Weather Watch

Tug of sun and moon drives tides

Looking out over the Morro Bay estuary during winter, it’s fascinating how quickly it changes from a brimming body of water during high tide to silvery mud flats at low tide.

The Central Coast usually experiences two high and low tides per day, but due to the tidal cycle being over 24 hours, some days have only one high or low tide.

The tides are caused by the gravitational forces on Earth exerted by the moon and sun.

These “tidal forces” are not the total gravitational forces exerted by the sun and moon on Earth, but the difference between these gravitational forces over the surface of the planet. The bottom topography or underwater bathymetry of our coastline also plays an important part in the changing tides.

I often wonder why the high tide does not occur when the moon is directly overhead or over our meridian, but actually happens hours later.

It may seem counterintuitive, but the gravitational forces of the moon and sun tug at the oceans primarily on the horizontal tangent and not the vertical plane to the Earth’s surface.

This is partly why the high tide lags between eight and nine hours on the Central Coast — also referred to as the lunitidal interval. This value can be used to program certain tide watches that you may have received during the holidays.

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 rotate, 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 high tide, this tends to be a nice time to be kayaking on the Back Bay.

During a full as well as new moon, the sun and moon are aligned, producing strong tidal forces on the Earth. This is also referred to as spring tide, even though it doesn’t have anything to do with the spring season. These are the highest and lowest tides of the month.

During the moon’s quarter phases, the gravitational forces of the moon and the sun are perpendicular to each other. Nearly canceling each other’s tidal forces out, the result is smaller high and low tides. This is referred to as neap tide.

Earth’s orbit around the sun is really an ellipse, a shape that can be thought of as a “stretched out” circle or an oval.

Tides are enhanced when Earth is at perihelion, when the whole planet comes closest to the sun. This occurs during winter in the Northern Hemisphere and summer in the Southern. Perihelion this year was last Sunday. Aphelion, when Earth is farthest from the sun, occurs during our summer, this year on July 6.

Because the Northern Hemisphere leans away from the sun when we’re closest to it, and toward it when we’re farthest from it, our axial tilt tends to moderate our temperature extremes, keeping our summers cooler and our winters warmer than they would otherwise be.

The opposite is true in the Southern Hemisphere. There Earth’s tilt compounds both the heat of summertime and the cold of winter.

When we look at the tide tables, you should remember that these are only predicted values utilizing gravitational forces.

During El Niño events, actual tides can be up to one foot higher due to thermal expansion of the water column. Storms can also affect sea level. Low atmospheric pressure associated with storms can allow ocean waters to expand, resulting in a temporary increase in sea level.

Strong winds can cause additional elevation of sea level due to storm surges.

The maximum tidal range along the Central Coast ranges from more than two feet below the mean low water mark, to more than seven 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!

This week’s forecast

A very weak but fast-moving front passed our area yesterday afternoon with cloudy skies and a few scattered sprinkles.

This front was followed by moderate easterly winds and mostly sunny and clear conditions.

A 1022 millibar surface high about 150 miles southwest of San Luis Obispo will remain nearly stationary through Monday. This high pressure ridge will continue to produce mild and dry weather.

Except for periods of variably cloudy skies and areas of fog this morning, clear skies are forecast to persist.

Temperatures today will range from the high 60s in the interior to the low 70s in the coastal valleys and beaches. Little change in these temperatures is expected through Monday.

This static weather pattern has allowed tule fog to develop in the San Joaquin Valley. This dense valley fog is expected to continue through Monday.

Clouds will increase Monday night as a cold front approaches the Central Coast. Rain and moderate to fresh (13-24 mph) southerly winds are forecast on Tuesday, turning to scattered showers on Wednesday morning, tapering off by Wednesday afternoon. Rainfall amounts should range from 0.5 to 1 inch. This is a relatively warm system, and snow levels should remain above 5,500 feet in the Sierra.

Skies will clear Wednesday night with the return of morning dense ground fog and dry weather Thursday through Saturday.

This morning’s longer-range models are in strong agreement that a wet weather pattern will develop the following week.

Surf and sea report

Today’s 10- to 12-foot west-northwesterly (285-degree deep-water) swell (with a 15- to 17-second period) will decrease to 7 to 9 feet on Monday.

A low-pressure system has swept across the North Pacific and will intensify to 951 millibars by the time it reaches the Gulf of Alaska this morning.

Hurricane-force westerly winds associated with this vigorous storm will produce fully developed seas of more than 55 feet about 1,800 miles to the west of Central California.

A very long-period west to west-northwesterly (270- through 285-degree deep-water) swell from this storm will arrive along our coastline late Tuesday at 12 to 14 feet (with an 18- to 22-second period) and will peak on Wednesday at 15 to 17 feet (with an 18- to 20-second period).

This swell will gradually decrease on Thursday through next Friday. A very active North Pacific will continue to produce fairly rough oceanographic conditions along our coastline. The next large swell is forecast to arrive along our coastline on Jan 18.

Conservation tip

Water is about 800 times denser than air. Waves and tides contain a great deal more potential kinetic energy than the winds for the same area or footprint.

This energy can be converted into electricity. Pacific Gas and Electric Co. will continue to evaluate different hydrokinetic devices that may play an increasingly important role in generating clean energy for California. For information, visit www.pge.com

John Lindsey is a media relations representative for Pacific Gas and Electric Co. and a local weather expert and has lived along the Central Coast for more than 22 years.

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