Posted on December 13, 2012

San Luis Obispo County: The Highs and Lows of a Phenomenon Called ‘King Tides’

By John Lindsey

AVILA BEACH — These past few days have seen the phenomenon of “king tides” along the California coast. “King tides” refers to tides that are the highest and lowest of the year that are usually separated by less than 12 hours. The term originated in Australia and has since spread throughout the rest of the nations that border the Pacific Ocean.

At 9 a.m. this morning, the tide reached 7 feet high at Avila Beach. Note that no sand is showing – just seawater. (Photo by John Lindsey.)

Today (Dec. 13), the predicted high tide reached 7 feet at Port San Luis in San Luis Obispo County at 8:54 a.m. The tide hasn’t been this high since Jan. 10, 2009. This afternoon’s predicated low tide will reach -1.8 feet at 4:15 p.m.

The maximum tidal range along the Central Coast can reach from more than 2 feet below the mean low-water mark to more than 7 feet above it, for a tidal range of more than 9 feet. The greatest tidal range that I know of is in the Bay of Fundy on the east coast of Canada, which can exceed 52 feet.

The tides are caused by the gravitational forces exerted by the moon and sun. The bottom topography — or underwater bathymetry of our coastline — also plays an important part in the changing tides.

This tugging produces a tidal “bulge,” or area of higher sea level on the ocean’s surface. As Earth rotates eastward on its axis, the Central Coast moves into this bulge, which produces a flood tide, and eventually a high tide. As Earth continues to spin, we move into an area of below-normal sea level, or nodes, which produce an ebb tide, eventually reaching low tide. The slack tide is when the tide is not coming in or going out.

During strong El Niño events, when seawater temperatures along our coastline are warmer than normal, water levels can actually be several inches higher than those predicted in the tide tables.

This is because of the thermal expansion of the upper levels of the ocean.

As you can imagine, global warming is also contributing to higher-than-predicted tides as sea levels continue to rise.

The California King Tides Initiative is encouraging the public to document these tides that occur along the state’s coast. The organization’s goal is to document changes to our shoreline and give the public a glimpse of the future. This month’s king-tide event runs through Friday (Dec. 14). The group says upcoming ones will occur on January 9-11 and February 7-9 early in the new year.

If you’re going to photograph these tides, please be safe! Never turn your back to the ocean, and always be aware of your surroundings and of weather and oceanographic conditions.

John Lindsey works for PG&E in San Luis Obispo County and is a longtime local meteorologist. His columns regularly appear in the San Luis Obispo Tribune. Email Currents at currents@pge.com.

 

 

 

Comments are closed.

"PG&E" refers to Pacific Gas and Electric Company, a subsidiary of PG&E Corporation.
© 2014 Pacific Gas and Electric Company. All rights reserved.