The Golden Gate Bridge, in its summer cloak of fog. Photo: kqedquest.

Another foggy morning. Why is the Bay Area so foggy in summer? To answer that question, look west—at the Pacific Ocean.

If you’ve ever tried to swim at the beach in Northern California, your chattering teeth will tell you that the water is cold. This cold water makes fog form. The cold water cools down the air above it, and cool air can’t hold as much moisture as warm air. The moisture condenses into fog.

The water along the coast of California is cold for a couple of reasons. First, the California Current brings cold water from Alaska southward along the coast. And second, cold water from the deep ocean comes up to the surface through a process called upwelling. From March through September, wind blows southward along the coast. This wind, combined with the rotation of the earth, creates surface currents that move water from the coast out into the ocean. Something has to fill in the space that was left behind when the surface waters moved out to sea. So water from the deep ocean is sucked to the surface.

The water from the deep ocean is full of nutrients. Upwelling is super important for ocean dwelling creatures—the nutrients in the water feed the phytoplankton and move on up the food web. The lush kelp forests along the California coast exist because of upwelling. And the water from the deep ocean is really cold, which makes fog form over the areas of upwelling.

The fog rolls in from the ocean onto land in the morning as the rising sun heats up the land. Warm air rises, and something has to fill its place—the foggy air that’s hanging out above the ocean.

So to summarize, summer winds create upwelling, fog forms over the cold water, summer sun heats the air above the land and makes it rise, and the fog gets sucked in.

However, the amount of fog has declined by 33% over the past 60 years. UC Berkeley professor Todd Dawson talks about this in QUEST’s Science on the SPOT: Science of Fog. Fog is declining in part because upwelling along the coast has weakened, thanks to global warming.

Warmer air temperatures are heating the surface layer of the ocean. As the surface layer gets warmer and thicker, it becomes harder for the cold deep water to mix with the warm surface layer. This weakens the upwelling. Weak upwelling means less fog is produced.

Someone (though maybe not Mark Twain) once said that the coldest winter they ever spent was a summer in San Francisco. That San Francisco summer was cold because of the fog. Which brings to mind another (potential) Twain quote: “Everybody talks about the weather, but nobody does anything about it.” San Francisco is getting less and less foggy, thanks to global warming, and so far we aren’t really doing anything about it.

The noise of all these nesting and breeding birds is almost overwhelming (check out the slideshow below for a firsthand look), but these birds speak for a lot more than themselves. Our guides, PRBO Conservation Science, have been studying these birds for 40 years. As Biologist Russell Bradley explained, these seabirds are environmental samplers. In order to raise their chicks, they depend on the food web that blooms in the spring when coastal upwelling brings nutrient-rich water to the surface. If that is disrupted or delayed, the first place scientists will see it is in these bird populations, who will either have poor or non-existent breeding seasons.

Those changes in the upwelling patterns can be due to natural variability in the system. But increasing, scientists are asking whether the changes are due to climate change. That's not an easy question to answer. There are a lot of different factors in the mix.

I spoke with Zack Powell, a professor at UC Berkeley who studies climate and upwelling, and he said it all comes down to the timing of natural cycles. First, there's El Nino – where warm water spreads across the equator and heads up the California coast. That can happen every two to seven years and when it does, it acts a barrier to upwelling, interfering with the marine food web. Scientists recently confirmed that El Nino will return this year.

Looking at changes on a longer time frame, there's the Pacific Decadal Oscillation. It's a pattern of ocean warming and cooling that can last 30 years. Powell says it can also have an effect on marine life and fisheries.

And finally, there's climate change, which comparably may cause changes on the longest time frame. Powell says there's about 100 years of historical data about the ocean conditions off the California coast and it's not much when looking at such long-lived patterns. Powell and others work on climate modeling to help answer these questions. Some of the models show that the seasonal winds may become stronger, meaning upwelling patterns could be altered. And ocean temperatures could rise significantly, changing the way warmer surface water and nutrient-rich deep water mix.

Powell says right now his focus is the granularity of the climate models. They simply can't predict changes on a small geographic scale. "For most models, the smallest footprint is about 100km and all the upwelling takes place closer to shore than that." But he's hoping there will be drastic improvements over the next few years. And if extreme changes do take place, for whatever reason, the birds will certainly tell us.

Listen to the Journey to the Farallones radio report online, and check out our Farallon Islands Interactive Map for the sights and sounds of the island. Or watch the audio slideshow below for a first-hand look.

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