What that article about El Niño solving California’s drought problem obscures

One of my Middle School teachers read an article about El Niño and the California drought. She shared it with me, asking for my insights as a graduate student in Princeton University’s Atmospheric and Oceanic Sciences Program.

Once my response reached page-length, a friend said I should share it more widely, which brings me here.

What the Accuweather article gets right is that there is currently an El Niño, which is expected (>90% chance, Climate Prediction Center) to persist into the winter, and possibly even the spring (>80% chance). Since ENSO (the El Niño Southern Oscillation) is one of the dominant drivers of interannual variability in the global climate system, the current El Niño is expected to have global consequences, including possibly in the western United States. Past El Niño events have been correlated with increased rainfall in southern California.

What the Accuweather article obscures is the statistical uncertainty of the entire business. There’s a reason for that: as with much sensationalistic reporting of weather online, which is incentivized by click-based ad revenue, the article overemphasizes unlikely extreme possibilities.

In particular, the article neglects to mention that the subtropical jet stream is highly variable on a weekly basis, in both latitude and intensity. Thus, I find the graphic of it flowing directly over Los Angeles (the second largest city in the US, and one currently experiencing extreme drought) to be sensationalistic. It would be wonderful if an El Niño event helped California recharge some of its reservoirs, but given the uncertainties in ENSO and the spatially and temporally inhomogeneous nature of precipitation, it would be unwise for hydrological planners to bank on it.

The current El Niño could be one of the strongest in the past 50 years, but with a sample size of only 17 El Niño events in the past 50 years, any new event can possibly set a new record. There is large support from statistical and dynamical models that suggest this year’s El Niño may indeed be strong. However, I caution that a couple years ago some of the same forecast models predicted a moderate-to-strong El Niño that ended up not panning out. It is foolhardy to selectively consult only the most dire model runs.

There are large differences among the forecasts of different ENSO models. Only 4 of the 25 forecast models predict that the current El Niño will break the record set by the 1997/98 El Niño, which clocked in at a 2.4ºC SST anomaly in the Nino3.4 region. Source: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf

I studied conceptual models of ENSO in two courses at Cornell University — Climate Dynamics and Tropical Meteorology. Through projects in those courses, I concluded that meteorologists know a lot about ENSO, but still have a lot to learn. Our understanding of El Niño is developing on multiple fronts: mathematicians are modeling the basic patterns that underly ENSO in one and two variables at the same time that meteorologists operate fully-coupled earth system models in tens of thousands of variables. Each group continues to sharpen our overall understanding of ENSO.

The Accuweather article refers to past rainfall records to suggest that California may receive much-needed rain and snow. It posits that the current El Niño may break records. However, as in this event and all others, there could overwhelming — or underwhelming — surprises in store. One thing I am very confident about is that humans have not yet witnessed the full range of Tropical Pacific variability. Hopefully, we will learn from this event to better forecast — and communicate our forecasts for — the next El Niño.

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