If you average the temperature of the blazing Sahara desert and
Death Valley with the chilling winds of Antarctica and Minnesota
you will come to 59 F. This number also happens to be about the
average temperature of Tam Valley. It is never extremely hot here
and it is never very cold.
In August when the rest of California is sweltering I’ve huddled
behind a sand dune at Muir Beach wearing a down jacket —just
barely comfortable— and yearning for long underwear. And one
New Years Day I stripped down to a tee shirt while watching
migrating gray whales from the Point Reyes Lighthouse.
The primary reason for this seemingly variable and capricious
temperature is the Pacific Ocean. This gigantic body of water
dominates all aspects of life along the coast, especially the
weather. One of the characteristics of water is its reluctance to
give up heat; it changes temperature very slowly. So the
temperature of the ocean determines the temperature of the nearby
air. During the course of one year the sea water only varies from
48 to 62 F. But in August it’s supposed to be hot, not cold! A
phenomenum that provoked Mark Twain to make that oft-repeated
statement about summer in San Francisco.
This maritime influence can be roughly divided into three
seasons— the upwelling phase, the oceanic period and the
Davidson Countercurrent. The cold water current that streams down
from Alaska is called the California current. It flows at 1/2
knot and averages about 54 F –cold. When the Beach Boys and Jan
and Dean were singing the praises of California beaches they
never mentioned wet suits.
And if this current wasn’t already cold enough to freeze a
walrus, spring and summer conditions chill the water even
further. Beginning in March a high pressure system usually
develops out in the Pacific. This system keeps rain storms away
from central California and it generates high winds. It is not
unusual on an April afternoon to have 50 knot winds howling in
from the northwest all along the coast. The Point Reyes
Lighthouse is one of the windiest spot in the West, winds have
been clocked at 133 mph. No wonder trees cannot get a root-hold
on exposed hillsides.
As these winds blow they push the surface water in front of them.
When anything moves in the northern hemisphere it has a tendency
to deflect to the right about 45 degrees. This is called the
Coriolis Effect. So the water being pushed by these northwesterly
winds, deflects to the right and moves away from the shore. This
surface water has to be replaced, so deeper ocean water flows in
and upwells along the coast.
This upwelled water is cold. And because it is bottom water it
has accumulated all the dead plants and animals that floated down
from above. It is chock-full of nutrients. You could think of it
as marine compost. As it upwells, it fertilizes the top layer of
the sea. In this upper zone where the sunlight penetrates, the
small plants (phytoplankton) bloom in enormous numbers. The small
animals (zooplankton) eat these plants and then bigger animals
eat those, et cetra. The result is one of the richest marine
ecosystems in the world — crabs, mackeral, herring, salmon, sea
lions, elephants seals, blue and humpback whales and, of course,
fishermen all thrive along the central California coast. And it
is all a result of the strong northwesterly winds that we all
curse in the spring.
The cold water also results in another prominent feature of our
area — summer fog. As warm winds move east off the open ocean
they cross over the band of cold upwelled water right near the
coast. The moisture in the air condenses out as fog. As the fog
moves inland, sometimes for miles, it is gradually rewarmed and
reabsorbed into the air.
In last weeks column I talked about the major influence the
Pacific Ocean has on our local weather. This effect can be
divided into three phases. The first is the upwelling phase.
Strong northwesterly winds of the spring and summer cause
upwelling of cold, nutrient-rich water. This is coupled with the
California current which carries cold water south from Alaska.
This results in sea temperatures hovering around 55 during the
summer, cold and productive water. One of the richest fisheries
in the world is off the central California coast.
In late summer the northwesterly winds weaken and upwelling
ceases; we begin the Oceanic period. The Pacific finally does
becomes peaceful and the sea temperature slowly increases. Calm
days and occasional east winds create smog conditions in West
Marin. The air temperature often reaches the 80’s and all over
the Bay area people call in sick to their jobs and flock to the
beaches. The tough even go swimming!
With the warming sea temperatures come increased numbers of
jellyfish, sharks and a dinoflagellate called Noctiluca .
Dinoflagellates are one-celled plant/animals. They can move with a
whiplike appendage like an animal. But some also have chlorophyll
and manufacture their own food like a plant. Noctiluca’s
biological claim to fame and the reason it is called “night
light” is that it bioluminesces. When it blooms in tremendous
numbers the breaking waves at the edge of the sea are filled with
These delightful coastal days end with the arrival of the third
major period, the Davidson Countercurrent. The high pressure
system that has deflected storms away from central and southern
California and shunted them north to Oregon and Washington slowly
disintegrates and weakens. This allows the wild Pacific squalls
to come rolling in. These winter rampages bring buckets of much-
needed rain and are usually accompanied by towering waves. Newts
emerge, steelhead run up the streams, the hillsides turn green
and water managers everywhere breathe a sigh of relief.
Every ocean current in the world has an undercurrent that flows
below it and in the opposite direction. Our local undercurrent is
called the Davidson Countercurrent; it flows up from the south
carrying warm tropical water. As the California current weakens
during the winter (for reasons not fully understood), this
undercurrent rises to the surface and brings the warmer water
along our central coast. This is the reason that the sea
temperatures are actually warmer in the winter than in the
summer. This warm current moderates the winter temperatures in
West Marin; it rarely freezes here and ice never forms along the
Then in late February or early March the winds begin to blow
again from the northwest. The Davidson Countercurrent weakens,
the California current takes over and upwelling begins again.
This scenario describes a perfect world, but perfect it ain’t.
Sometimes the winds fail and upwelling doesn’t occur and we have
an El Nino. The last big one occurred in 1983 and 84. Several
amazing things happened. Sea level was 6″ higher than normal and
houses along the Malibu coast were swept away. I went swimming
every day at Muir Beach. The water was 67, a veritable hot tub.
The salmon moved north and fishing boats were repossessed by the
banks. There was a near total breeding failure of sea birds on
the Farallon Islands. Divers reported visibility of 150′ at the
Cordell Bank. Marlin were caught in Monterey Bay. We had a very
There are some indications that we may be headed for another El
Nino period. The sea temperatures are above normal and the
seabirds at the Farallons have had a poor breeding year. Maybe,
just maybe, we will get some rain this winter.