wave gets so tall it can't
support itself and the water crashes over the top. This is called a breaker, and breakers form in
an area called the surf zone.
The wave loses most of its energy by breaking (it actually gives off some light and heat),
and the remaining energy causes the water to rush up the shore. It loses the rest of its energy to
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friction in this manner, then gravity pulls the water back out to sea. The surge onshore is called
swash; the slump back to sea is called backwash. Swash and backwash occur in the swash zone.
Figure 4. Waves at the shore.
Sediment Transport
All waves that reach the shore have a direct effect on the shape of beaches with which
they come in contact. Large winter waves, and waves related to storms, can move hundreds of
tons of sand from one place to another in a few hours (high energy results in erosion). Calm
weather and small waves, typical of a summer environment, transport sand back onto the beaches
(low energy results in deposition).
Sand is transported along beaches as a result of the ebb and flow of waves that hit the
shore at an angle. The effects of this can be seen well at the entrance of the Santa Barbara
harbor. The wave energy that moves sand down the coast, along the shore, essentially disappears
at the harbor entrance at the end of the breakwater. The absence of the energy of breaking waves
results in the sand being dropped at the mouth of the harbor. This action of depositing sand at
the harbor entrance happens everyday, but is most noticeable during the winter months when
large storms transport a lot of sand along the shore. When it gets to the harbor entrance, the
abrupt decline in breaking-wave energy can cause a rapid build up of sand at the harbor entrance.
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Impacts of large El Niño storms on the coast of Santa Barbara
The last El Niño event occurred between the years of 1997-1998. There were
many storms that pounded the California coast with rain, but the month of February
supplied the most precipitation. The week of February 1-9th supplied the most rain out
of the entire month. The rain fills up riverbeds making them flow faster and carry more
sediment down to the river mouth and into the ocean. This rain caused 19 million metric
tons of sediment to flow down into the ocean from river runoff and beach erosion.
The pictures below show how much extra sediment has run off into the ocean by
the brown and green colors just off the shoreline.
SeaWiFS true color image for 2/9/98 showing
(A) The California coast.
(B) The Santa Barbara Channel. Between 2/4/98 and 2/8/98, 28 cm of precipitation fell
in the City of Santa Barbara.
(C & D) Aerial photographs of plumes in the Santa Barbara Channel taken on 2/10/98.
Plumes & Blooms: UCSB, NOAA CINMS, ONR, SeaWiFS (L. Mertes and J. Warrick,
unpublished data).
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CHAPTER 6
IMAGE OF SANTA BARBARA CHANNEL SEA SURFACE TEMPERATURES
10
SANTA BARBARA CHANNEL BATHYMETRY
(source: http://www.sccoos.org/data/bathy/?r=2)
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