Weather Watch

I can see seawater clearly now, the rain is gone

Seaweed visible through exceptionally clear ocean water.
Seaweed visible through exceptionally clear ocean water.

When I was young, my friends and I used to go abalone diving off Bird Rock along the Point Reyes shoreline near Tomales Point.

This region was in the so-called “Red Triangle” that stretches from Bodega Bay southward to Monterey out to the Farallon Islands. This area is known for its abundance of great white sharks that feast on marine mammals.

Usually at this location, the seawater visibility ranged between 5 and 15 feet, not nearly enough range to see the “Man in the Gray Suit” until he was right on you. At this dive spot, you free dive in a blurred expanse of fear, only limited by how long you could hold your breath.

Nevertheless, legal-size red abalone was plentiful, and your time in the cold water short.

However, one late September day back in 1980, all oceanographic conditions came together to produce unheard-of Caribbean-like seawater visibility of well over 100-plus feet.

That day, we dove for hours, mesmerized by the beauty of the ocean environment. Despite the first signs of hypothermia, we just didn’t want to leave.

Over the past week, seawater visibility along the San Luis Obispo County coastline has increased, which leads to the question, 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 sediment 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.

Combined with the rotation of the earth, these winds push the surface waters to the right of the wind direction, thus pushing these waters out to sea, which creates a void. Cold, clear and nutrient-rich water is drawn from below to fill this void.

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

This forms the foundation of the food web.

The zooplankton population can explode as they eat phytoplankton and other zooplankton, creating abundant food for small baitfish such as anchovies and also further reducing seawater visibility.

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 wind 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.

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