Weather Watch

Clearing up ocean’s murky water

Last year, Tom Messenger from Los Osos was shooting aerial photographs of the ocean near Morro Rock. In several shots, the photos clearly showed the shadows of the surfers on the sandy bottom.

In all his life, he said, he had never seen the ocean so clear. He asked, “Is there a connection between particular weather patterns and water clarity?”

Along our scenic Central Coast, many factors affect seawater visibility. The greater the amount of suspended solids in the water, the murkier it becomes.

A major cause of poor seawater visibility is runoff caused by rain showers and seasonal storms. During these periods, large amounts of sediment from eroding creeks and rivers flow into the ocean.

Another culprit that affects water clarity is tides. As tides flood in and ebb out of our bays and estuaries, sediment and debris from the bottom become stirred-up in the water column.

The greater the tidal range, the more likely that debris from the shore will enter the water. That further compromises visibility.

Large wave events also come into play, especially long-period swells that mix up sediments in the same fashion as tides.

Upwelling along our coastline also plays an important role in seawater visibility.

As the northwesterly winds blow parallel to our coastline, the friction of the wind causes ocean surface water to move.

Because of the Coriolis force, the surface water flows to the right, or offshore.

This causes upwelling along the coast as cold, clear and nutrient-rich water rises to the surface along the immediate shoreline.

When the northwesterly winds relax and upwelling diminishes, phytoplankton can multiply rapidly, especially during the longer days of late spring and summer, further diminishing clarity.

Great seawater visibility can happen at any time of the year, but it seems more likely to occur along our coastline during the late fall and early winter months. This is also when we have the warmest seawater temperatures.

During late fall, the days become shorter and heavy plankton blooms become less common.

The winter rains have not yet begun and long-period swells from the Southern Hemisphere are winding down for the year. In addition, big waves from the Northern Hemisphere have yet to appear.

When all these conditions come together, water visibility along our coastline can exceed 60 feet near shore and even greater farther out to sea. That’s why many scuba divers in California look forward to the fall season. And on a clear day, the Central Coast has some of the most beautiful dive spots in the country.

Because of the great amount of upwelling, large kelp forests flourish in our region. If you ever have a chance to dive and experience it first-hand, you will see rays of sunshine beaming through this underwater forest illuminating fish, invertebrates and the occasional marine mammal.

It has been rightfully compared to flying through a redwood forest in Big Sur.

This week’s forecast

An Eastern Pacific high will remain nearly stationary about 600 miles to the southwest of San Luis Obispo, while a 1,024 millibar high will move over the Great Basin.

This condition will produce gusty northeasterly (offshore) winds and areas of dense ground fog north of the Cuesta Grade during the morning hours.

The strongest offshore winds are expected near Morro Bay High School, the eastern parts of San Luis Obispo near Cal Poly, San Luis Obispo High School and French Hospital Medical Center, as well as our coastal canyons and passes.

The winds will shift out of the northwest (onshore) during the afternoon hours, producing coastal low clouds along our northwesterly facing beaches (Los Osos) during the afternoon hours.

Little change in this pattern is expected through Wednesday.

Today’s high temperatures will range from the mid- to high 60s and maybe even low 70s in the coastal valleys and beaches while the interior will mostly be in the mid-60s.

High pressure will build into the upper atmosphere on Thursday with temperatures approaching the mid-70s.

Today’s longer-range charts and models indicate the high-pressure ridge will move northward. This would allow for temperatures to cool to near seasonal normal ranges Friday, and then the storm track will move southward.

It could produce rain Saturday, with increasing chances for rain the following week.

Surf and sea report

The swell peaked Saturday afternoon at 12.4 feet with a 17-second period at the Diablo Canyon Wave-rider Buoy. The Half Moon Bay Marine Buoy near the Mavericks surf contest reported a significant swell height of 22 feet with a 17-second period Saturday afternoon.

Today’s 10- to 12-foot west-northwesterly (285-degree deep-water) swell (with a 15- to 17-second period) will decrease to 9 to 11 feet Monday.

This swell will further lower to 7 to 9 feet (with an 11- to 14-second period) Tuesday, lowering to 5 to 7 feet Wednesday.

Combined with this west-northwesterly swell will be 2- to 4-foot northwesterly (310-degree shallow-water) seas during the afternoon hours through Wednesday.

A 3- to 5-foot northwesterly (290-degree deep-water) swell (with an 11- to 13-second period) will develop along our coastline Thursday, decreasing to 3 to 4 feet by Thursday night.

A 9- to 11-foot west-northwesterly (285-degree deep-water) swell (with a 15- to 17-second period) will arrive along our coastline Friday.

Preliminary analysis:

A 964-millibar storm is forecast to develop off the coast of Russia on Wednesday and move toward the Gulf of Alaska. A very long-period northwesterly swell from this storm is forecast along our coastline on Feb. 22.

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 Company. He is also a local weather expert and has lived along the Central Coast for more than 22 years.

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