Q: What factor keeps the California coast from becoming unbearably hot?
Q: What keeps most people north of Pt. Conception from swimming in the ocean for longer than just a few minutes (without a wet suit)?
Q: What has provided the environmental pressure that
has resulted in the thick fur of
sea otters,
the blubber of
pinnipeds and
cetaceans,
and the mammalian dive reflex in many living mammals, including humans?
If you answered A: cold water, you are right. Specifically, I’m talking
about cold ocean water, and there are two major reasons that the ocean is cold
for most of the year off the California coast north of Pt. Conception.
The first reason is the California Current. This current is part of the North Pacific Gyre that distributes warm tropical water northward and cold polar water southward. The California Current actually flows south past British Columbia, Washington, and Oregon, and all of northern and central California before veering westward at Pt. Conception. South of this point is the Southern California Bight with its own gyre of warmer waters circulating along the coast from northern Mexico and San Diego to the Santa Barbara Channel and the Channel Islands.
The second reason is the one we will focus on today: coastal upwelling, a set of complex motions in the ocean which, however, can be explained with a relatively simple model. Along the west coast of North America, the prevailing winds are from the northwest. These winds intensify in spring and through summer. Logic would tell us that winds from the northwest would move water onshore along with it. However, because the Earth is rotating on its axis, the ocean water is deflected away from the coast at roughly 90 degrees to the right of the prevailing wind direction. This deflection to the right, in the northern hemisphere, is said to be due to the Coriolis force, which derives from the planet’s rotation. Demonstrate with globe.
Near the surface, the waters are deflected only slightly to the right, and a majority of the wind energy is transferred to the water. Deeper in the ocean, increasing pressure, as well as other factors such as temperature and salinity, result in layers of water of varying, and generally increasing, density. Each progressively deeper layer of water is deflected somewhat more to the right, but friction causes less and less of the energy to be transferred to lower layers. A model or drawing of the resultant energy vectors looks like a descending helix and is known as Ekman’s spiral. At a certain depth so little of the energy is transferred to these deeper layers that essentially the winds are not influencing their movement. The maximum depth at which this occurs is about 150 to 200 ft. This depth varies depending on several factors, including wind speed and seawater density.
The net result of these motions and their sequential
deflections is that, during periods of strong northwest wind, the upper 150-200
ft. of seawater is moved offshore roughly to the southwest. Since water is
fluid, water must come from somewhere to replace the great volume that is moved
offshore. It comes up from the deep ocean, along the continental slope and over
the continental shelf. The movement is most pronounced where there are
submarine canyons, or deep gulches leading from the continental shelf down to
the abyssal plains, or deep ocean basins. Water from the deep ocean is cold,
and it is rich in nutrients from organisms that have died and drifted downward,
and from sediments that have washed off the continents and been deposited
there. It is also a physical fact that cold water can contain more dissolved
gases than warmer water.
The rising of cold deep water to replace water that has moved offshore is called
upwelling.
One place on the west coast where upwelling is especially pronounced is at
Monterey Canyon, a great deep canyon that cuts across the continental shelf from
several hundred feet deep to 8-10,000 ft. This canyon in fact is deeper than
the Grand Canyon. It would be a National Park if it were not covered with ocean
water. However, upwelling also occurs along great stretches of the west coast,
being most pronounced at points and headlands such as Cape Mendocino, Pt. Arena,
Pt. Reyes, Pt. Lobos, and Pt. Buchon, and in other areas where there are smaller
submarine canyons.
The welling up of cold, nutrient-rich water takes place primarily in spring and summer, when the days are lengthening to the greatest number of daylight hours. The combination of abundant nutrients with long hours of sunlight creates the perfect conditions for plant growth, in particular phytoplankton, or single-celled plants. These grow and multiply so fast that marine biologists speak of plankton blooms. They are literally population explosions.
Very soon afterward, zooplankton, or drifting animals, both single-celled and larger larvae and adults, come to feed on the superabundant phytoplankton. These in turn feed small fishes and birds, which in turn feed larger fishes, sharks, and a variety of larger marine birds and mammals. After the phytoplankton blooms die off, bringing a temporary end to the explosive feeding cycles, marine animals rest and wait for the next cycle of blooms. They may move elsewhere to find other food sources, and may even migrate to other parts of the world to take advantage of plankton blooms in spring and summer in the southern hemisphere.
Upwelling also keeps cold water near the coast at a time when increasing day length has the potential to heat the land to searing temperatures. As we know, this happens in inland California and in the American deserts during summertime. But the water acts as a moderating influence on coastal summer weather, partly just because it is colder than the surface water that is moved offshore, and because water has a higher heat capacity than the land: it takes more sunlight, and more time, to heat water than to heat land.
As inland areas heat up during summer, the air expands and rises. This causes air to be pulled in from areas over the ocean to areas onshore. The summer sun causes huge volumes of water to evaporate from the sea surface. However, when air moves over the cold waters near shore, it cools down and reaches the dew point—the temperature at which water will no longer remain in vapor form and begins to condense. This is why summer fog, so famous in San Francisco, is so widespread throughout coastal California, Oregon, and other parts of the west coast. This also explains why coastal fog can form on quite windy days, a condition known as convection fog. In inland areas, fog tends to form in calm air from condensed water vapor that has come from ground moisture. This is known as advection fog. The two processes are quite different.
Upwelling moves cold, nutrient-rich seawater near the coast in spring and summer, moderates coastal temperatures, and feeds plankton blooms that are the basis for huge, complex ocean food webs. This ends our discussion of Physical Oceanography 101.
Now let’s turn our attention to a few ocean hazards and ocean safety tips, tying them into a broader understanding of weather and ocean dynamics.
How does surf form? What are swells? First a bit of vocabulary. On calm days with no wind, the sea surface is relatively smooth. The Pacific Ocean is never completely calm, but can be mostly flat during a windless morning in spring, summer, or early fall.
A light breeze or wind will form small capillary waves, just tiny ripples on the water surface. Winds up to about 15 knots (about 17-18 mph) blowing for a few hours will kick up waves about 3-4 ft. high. Choppy waves in the area where they are being generated by wind are known as seas. Past 15 knots, the tops of the waves begin to blow off, which are the familiar whitecaps, essentially a rougher, more energetic form of seas.
As seas move away from the area of wind, such as away from a storm at sea, they become sorted out into long parallel lines of wave movement known as swells. Swells cause the sea surface to move up and down rhythmically. The speed and amount of movement, as well as the height of the waves, are determined by three factors:
For example, a 10-knot wind blowing onshore for a few hours over nearshore coastal waters will not generate swells and only small to moderate seas. A 35- to 40-knot wind blowing for 2 days in mid-ocean over hundreds of miles will generate very large powerful swells. Swells do not cause water to move across the sea surface, but they can transfer enormous amounts of energy from distant storms to coastal areas.
As swells approach the shore, they begin to drag on the bottom. This is because water movement in swells extends well below the surface, and when the area of subsurface movement touches the bottom, friction drags on it and holds it back. This causes the wave to slow down, but also to “pile up” and become steeper. As the wave advances toward shore, this process continues until the crest, or top of the wave is moving faster than the bottom of the wave. At this point, the wave collapses forward or breaks, forming surf, and the point at which the wave breaks is known as the impact zone. The height and power of the surf is directly related to the size of the swells moving toward shore. Surf often comes in groups of larger waves called sets, followed by smaller waves.
As surf hits the shore, water is thrown up onto the land. What goes up must come down, so the water returns to the ocean, either in a solid sheet flow known as backwash or along low points in the beach, forming rip currents. It’s best not to call them “rip tides,” as they have nothing to do with tides and tidal movement.
Surf along the shore is an inherently unstable area where people can get into trouble. Boats at sea, even small kayaks, can handle quite large swells as long as they are not breaking and forming surf. At beaches, however, surf can knock people off their feet, backwash can drag them into the impact zone of the next breaking wave, and rip currents can pull them through the impact zone out into much deeper water. Waves larger than the average waves for that time of day can surprise people and are known as sleeper waves or sneaker waves. A true rogue wave is a massive mid-ocean wave that can overwhelm even a moderate-size fishing boat. They often flatten out by the time they reach shore, but still can carry enormous energy.
OK. I hope you’re not overwhelmed with all the vocabulary, which may be new to some of you. How can we help visitors stay safe near the ocean? A few basics:
This is serious business. It isn’t our job, and we aren’t trained as interpreters and Docents, to be lifeguards. But drownings do occur along the coast because people get in trouble with some of the natural hazards I’ve discussed. We can explain them as best we can, and it is our job to use an educational approach to protect people from dangers they may not understand. We can do this any time we are at a beach, tidepool, or bluff.
What else can we do?
You can contact Rouvaishyana at r at hearstcastle dot com