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I'm Erika Zavaleta and this is Ecosystems of California.

And I'm at Seacliff State Beach on the northern end of Monterey Bay.

Behind me you can see the curve of the bay as it goes first east, then south,

and curves back around far behind me, but that's mostly obscured by the fog.

So, California's climate is really diverse for a bunch of reasons.

It's north-to-south length, which is 800 miles, or about 1,300 kilometers.

Its really strong gradient from the ocean, going inland, and

its many and varied mountain ranges and elevational diversity.

So today we're going to travel a transect.

We're going to travel from west to east across California, starting at the ocean

and ending in the desert, to have a look at some of that climate diversity.

We're starting here at the ocean, early on a summer morning.

Temperature is about 59 degrees, the winds are calm.

The temperature is incredibly stable at the California coast.

And that has to do with the incredible thermal mass of the Pacific Ocean.

So, in the summer the Pacific is cooling the air.

In the winter, the Pacific is keeping it relatively warm.

And the temperature only swings five to ten degree Celsius on average between

summer and winter here.

Another thing you can see behind me is this marine stratus layer, or high fog.

And this is typical on the California cost, especially in the summer.

And a way that that advective summer fog works is kind of like this.

So, the Pacific Ocean here in California is marked by strong upwelling

most of the year.

And with that means is that long shore winds push the surface waters away from

the coast, and that pulls up cold water from down deep.

So, because that water is upwelling right at the coast,

it turns out there's a temperature gradient.

As you come in to the coast from out at the ocean, it actually gets colder.

And so, in the morning when the sun comes up over California and

the air starts to warm and to rise inland,

it pulls moist air over the surface of the ocean and the marine layer.

That moist air encounters these colder waters near the shore.

And it gets cooled by that water surface, reaches its dew point, and

some of that water vapor condenses out as fog.

It happens up high, when the marine layer's thick, when the temperature

inversion over the ocean that's driven by the cold surface is deep.

And so, today that marine stratus layer,

that moisture is coming out towards the top of the inversion.

That fog can get drown inland.

And so on some summer days when it's really hot in the interior,

it'll enter through the Salinas Valley in the San Francisco Bay and

stretch all the way to the Central Valley.

So let's leave the ocean behind and head east.

So we're 25 miles from the coast now.

And you can see behind me that we're nearing the end of this fog layer,

this marine stratus layer.

So, the coast ranges can block that stratus layer from entering inland.

But where we are now, near Hollister, California, there are some valleys.

And so as that warm air behind us is rising, it's pulling air in from the ocean

inland and bringing that fog in many miles from the coast.

As we move that way, we're going to be moving into an area of much, much higher

temperatures and drier air that's more typical of California in the summer.

So, why doesn't it ever rain in California in the summer anyway?

The main reason is that there's something called the North Pacific High.

It's a big atmospheric system, and it's always out there but

sits in different places at different times of the year.

And in the summer, it just parks itself over California.

It's a high pressure system.

And it basically just rebuffs any storm system that comes into the area.

It bounces off of that high pressure and gets held off of the coast.

And it sits there until some time in September, October maybe.

And then it'll move out offshore.

And the storms can come down and in from Alaska and

other places in the North Pacific.

So that system of that high moving up over California is really stable.

It's been doing that at least a few thousand years.

And as far as we know, it's going to keep doing it into the long future.

And until it stops doing that, we'll continue to have these totally dry,

rainless summers over much of the state

We're now at San Lewis Reservoir on the edge of the Central Valley to the east.

And just over Picacho Pass out of the most interior ridges

of the Coast Range to the west.

And we've come out of the fog.

It's pretty windy up here.

The wind is funneling through Picacho Pass from the ocean.

And because the air has been forced up the mountains, it's left the fog behind.

So, the San Lewis Reservoir is part of the state water project.

It was built in the mid-1960s and first filled in 1969.

It's a storage facility.

So water is pumped here from Sacramento San Joaquin Delta.

And then it's stored here and

conveyed from here to agricultural users throughout this region for irrigation.

It's an interesting part of this incredible infrastructure of water

delivery that marks California.

Partly because it has this summer drought seasonality.

And one of the things that you can see behind me is that the reservoir is

really low.

So, there's been quite a bit of drought lately.

And the long-term trend in California has been of decline in precipitation.

And San Luis Reservoir has dropped to as low as 17% of its capacity.

So, climate change is not the main thing that we're talking about today and

looking at today.

But it is this backdrop against to which we look at the climate of California and

its long-term trends.

The other thing you can see behind me is that the hills now are really dry.

It's a lot drier here than where we left the coast this morning.

And that's because now we've come over a few ridges of the Coast Range.

So we're in the range shadow of the Coast Range here.

Precipitation here is about half of what it was in Hollister at our last stop,

which is in turn about half of what it is at the coast.

And what's happening is that those same moist air masses that create a fog

at the coast, in the winter here, are moving in from the ocean.

And when they hit mountains, they begin to rise.

As they rise, they cool adiabatically.

Adiabatically means that they're not giving off heat,

they're just losing energy because they're expanding from a decrease in pressure.

So as those air masses rise and cool, they can reach their dew point so

that the moisture, the water vapor in that air, condenses.

And it can form mountain clouds and rainfall.

That's one of the reasons why as you go up in elevation on the windward side of

the mountains like the Coast Range and the Santa Cruz mountains, you can get

precipitation levels that are much higher than they are down closer to sea level.

So the air masses come up the mountains, they cool, they drop moisture.

They come over the back side of the mountains and then they begin to sink.

And they've had all the moisture wrung out of them, or

a lot of the moisture wrung out of them.

From that rise, they drop.

As they're dropping, they warm now, because they're increasing in pressure.

And so they hold on more tightly to their water and

they're less likely to drop it as precipitation.

So that's happened once over the coast ridge.

It's happened again over the interior ridge of the Coast Range.

And now we're back on the back side and the sinking warm air masses are hanging on

to their water and much less likely to rain.

And we'll see the most dramatic rain shadow in California at the eastern end of

the state when we cross over the Sierras into the Great Basin Desert,

which itself marks the rain shadow of that big mountain rage.

So we started at the Pacific.

We stopped near Hollister, California and

over the Interior Coast Range at San Luis Reservoir.

We've crossed the Great Valley now and

we're in the foothills of the Sierra Nevada.

And behind me you can see them rising up with some oak woodlands

scattered into these grasslands.

It's a little bit of savannah.

So as we climb the Sierras,

we're going to climb out of what's right now a pretty hot, dry place.

Temperatures in the high 80s.

It's mid-day in August.

And as we rise, we'll be rising with that air that's coming

up out of the valley and adiabatically cooling.

So the temperature will drop as we go up.

For the same reasons that it gets cool as we go up,

precipitation levels are going to go up as we climbed into the Sierras.

So air masses from Pacific, by the time they reached the Sierras,

have lost some other moisture in the Coast Range mountains.

But they get to the Sierras still laden with a fair bit of moisture, and

of course the Sierras are a lot higher.

Now we're going up to over 4,000 meters in some places.

So, as that air rises up the Sierras, it drops a lot of moisture.

And so, that mountain precipitation, which falls mainly in the form of snow because

we're in that Mediterranean climate system,

can accumulate to levels on average of about ten meters a year at Donner Summit.

A ton of snow pack, with some of the sites receiving higher levels than anywhere else

in the continental US every winter.

So, we're going to head up into the mountains now.

So, now we nearing the top of Tioga Pass,

near the top of the Sierras at 3,000 meters in elevation.

And even though we are at the top of the Sierras,

we're still in a region heavily influenced by the Pacific Ocean.

We started at the Pacific 300 kilometers ago and have come over the Coast Range,

across the Central Valley, up into the foothills, and up the Sierra.

Behind us are the mid-elevations of the western slope of the Sierras,

where most of the precipitation gets wrung out of the air as it's rising up.

And at the top here,

at this point, the main features of the climate have to do with cold.

Most of the precipitation up this high is falling as snow,

because there isn't summer precipitation for the most part.

And so organisms up here have to deal with a real mismatch between the growing season

and the timing of water availability.

The temperature here is much colder, even now in August.

It's probably 60 degrees out right now, dropping into the 50s as the sun is

setting, but also because we're above 3,000 meters.

And then finally,

the temperature variability of the range up here is really high.

In the middle of the day here in the summer we're hitting pretty hot

temperatures.

And at night, even in the summer, temperatures can drop down to freezing.

The annual variability in temperature at this site is also totally different

from at the ocean.

Instead of a fluctuation on the order of 10 degrees, here the fluctuation and

the difference between mean temperatures in the summer and

the winter are 40 or more degrees.

So now that we've crested the Sierras and come down their eastern side,

we're about 220 miles from the ocean, or 350 kilometers.

Almost as far from the ocean as you can get and still be in California.

And we've entered the enormous rain shadow of the Sierra Nevada, which extends not

only across Eastern California but also across most of Nevada and into Utah.

That enormous area of arid lands that's shadowed from the ocean by

the Sierra Nevada is the Great Basin,

which you can see the beginning of behind me.

So, the other thing that's changed dramatically as we've

gone from the coast to here, is that at the coast,

annual temperature fluctuations are on the order of five to ten degrees Celsius.

Whereas out here in the desert, they're much, much larger.

35 to 40 degrees Celsius on average between the July maximum and

the January minimum mean temperatures.

And so, you can have temperatures well below freezing here in the winter and

well above 40 degrees Celsius or a 100 Fahrenheit in the summer.

Ocean, altitude, mountains and wind shape California's climate.

And we haven't even looked at winter or

about the variation that stretches from the north to the south.

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Climate Transect

Ecosystems of California

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