I have been interested in weather since the sixth grade. The atmosphere has always fascinated me. After attaining three degrees in meteorology, one of the most essential things that I have learned is that the atmosphere is destined to show you new things all of the time. Just after Memorial Day, a meteorology colleague of mine shared images of a fascinating circulation off the coast of California. It was an eddy-like feature that actually developed an eye, but it was not a hurricane or tropical storm. While familiar to the meteorologically-inclined in that part of the country, the Catalina Eddy and related circulations may not be familiar to many people. Here’s a “101” discussion on them.

Curt Kaplan is a meteorologist at the National Weather Service Oxnard. During the week of Memorial Day, he tweeted, “Hurricane “ Catalina Eddy”….This was the early morning vis (visible) satellite (image) on Memorial Day showing an eddy circulation which is typical across the SoCal Bight.” Kaplan said a 3-mile wide eye developed, which made the system look like a well-defined hurricane. However, he was quick to point out that this feature off the California coast near Santa Catalina Island is just indicative of a shallow marine layer. Whew, there is a lot of meteorological jargon to chew on there so let me decipher it for you.

If you have ever seen dramatic pictures of fog rolling in under the Golden Gate Bridge, that is an example of the marine layer. According to the National Weather Service Jet Stream website, the marine layer represents, “a difference between a cool, moist air mass and a warmer air mass.” The coastal waters of California and much of the west coast have their origins in the Gulf of Alaska and are significantly cooler than water at similar latitudes along the east coast. Air that moves over these colder water are denser and can be chilled to the dew point temperature at which condensation (clouds or fog) forms.

The Catalina Eddy that Kaplan mentioned forms because of upper atmospheric flow interactions with the islands and drought coastline of Southern California. The eddy system often results from offshore winds associated with high pressure and coastal winds associated with low pressure. According to the NASA Earth Observatory website, these flows “combine with the topography to give the marine stratus clouds a cyclonic, counter-clockwise spin.” The image below was captured by the NASA Aqua satellite in 2013.

Kaplan, an expert in regional meteorology of the Southern California Bight region, told me that this eddy set-up is common when northwesterly winds (from the northwest) pass to the west of Santa Catalina Island. The dynamics and rising motion associated with the cloud system can send clouds up the coast towards Los Angeles. This brings the aforementioned marine layer into the populated regions of Southern California. University of California Los Angeles (UCLA) atmospheric sciences expert Roger Wakimoto published an account of the Catalina Eddy and its impact on pollution more than 30 years ago in the journal Monthly Weather Review. His study also highlighted Lance Bosart’s landmark 1983 study on the lifecycle of the Catalina Eddy. Scholars at Embry-Riddle Aeronautical University also provided a more contemporary discussion (2015) of eddy circulations in marine stratocumulus clouds.

Isn’t the atmosphere cool?