Spring means cold waves and rough seas off SLO County coast. This is why

As most beachgoers will tell you — especially surfers and scuba divers — the seawater temperatures along the Central Coast have turned downright cold.

On Friday, seawater temperatures dropped to 49.3 degrees Fahrenheit at the Diablo Canyon Waverider Buoy.

Scuba diving in water temperatures like that is challenging.

You can’t seem to stay warm even with the thickest wetsuit or a dry suit. The freezing water feels like hundreds of needles pressing against your forehead.

Surfers report numb fingers and toes despite gloves and booties.

Not only has the seawater been cold, but the chaotic northwesterly seas have created hazardous boating conditions.

So why is the ocean near the coastline so cold and the seas so rough?

The answer is in the winds, and here is why.

The Eastern Pacific High often becomes firmly anchored off the Northern California coastline in spring. This area of high pressure keeps the storm track far to the north and helps produce the gale-force northwesterly winds and high seas along the Central California coastline.

Unfortunately, these relentless springtime northwesterly winds often signal the end of the rain season.

The northwesterly gales of spring produced vast amounts of upwelling along the coastline.

As the northwesterly winds blow parallel to our shoreline, 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 to the surface along the immediate shoreline.

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.

This is one of the main reasons that spring historically has the coldest seawater temperatures.

In fact, decades of seawater temperature data from subtidal temperature monitoring stations along the Pecho Coast near Diablo Canyon Power Plant indicate that the average yearly seawater temperature is 55.5 degrees.

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.

Not only do the northwesterly winds create frigid seawater temperatures, but afterweeks of these gales, many people in the coastal regions have grown wary and asked the question, will they ever end?

The good news, an atmospheric phenomenon south of the Central Coast can deliver a welcome break from these northwesterly winds.

It is called a Catalina eddy. No, that is not someone you don’t want to meet in a dark alley by a pool hall.

It is a circular counterclockwise airflow in a small area in the Southern California Bight, the coast from Point Conception to just south of San Diego.

When a Catalina eddy forms, it can quickly decrease the relentless northwesterly winds and allow low marine clouds to develop over the coastal regions.

The formation of the Catalina eddy is still not completely understood. During a period of gale-force northwesterly winds, coastal eddies seem to develop as these winds travel down the rugged California coast past Point Conception and interact with the topography of the Southern California coast.

That interaction causes the winds to flow in a counterclockwise direction and creates an area of low pressure in the vicinity of Santa Catalina Island.

When coastal eddies develop, the winds will shift out of the south and usually produce a rapid increase in the depth of the marine layer, resulting in a thick blanket of fog and cooler temperatures.

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