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What causes the dreaded "catalina eddy" that routinely plagues us with early morning south winds, especially in the spring/early summer? Seems like it's been happening a lot recently.
And why do those days tend to glass off in the afternoons?
asked by Todd Smith, Newport Beach
The Surfline Forecast Team replies:
You have it right, this is the peak season for the Catalina Eddy.
Simply, the Catalina Eddy is a counterclockwise circulation that forms in the bight of Southern California. It can occur all year, but happens most often from may through July.
As the desert heats up this time of year, thermal low pressure forms there -- at the same time Pacific High pressure builds off the coast. The resulting NNW wind flow barrels down the California coast turning towards the coast and interacting with the islands and shape of the coast, which guides the air into a loop, with a center often near Catalina. This is the same as swirls and eddies around rocks in a stream.
If low pressure is near Las Vegas a complete circulation forms best. If the low center is near Yuma, AZ, a W-SW wind will often blow during the afternoon.
As for the glass off. At the same time this whole eddy business is going on, a sea breeze is at work too. This wind begins perpendicular to the coast (SW), and veers by evening to the W-NW. When these two winds collide, there is sometimes a calm -- or glass off. There are other things at work too, but that is the simple explanation.
There's much more to the formation of the Eddy, but that's a simple surface description.
See the image attached below.
Thanks for the excellent question.
Image: Sam Pepke
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