Ocean Motion

Wind and water--two of nature’s most elemental forces--combine to produce one of Southern California’s best-known attractions, the waves that reach its beaches. Here’s how, whence and why swells reach the beaches of Ventura County.

Storms That Send Us Swells

Storms driving waves to Ventura County beaches normally take place at three distant points. Wind-driven swells travel thousands of miles in some cases, depositing waves on local beaches. Here’s where waves are wrought:

A) Northern Hemisphere Storms

They typically develop in the northeast Pacific, more than 2,000 miles away. The fetch--the area across which the wind blows to build a swell--can be 1,000 miles in diameter, with winds in excess of 75 mph. Intervals vary because storms are relatively close and swells mix.

B) Tropical (Mexican) Storms

They need warm water to gain strength, typically forming 1,500 miles from Ventura County, off mainland Mexico, with up to a 300-mile-diameter fetch. When they drift northwest into cooler waters off Baja and Southern California, they lose some wave-making potential. Wind speeds are higher--they can reach 150 mph--but these storms are smaller.

C) Southern Hemisphere Storms

They travel the greatest distance to Ventura County--some more than 7,000 miles, taking up to two weeks. They form as far away as the antarctic ice pack, south of New Zealand. The point-of-origin fetch can be more than 1,000 miles in diameter, with wind speeds exceeding 75 mph.

Sheltered From the Storms

The Southern California coastline is partially sheltered by a series of natural breakwaters, so we don’t feel the full force of remotely generated swells. Point Conception, offshore islands and shallow banks off the islands can contribute to reducing swell size.

Ventura Surf Outlook

And there’s more on the way...Today’s surf forecast for county beaches is 5 feet and bigger, swell direction from the west. The swell was generated by a major storm that marched across the Pacific to pound Northern California. It will result in some breakers cresting to 10 feet or more on beaches, before diminishing on Thursday.

Northern Hemisphere

Swells generated by storms in the northern Pacific often have a clear path through the Santa Barbara Channel to hit Ventura County beaches. These swells produced the big surf this week, as they do every fall and winter.

Southern Hemisphere

Swells travel from the far South Pacific but can be blocked by Santa Catalina and San Clemente islands. The approach to Ventura county uses an avenue that skirts the east side of San Clemente Island.

Tropical (Mexican) Storms

These swells use the same approach as those from the Southern Hemisphere, but travel closer to shore.

Whipping Up the Water

The waves we see breaking on Ventura County beaches are mostly generated by winds. The height and length of these waves depend on three factors: wind speed, the distance it blows across the ocean surface and the length of time it blows. Here’s how waves are created:

Maturity

As newly developed waves leave their point of origin, they organize into lines of swell and move downwind. The original wind-driven waves disperse and new ones of greater length and speed form.

Landfall and Break

As waves approach land, they decelerate; lengths shorten and peaks form. As the ocean bottom becomes shallower, waves become critically steep and they break. The result: surf.

Genesis

Storm winds blow across the ocean’s surface, raising ripples, then chop. Waves continue to grow if wind continues, transferring energy by pushing directly against the wave backs. As waves grow steeper and higher, energy transfer between wind and water becomes more efficient.

Waves and Measures

Surf forecasters use varied forms of technology, including international weather services, ship reports and National Oceanic and Atmospheric Administration weather charts. Data from ocean buoys are part of the data chain, helping clarify wave direction, height and interval. Here’s how they work:

1) Buoys move up and down and to and fro, recording and transmitting swell information.

2) Satellites receives size, direction and interval data every one to six hours.

3) Satellite data are relayed to the National Data Buoy Center in Mississippi.

4) Data are sent to the National Meteorological Center in Maryland, then back to another satellite, whence it is available to forecasters.

Source: Sean Collins, Surfline / Wavetrak, “A Book of Waves,” by Drew Kampion, “Waves and Beaches; The Dynamics of the Ocean Surface,” by Willard Bascom

Researched by DENNIS LOWE and TOM REINKEN / Los Angeles Times