![]() So in this region, a declaration of El Niño versus ENSO-neutral does not historically mean a large change in rainfall probabilities. It is only during La Niña (when the tropical Pacific is colder-than-average) when the number of wetter years diminishes. Neutral and El Niño years in this region both show a large range of possible outcomes, which includes some pretty wet years (2). In other words, the onset of El Niño might not lead to a strong increase of wintertime precipitation compared to Neutral years. One interesting aspect is that rainfall possibilities during neutral and El Niño are somewhat similar. This holds true for areas that show even stronger relationships between SST anomaly and rainfall, such as southern California (Figure 2a). Thus, we can conclude that while El Niño might in general lead to higher rainfall totals, there certainly is no 100% guarantee. But from looking at Figure 2b, we know that there is a lot of variability in the historical record. Yes, if we took an average of all the El Niño, La Niña and Neutral years together, we would discover that, on average, El Niño years meant higher rainfall totals. and neutral years all over the place (1). In fact, the relationship in the San Joaquin Valley can best be described as “kinda messy.” There are El Niño “years” with lighter amounts of rain. Not surprisingly, the two regions have historically seen different impacts from El Niño. Can the past give us a hint at what the future may hold? The black line represents the 2014 August-October ONI value. Red dots show ONI values greater than 0.5 (El Niño), blue dots show values less than -0.5 (La Niña), and black dots are neutral ONI values. Part (a) is Climate Division 91, San Joaquin Valley, while (b) is Division 93, the southern California coast. Total precipitation for December-March (vertical axis) compared to Oceanic Niño Index (ONI) values in August-October (horizontal axis). ![]() The dashed line points out the value of the August-October 2014 ENSO index value (called the Oceanic Niño Index).įigure 2. Part (a) shows conditions across southern California and part (b) shows the San Joaquin Valley, where most of the agriculture in California is grown. ![]() (Although personally, I think part (b) looks like a bunny.) Figure 2 shows how, in the past, sea surface temperature (SST) anomalies during the August – October (ASO) time frame were related to the subsequent December-March precipitation totals for certain regions in California. And no, Figure 2 is not a Rorschach test. To show this, instead of giving you probabilities and numbers, I am going to let your eyes tell you the story. The second issue is that, as mentioned previously on the blog, no two El Niño events are the same, and thus their impacts aren’t either. You wouldn’t expect the climate to be the same in northern and southern California, so you shouldn’t expect the impacts of El Niño to be the same either. The first issue is that California is a BIG place. So you may wonder, is it a slam dunk that a developing El Niño causes above-average rain for California? And finally we have discussed exactly what we mean when we use probabilities in these types of forecasts. We have even discussed what the upcoming winter forecast will be for the United States. We have previously touched on the large-scale impacts an El Niño could have on the United States, namely shifting storm tracks around North America, potentially resulting in an abundance of rainfall for parts of California. The issue since drought first arrived back in December 2011 is that Mother Nature missed this memo. Drought Monitor.ĭuring a normal year in California, rainfall picks up in intensity throughout the late fall into winter. Map by NOAA, based on data provided by the U.S. Currently, 99.72% of the state is under drought with 55.08% of California experiencing “D4,” or Exceptional Drought. Drought conditions as of November 18, 2014.
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