Weather Watch

The ocean in SLO is so cold it’s like ‘hundreds of needles’ on your skin. Here’s why

The northwesterly wind gust reached 55 mph on Tuesday evening, followed by 54 mph gust on Thursday evening at the Diablo Canyon Power Plant’s meteorological tower.

An anemometer reported 79 mph wind gust at the Morro Bay Harbor Department on Tuesday evening. Farther inland, both the Santa Maria and Lompoc airports reported 44 mph northwesterly wind gusts. These winds toppled over a few eucalyptus trees at the Morro Coast Audubon Society Sweet Springs Nature Preserve in Los Osos.

More importantly, these relentless spring time northwesterly winds, which often signals the end of the rain season, have produced frigid seawater temperatures along the coastline. Seawater temperatures at the Diablo Canyon Marina dropped to 49 degrees Saturday morning.

Scuba diving in water temperatures like this is difficult.

You can’t seem to stay warm even with the thickest wetsuit or a dry shirt. The freezing water feels like hundreds of needles pressing against your forehead. So why does the water get so cold when the northwesterly winds blow? As the northwesterly winds blow parallel to our coastline, the friction of the wind causes ocean surface water to move.

Because of the Coriolis Effect, the surface water flows to the right, or offshore. This, in turn, causes upwelling along the coast as cold, clear and nutrient-rich water rises from the ocean’s depths to the surface along the immediate shoreline to replace the shallow water that is pushed out to sea.

On days with plenty of sunshine and lots of upwelling, California giant kelp (macrocystis) can grow up to 24 inches in just one day, ultimately reaching more than 150 feet in length. At that rate, you could almost see this type of algae grow in front of your eyes. Usually, the peak growing season for giant kelp occurs in spring during the height of the upwelling season.

Occasionally during spring, the northwesterly winds relax, and upwelling diminishes. When this condition occurs, the longer days produces enormous phytoplankton blooms due to photosynthesis. These plants, in turn, provide food for the zooplankton, which also rapidly increase in numbers providing food for baitfish, such as Pacific sardines, anchovies, smelt, Pacific herring and grunion, which provides food for larger creatures such as seabirds, pinnipeds, bigger fish and even whales.

For many years, one of my jobs at the Diablo Canyon Power Plant was to monitor and record seawater temperatures along the Pecho Coast as part of the plant’s environmental monitoring program. This job was accomplished by electronic waterproof seawater temperature recorders, which are currently about the size of a large Sharpie.

These temperature recorders are housed in steel canisters and placed at fixed locations in the intertidal and subtidal zones. Depending on the tidal cycle, intertidal canisters are exposed to the air at minus low tide and underwater at high tide; the subtidal locations are always underwater.

Depending on the weather, waves and tides, the temperature recorders are swapped out every few months. During the summer, the minus low tides occur during the dark hours before dawn. Over the decades, an anthology of marine biologists have journeyed to the tide pools and swapped out these temperature recorders by flashlight or by the light of the of the moon. If the conditions are right, bioluminescence from plankton blooms can mark your track across the cold, wet and slippery rocks of the littoral.

During swap-out, the new recorders are installed in the canisters and the old recorders taken back to the Diablo Canyon Ocean Lab to be downloaded to a computer database, serviced and calibrated. This process has been going on since 1976 and has produced an amazingly rich temperature database containing millions of recordings.

Data from the Patton Cove subtidal temperature monitoring station, which is just south of the power plant, is in approximately 10 feet of water. This station has been in use since 1976, and more than 900,000 temperature data points have been recorded there. The average yearly seawater temperature at this location is a chilly 55.5 degrees Fahrenheit.

The average minimum temperature usually occurs during the spring and reaches a bone-chilling 48.8 degrees. During the fall, the average maximum temperature can rise to 62.7 degrees. As we move toward summer and fall, the seawater along our coastline will warm, allowing for more comfortable conditions for the surfers and scuba divers, with the promise of albacore gathering closer to the shoreline.

Climate change presentation

At 4:30 p.m. Wednesday, I’ll be giving a presentation about climate change called “The Seven Deadly Sins of Climate Change” on the expected effects of it on our local weather, seawater temperatures and waves along the Central Coast at the Morro Bay Library, 625 Harbor St.

For more information, please visit or call 805-772-6394.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.
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