El Niño and our beaches | Gary Griggs, Our Ocean Backyard
Our beaches come and go seasonally in response to changing wave conditions. The larger and more energetic winter waves stir up the beach sand and carry it offshore, eroding the shoreline back. Six months or so later, the smaller and less energetic spring and summer waves gradually move that sand back onshore, building the beach just in time for all the summer visitors.
Some beaches change over longer time periods as well. For those who walk, bike, skate, or drive regularly along West Cliff, and who have been here for a number of years, may have noticed that a beach will form above Cowells from time to time. This begins to form in some winters and may persist into early summer.
This beach is easy to see from the sidewalk and appears as a large sand bar and beach that extends along the shoreline from below the Sea and Sand all the way into the cove below the wooden stairway, and often several hundred yards along the base of the cliff toward Steamer Lane. For surfers, paddlers and others trying to get to the water from the stairway, which can be challenging at high tide when a significant swell is running, when this sand bar and beach are present you can step safely from the stairs onto the sand.
This occasional beach becomes a great place for families during the years when it forms because there is sand beach along the shoreline in front of the rip-rap, and shallow water extends hundreds of feet offshore. It also makes for gentle waves that are ideal for inexperienced surfers. You may have wondered why this beach periodically appears and then disappears for several years before it returns again. From many years of observation it appears that this beach usually forms in El Niño years.
The periodic change in ocean circulation and weather known as El Niño affects the entire Pacific Basin and its effects range from heavy rainfall, floods and landslides along the eastern edge (California, Central and South America), to droughts and wildfires along the western side of the Pacific (Australia, Indonesia, and India, for example).
Along the Monterey Bay shoreline, waves during El Niño years tend to come from the southwest and west rather than from the northwest, which is the more typical winter pattern. When combined with heavy rainfall and runoff from the San Lorenzo River, which delivers a lot of sand to the shoreline, these more southerly winter waves often transport the beach sand into the cove above Cowells.
The sand settles and gradually builds the beach seaward and into the cove below the stairway. It may remain for four to six months until the more typical swell from the northwest returns and carries the sand south and east. It moves towards Main Beach, Seabright and then into the harbor mouth where about 250,000 cubic yards on average each year has to be dredged out and placed on Twin Lakes Beach. But for those few months, Santa Cruz has a special beach that only locals tend to know about; and then it’s gone again.
El Niño events have been observed along the coast of Peru at least as far back as the 1600s. Geologic evidence from the sea floor sediment records indicates El Niño events have been around for at least thousands of years.
Peruvian fishermen, as well as the fertilizer industry, were painfully aware of these events because of the impact on their livelihoods. The bad news started with the intrusion of warm water into the coastal zone, which shut down the upwelling of the nutrient rich subsurface water. Without the nutrients or fertilizers, the plankton didn’t bloom, which broke up the food chain. The fish left town, and the coastal birds that fed on the normally plentiful fish also took off and failed to leave their nitrate and phosphate rich guano on the coastal rocks. Fishing and fertilizer industries ground to a halt every two to seven years, hurting the local economy.
In “normal” years, although it’s not clear just what normal is any longer, the trade winds blow towards the equator from the northeast and southeast. These winds push the warm surface water in the equatorial region towards the western Pacific, to the area near The Philippines and New Guinea. Some of this warm water then moves north and circulates clockwise around the North Pacific as the Kuroshio Current, which transitions to the North Pacific Current, and then finally becomes our offshore southward flowing California Current. The rest of the warm equatorial water turns south and completes a counterclockwise journey around the South Pacific as the East Australian Current, the Antarctic Circumpolar Current, and finally the Peru Current, which flows north along the coast of South America.
Now, here is the mystery that no one has yet figured out. Every two to seven years the trade winds die down, all of that warm water in the western Pacific reverses its direction and flows back towards the coast of South America along the equator, producing an El Niño. But why?
Despite all of our observations from ships, buoys and satellites, we still don’t know why El Niños occur. If you can figure this one out you will be famous; probably not rich, but famous.
Gary Griggs is a Distinguished Professor of Earth and Planetary Sciences at UC Santa Cruz. He can be reached at firstname.lastname@example.org. For past Ocean Backyard columns, visit http://seymourcenter.ucsc.edu/about-us/news/our-ocean-backyard-archive/.