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. If the developing El Niño event reaches a strong or very strong intensity and maintains its strength through winter 2015-2016, the odds of experiencing persistently wet conditions next winter will increase. The occurrence of frequent precipitation events during significant El Niño winters increases the probability that antecedent hydrological conditions will be moist if and when heavy precipitation events do occur, increasing the risk of flooding. Also, since the trajectory of Pacific storms during strong El Niño winters tends to be from a much lower (more southerly) latitude, air masses during rain events tend to be warmer and moister overall. This can have several effects, including higher snow lines and more rapid runoff, greater precipitation intensity overall, and an increased risk of deep moist convection (which can produce very high rainfall rates even in the absence of mountainous topography). It’s important to note that almost all of California’s major flood events result from landfalling “atmospheric rivers,” which tend to be more frequent (but not necessarily more intense) during El Niño years. Therefore, a major flood can easily occur during any winter, El Niño or not. Still, for upper-tier El Niño events, there is definitely an increased risk of above-average precipitation and flooding during the cool season. Since strong El Niño events increase the likelihood of wet California winters, it does stand to reason that a strong El Niño in 2015- 2016 could provide at least partial (and perhaps substantial) drought relief. A wet winter would most likely allow most of California’s major reservoirs to fill, though those who operate California’s dams and reservoirs are heavily constrained by flood control mandates. Surface soil moisture would increase, and drought-stressed forests and ecosystems would benefit substantially in the short term. Stress on urban water supplies would be reduced as demand decreases, and supply increases.

The latest dynamical model forecasts continue to suggest the potential for a very strong El Niño event in 2015. (CPC) But even a tremendous amount of water falling from the sky won’t completely alleviate all of California’s drought impacts. And if much of this hypothetical precipitation were to fall as rain rather than snow in the Sierra Nevada, longer-term water storage wouldn’t be boosted nearly as much as it would otherwise. California is currently witnessing firsthand what happens during its first year in recorded history without a measurable springtime snowpack, and it’s becoming quite clear that warming temperatures aren’t very compatible with the snowmelt-dependent water storage infrastructure currently in place. The Pacific Ocean, on the whole, remains extraordinarily warm (even in regions geographically far removed from those used to define El Niño), and is expected to remain so for the foreseeable future. This means that even if heavy precipitation does return to California next winter, temperatures will likely remain well above the long-term average. And just to reiterate a key point from above: we still don’t know for sure whether strong or very strong El Niño conditions will ultimately develop (nor whether they will persist until winter, when they are most relevant for California). Confidence is starting to increase in current projections, since we’re now emerging from the Spring Predictability Barrier and most dynamical models are still suggesting the potential for a powerful event. But when we concatenate all the various uncertainties discussed above, there’s still something of an open question regarding what happens in California next winter. At this point, it’s fair to state that the likelihood of experiencing a wetter-than-average winter (and, perhaps, flooding) is increasing, but simultaneously that the risk of the California drought continuing into 2016 is nearly 100%. Needless to say: it will probably be a very interesting year to come for weather and climate-watchers in the Golden State. Stay tuned!

http://www.scpr.org/news/2015/08/10/53627/el-nino-versus-the-blob-which-will-win-out-this-wi/ You may have heard a strong El Niño could bring lots of rain this winter, but there’s another weather pattern in the region that might keep things dry.

It’s a ridge of high-pressure air that’s created a huge patch of warm water off the coast, nicknamed “the Blob.” This ridge and blob tag-team is believed to be deflecting storms and keeping California locked in drought. Which pattern will win out? Let's size up the players. El Niño: The Wet Wonder

(This NOAA image shows warmer than average sea surface temperatures from November 1997 and July 2015. You can also see the warm temperatures off the West Coast in the 2015 image.)

Location: A stretch of the Pacific Ocean near the equator. Size: About six times the size of the continental United States. Temperature: 2 to 3 degrees Celsius above average. Age: Less than a year old (researcher have been watching it since December of 2014).

Background: An El Niño occurs when trade winds across a large part of the Pacific Ocean die down. This allows the water there to warm up more than average. Since the Pacific covers a third of the globe, changes there affect weather around the world. For instance, a strong El Niño typically alters the flow of a jet stream that carries storms across the Pacific from the tropics to the U.S., said researcher Nate Mantua with the National Oceanic and Atmospheric Administration. In most winters that storm track is aimed at the Pacific Northwest, supplying the region with months of wet weather. "During the El Niño year, it tends to alter the storm track in a way that makes it much more frequently pointed at California," said Mantua. That happened during the 1997-98 El Niño when parts of the state saw double the average rainfall, leading to floods and mudslides that caused $550 million in damages. This year’s El Niño could be even stronger, says Bill Patzert, a climate researcher with NASA's Jet Propulsion Laboratory. "The intensity of this El Niño is larger than anything at this point than I have seen in my career," said Patzert. But he cautioned, the trade winds could kick back up and cool the water down, resulting in a weak El Niño and not as much rain. Still, at the moment all signs are pointing to a monster weather pattern this winter.

The Buoyant Blob

(Map showing the sea surface temperature (SST) anomaly spread along the West Coast as of March 2015. Image via NOAA/ESRL Physical Sciences Division at Boulder, Colorado.) Location: The Gulf of Alaska and stretching down to Mexico. Size: Stretches 2,000 miles down the coast and more than 1,000 miles out from the West Coast. Temperature: 2 to 3 degrees Celsius above average. Age: 1 year old (it was first noticed in 2014). University of Washington meteorologist Nick Bond coined the name “the Blob” during an interview last year. "I was remarking upon the unusually warm waters off the coast in an unusual pattern and I referred to it as the Blob and the name kind of caught on," he explained. The Blob is likely a result of a ridge of high-pressure air, sometimes called the Ridiculously Resilient Ridge, that has stayed parked above the North Pacific on and off for the past couple years. That ridge deflected winter storms away from the West Coast, keeping things dry on land. It also blocked the strong winds that usually cool the ocean over the winter, said Bond. "And so less heat was leaving the ocean. Also the weaker winds meant less stirring of the upper part of the ocean and kind of mixing up of colder water from below." All of this allowed the ocean off the West Coast to warm up, resulting in a Blob of warm water that eventually stretched from Alaska to Mexico.

The Match Up Researchers say there’s no precedent for something like the Ridiculously Resilient Ridge and the Blob interacting with an El Niño. It’s not clear which one will win out, but here are some scenarios. 1. EL NIÑO DOMINATES. Sure, the Ridge and Blob are big, but El Niño is bigger. It is known to have a much larger impact on weather around the world. In fact, Stanford researcher Daniel Swain thinks El Niño will likely mess with the climate patterns that have kept the Ridge and Blob in place for this long. They'll break apart and the entire West Coast could get a good soak. 2. THE BLOB AND RIDGE HOLD STRONG. Another possibility is El Niño brings the rain, but the Ridge and Blob stay put over the Northern Pacific, deflecting storms. In this scenario, Swain says Southern California could see a wet winter but there would be less precipitation in the north, where many vital reservoirs are. "If indeed this ridge and this warm water mass persist that could have an opposing effect of a wetting effect of a strong El Niño event," he explained. This outcome is more of a wild-card, Swain said. He thinks the most likely result is that El Niño trumps the Ridge and Blob this winter. 3. EL NIÑO AND THE BLOB TEAM UP. There's a chance these patterns could work together, said Nate Mantua with NOAA. The Blob is made of warm water, which easily evaporates, he explained. That means the Blob could end up pumping more moisture into the air and fueling rain clouds as they pass over the ocean on their way to land. "So you’d expect this kind of boost from exceptionally warm ocean to the strength of the storms and the amount of rainfall that comes out of them," Mantua said. Luckily, you won’t need Pay Per View to find out who wins this showdown, just tune into your local weather forecast this winter.