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By Daniel Swain: (19/Sep/15) By Daniel Swain: http://www.weatherwest.com/ (19/Sep/15) El Niño: still extremely impressive, and not done strengthening quite yet The strongest El Niño event since at least 1997-1998 continues to intensify in the tropical Pacific. A very impressive, classic El Nino signature exists in recent sea surface temperature anomaly plots. (NOAA Coral Reef Watch) The latest oceanic Kelvin wave has finally reached the coast of South America, and very rapid near-shore ocean warming has occurred over the past week. Nino 3.4 region sea surface temperature anomalies have now reached their highest August/September values ever recorded, and a new westerly wind burst appears to be ongoing in the West Pacific. All observational signs point to further strengthening of an event that is already in the top three since at least 1950. The present event is still expected to peak (in terms of maximum tropical ocean temperature anomalies) sometime between the late autumn and early winter months. Maximum effects upon California precipitation probably won’t occur until the January-March period, however, which is during the peak of our typical rainy season. Moreover, the September update of both the North American and International Multi-Model ensembles continues to suggests a high likelihood of a wet January-March period during the coming winter. Well above average temperatures are also expected to persist through most of the winter for most of California, which may have significant implications for lower-elevation snowpack. Weekly Nino 3.4 anomalies have reached all-time record high values for Aug./Sept. (NOAA CPC) It does not necessarily appear, however, that California will see an early start to the rainy season. Seasonal forecast models continue to suggest that November and perhaps early December will be drier and much warmer than average. This forecast has been fairly consistent in recent months, and suggests that our drought-worsened fire season may last for at least a couple more months in the north. One exception appears to be in the southern third of the state, which will have received three rounds of very impressive, drenching rains so far this summer by the end of the coming week. These intense tropical downpours are very likely linked to the extreme warmth of the Pacific Ocean near and south of the Southern California Bight, where water temperatures have recently approached an astonishing and record-breaking 80 degrees F! Fire risk in the far southern part of the state will still be greatly elevated during any Santa Ana wind events that develop, but recent and ongoing rains will partially mitigate the regional fire danger in the south. By Daniel Swain: http://www.weatherwest.com/ (03/Jun/2015) Strength matters: not all El Niño events have same effect in California, but watch out for the big ones. There is currently quite a bit of excitement surrounding current expectations of a “significant” El Niño in 2015-2016, especially given California’s extraordinary multi-year drought and model forecasts suggesting the potential for a particularly strong El Niño event. I’ve seen a number of rather hyperbolic (and seemingly mutually exclusive!) news headlines suggesting, that either California is headed for an epic drought-ending flood disaster or that El Niño cannot (and will not) provide drought relief since such the vagaries of the tropical East Pacific are essentially irrelevant to precipitation here. As the astute reader might have guessed, neither of these headlines is particularly accurate–but the reality is far more interesting than the dichotomy above would suggest. California, of course, receives the vast majority of precipitation during the cool season (the late fall through early spring months, or November through April). During most years, the months of December though March are most critical–and these are also the month during which most of California’s major floods have occurred. The fact that much of California has a well-defined, rarely-broken summer dry season means that a wet winter (and perhaps spring) will make or break the entire year–and also highlights the fact that any oceanic phenomenon (like El Niño) that might have the potential to influence precipitation on the annual scale would have to be acting during the cool season months. Thus, if we want to examine El Niño’s influence on California, it makes sense to focus on the core rainy season months of December through March. That’s not to say that El Niño can’t or won’t influence the risk of relatively rare summer rainfall events in the Golden State (I had a post focusing on this topic last year), but from the perspective of California drought relief, warm-season precipitation is highly unlikely to play a big role even during a strong El Niño year. Weak El Niño events have highly variable effect; strong El Niño events usually increase winter precipitation Note: for the purposes of this discussion, I have binned “weak,” “moderate,” “strong,” and “very strong” El Niño events according to Jan Null’s definitions, which rely on the Oceanic Niño Index. El Niño has achieved broad popular recognition in California, largely a result of the substantial impacts associated with two exceptionally wet winters linked to the strongest El Niño events in the historical record (1982-1983 and 1997-1998). When discussed in the media, however, the conversation usually centers around the categorical question surrounding the effects of “El Niño” vs. “no El Niño.” As it turns out, this binary framing not a particularly useful way to structure the discussion of El Niño’s impacts in this part of the world. Why is this the case? El Niño, on the whole, represents a weakening or reversal of the prevailing Walker Circulation, with typical east-to-west trade winds weakening or even becoming west-to-east winds (see this post from last year for more details). Because El Niño is defined so broadly, it actually encompasses a fairly wide range of atmospheric states– including a complete reversal of the direction of tropical wind patterns on seasonal timescales! It’s not all that surprising, then, that the atmospheric teleconnections associated with an El Niño event depend very much on the actual strength of the event. For weak events, the average atmospheric state over the Pacific Ocean isn’t all that well-defined, and historically California has experienced a very wide range of outcomes (including both very dry and very wet winters, along with everything in between). In general, El Niño brings about a strengthening of the subtropical (low-latitude) branch of the jet stream, typically at the expense of the polar (mid-latitude) branch. California usually depends on undulations in the polar jet to bring periodic storminess in the winter months, though even during average winters the subtropical jet does occasionally make an appearance. During weak El Niño events, this effect is less profound, and the end result can often be relatively weak versions of both the subtropical and subpolar jet vying for influence over the East Pacific. The net effect can be quite variable; if California’s lucky, we see moist storms originating from both regions, but if we’re unlucky, we can largely miss out on storm systems taking both trajectories. If we composite the most recent weak El Niño episodes, the average effect in California actually appears to be a slight drying during the winter months–directly contrary to the El Niño mythology that pervades the Golden State. However, things are a quite a bit different during a strong El Niño event. When East Pacific sea surface temperatures become sufficiently warm, large-scale atmospheric temperature differences between the tropics and the mid-latitudes are big enough to strengthen the subtropical jet quite substantially over the portion of the East Pacific that is most relevant for California wintertime precipitation. This enhanced subtropical jet can greatly enhance the strength of low-latitude storms west and even slightly south of California, and also makes it easier for such systems to tap into the rich tropical and subtropical atmospheric moisture reservoir that exists at lower latitudes. Additionally, storms during strong El Niño years have the potential to be more convectively unstable due to increased lower-atmospheric temperature and moisture, leading to an increased likelihood of intense localized downpours. In other words: a strong El Niño event tends to result in a jet stream structure that 1) steers more storms toward Southern California, 2) is favorable for stronger storms at a lower latitude in the East Pacific, and 3) affords pre-existing storms greater potential access to warm, moisture-rich air masses. Will El Niño end California’s extraordinary, multi-year drought during Winter 2015- 2016? Almost certainly not. Over the past four years of very low precipitation and record-shattering warmth, truly enormous water deficits have accumulated throughout California. On a statewide basis, the Golden State would need to see substantially more than an entire year’s worth of extra precipitation fall to eliminate the long-term deficit in a single year (in other words, a year with much greater than 200% of average). Since California’s all-time wettest years (typically associated with very strong El Niño events) have historically involved a doubling (200%) or less of annual precipitation, California would probably need to experience its wettest year on record (by a fairly wide margin) to erase ongoing deficits in a single year. While it’s not physically impossible, that would be a very tall order, indeed. And a winter like that would most likely bring a whole host of other problems (see below). Could a strong or very strong El Niño in 2015-2016 substantially mitigate the California drought and/or lead to serious flooding? Absolutely.
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