Gravitational forces exerted by the moon and sun create the timeless tides. This tugging produces a tidal “bulge,” or area of higher sea level on the ocean’s surface.
As the Earth rotates eastward on its axis, California moves into this bulge, which produces a flood tide, and eventually a high tide. As the Earth continues to spin, we move into an area of below-normal sea level, or nodes, which produces an ebb tide, eventually reaching low tide. The slack tide is when the sea is not coming in or going out. Typically, we experienced two low and high tides per day.
Starting Thursday and continuing through Christmas Day, California will see the phenomenon of “pre-king tides and king tides” along the coast. These tides are some of the highest and lowest of the year, separated by less than 12 hours.
The highest will happen at 8:44 a.m. on Saturday and again at 9:27 a.m. on Sunday, when the predicted tides will reach nearly 6.8 feet. That will be accompanied by a low tide of -1.5 feet at 4:50 p.m. later that day on Saturday. In other words, the level of the sea will shift 8.3 feet in a little over seven hours.
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These flood and ebb tides will create ripping currents in California’s bays and estuaries. 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.
However, actual tides may be higher than anticipated, and here’s why:
In 2015, one of the strongest El Niño events on record was brewing in the Pacific. Combined with global warming, it contributed to some of the warmest seawater temperatures ever recorded. Usually, seawater temperatures along the Central Coast average around 58 degrees in November. But that month in 2015, the temperature averaged nearly 62 degrees.
When the water warms, it causes thermal expansion in the upper levels of the ocean. Consequently, seawater levels can be several inches higher than those predicted in the tide tables.
In fact, the observed water level that month was about 6 inches higher than predicted tides at the Port San Luis tide gauge. In other words, if the tide tables predicted a 6.7-foot high tide on Nov. 25, 2015, the actual tide was 7.2 feet. So far this year, the seawater has been warmer than normal and actual tide levels have been running higher than predicted.
Other factors like storm surge — when the winds blow across the surface of the ocean toward the coastline — can also increase tides. If winds are powerful enough, they can push and pile water on the beaches and increase sea level many feet — or reduce it if the winds blow out to sea.
Another factor is atmospheric pressure.
Back in January of 2017, air pressure readings reached 1,033.9 mb, or 30.53 inches of mercury (inHg), at the San Luis Obispo County Regional Airport and 1,035.6 mb (30.58 inHg) at the Paso Robles Airport. These readings were some of the highest I’ve ever seen.
The higher-than-average weight of the atmosphere produced lower-than-expected tide levels; the higher pressure pushed down the water surface. On Jan. 27, 2017, the predicted tide level was -0.7 feet, but due to the higher pressure, the actual level was -1.1 feet.
Lower atmospheric pressure, during storms, for example, can produce higher sea levels. Also, storm runoff from flooded creeks and rivers that flow into estuaries and bays and high wind-generated waves can also create higher-than-predicted sea levels.
A storm event can bring all these factors together: “king tides,” warmer seawater, storm surge, low pressure and storm runoff combined with high swell can produce much higher water levels than expected.
This week’s high swell may cause flooding during the morning hours with the high tide. A 10- to 12-foot northwesterly (300-degree, deep-water) swell (with a 24- to 26-second period) is forecast on Wednesday, peaking at 11 to 13 feet (with an 18- to 20-second period) on Thursday.
More concerning, climate change will rear its ugly head at an ever-increasing rate.
“Global warming is driving actual tides to be higher than predicted on the tide charts, as warming waters and melting ice sheets on Greenland and Antarctica raise sea levels across the planet,” said Josh Willis, NASA oceanographer and climate scientist.
Earlier this decade, PG&E brought together a team of atmospheric scientists, oceanographers and meteorologists. Their consensus is that sea level will rise around 7 inches along the California coastline by 2030 and nearly a foot by 2040.
To get an idea of what this will look like in the not-too-distant future, the California King Tides Initiative https://www.coastal.ca.gov/kingtides/ 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 what’s to come.
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, weather and oceanographic conditions.
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