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El Niño’s Here. What Does That Actually Mean?

by on August 25, 2015

Sea surface temperature anomalies for the first two weeks of August 2015, relative to the average from 1971-2000. (Image courtesy NOAA)
Sea surface temperature anomalies for the first two weeks of August 2015, relative to the average from 1971-2000. (Image courtesy NOAA)

A

strong El Niño’s headed our way and its effects on California this winter could be significant, with heavy rains, abundant Sierra snow, devastating flooding, and destructive coastal erosion. Or maybe not.

El Niño
Beyond the Hype

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Bay Nature goes beyond the headlines to explore what the strongest El Niño in recorded history might mean — or not — for Northern California. Starting next week, we’ll post new articles here every Tuesday through the fall.

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Either way, wet or dry, the 2015-2016 winter season will go down in the record books. If wet, this year’s ENSO (the science acronym for El Niño Southern Oscillation) could replicate what happened decades ago during two other strong El Niños when rainfall records were set; if dry, because the forecast ENSO fizzles, well, that too will be record-setting but in very different ways.

Right now, all meteorological and oceanographic organizations agree the global stage is set for a strong—perhaps even a very strong—El Niño episode over the next six months. While El Niños of the weak or moderate category happen fairly regularly, coming and going with largely insignificant effects on central California, we’ve had two ENSOs in the last half century of the strong category: the winters of 1982-83 and 1997-98. And both were notable in the Bay Area.

San Francisco’s average (or “normal”) winter rainfall is 23.64 inches, resulting from an average of 68 rainy days. During the El Niño winter of 1997-98, more than twice that amount — 47.19 inches! — fell in a record-setting 119 days of rain. Slightly less rain fell during the 1982-83 ENSO, with 38.17 inches in 100 days of rain.

It’s these two strong historical ENSO wet winters that nurture hope for relief from our current drought. However, there are several good reasons to hedge about the coming winter. First, just two wet ENSO winters out of 50 years is not a compelling sample size for establishing causality, particularly when we see that out of San Francisco’s five wettest rainfall seasons only two of them were El Niño connected. Second, historically there’s a stronger connection between El Niño and rainfall in Southern California than in Central or Northern California. True, SoCal also needs the rain, but remember that the state’s north-south water system is based upon collecting water in the north before shipping it south. So even if Southern California receives decent rainfall this winter, only a small amount will be captured in local reservoirs while most will simply flood LA’s streets before draining into Santa Monica Bay.

Third — and this is a big one — is that while there are many atmospheric and oceanic ENSO conditions this year that are similar to the 1997-98 El Niño, there are also some significant differences that may (or may not) affect this year’s winter. Topping that list is the vast area of warmer-than-normal ocean water off the coasts of Oregon and Washington, aka “The Blob.” While a defining characteristic of a global El Niño is the immense areas of anomalous warm water covering the eastern Pacific, scientists don’t know what will happen when this warm water meets The Blob. These three points — small sample size, a history of El Niño’s affecting southern California more than Central and Northern California, and the unknown outcomes of the Battle of the Warm Water Blobs — are why many scientists are hedging their bets about the effects of this winter’s El Niño forecast.

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one comment:

Les Rowntree on August 26th, 2015 at 4:34 am

Here’s a fairly good ENSO article from this morning’s Washington Post

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