19 July, 2015

Dolores Deluge: Rare July Precipitation Event with an El Niño Fueled Tropical Connection

19 July 2015. For the past two days, conditions have seemingly gone topsy-turvy for some here in Southern California during what is typically the driest month of the year. An Angels game had to be cancelled due to "inclement weather" for the first time since 1995. Two formerly raging wildfires, including one that scorched 20 cars on the 15 freeway, are now 100% contained thanks to extremely high humidity and rare July rainfall. People in an assisted living community had to evacuate, not due to fire, but due to flash flooding ― and this is in July, when average annual precipitation is only 0.02 of an inch. People have been quick to directly blame El Niño, but in actuality, it's really only indirectly related.

Hurricane Dolores as a Category 4 storm Wednesday evening, hammering Socorro Island. Eventually, after dissipating over cooler waters, this system shot a plume of moisture up the coast as a tropical storm, then made landfall in SoCal as a remnant low
The real source of this rare July bonus moisture was, yes, that's right, former Category 4 Hurricane Dolores. On Wednesday, Socorro Island, a volcanic island about 200 miles southwest of Cabo San Lucas that contains a Mexican naval base, got hammered by sustained winds clocking in at 130mph, coupled with a 15-foot storm surge and horizontal rain, all from this beast. After that, the storm began to move into cooler waters and, naturally, weaken... ah, but slowly. Dolores remained a weak tropical storm as far north as Vizcaíno ― rare for July ― and produced tropical storm force winds even after becoming post-tropical, as far north as San Clemente Island. The result? A boatload of tropical moisture streaming over Southern California during what is usually the driest month of the year.

El Niño years tend to make this more likely to happen, for several reasons. One is the weakening and/or reversal of the trade winds. Normally, they blow from east to west ― that is typically why hurricanes also move in that direction. When the trades weaken or reverse, westward movement slows. Second is the large-scale collapse of blocking patterns that typically dominate over much of the North Pacific during the summer months. This allows low pressure systems to form in the North Pacific even during the dry season ― troughs that can grab tropical cyclones and pull them north. Third, with the resulting overall lack of upwelling, waters immediately off the California and South American coasts become much warmer than normal, giving tropical cyclones more overall fuel that can sustain them further from the tropics than usual. All of these factors put together can cause some rather interesting effects as the hurricane season in the eastern Pacific basin (which happens to be the very source of the wind shear that suppresses Atlantic activity) rolls on up.

Although this kind of situation is definitely the first of its kind for July in the known historical record, it's not the first of its kind period. In September 1997, for example, moisture from Hurricane Linda ― which currently holds the record for strongest in Eastern Pacific history, although probably not for long ― streamed across California, causing torrential rains and even hail the size of golf balls in some locations. That same year, moisture from the much weaker Hurricane Nora also managed to cause some interesting totals, especially in the Inland Empire, where flooding was rampant. Going further back into history, one of these eastern Pacific behemoths made landfall in Long Beach as a strong tropical storm back in 1939 ― also an El Niño year ― and even further back, in 1858 — again, El Niño — a Category 1 hurricane brought 85mph sustained winds and 10 feet of storm surge to San Diego.

Given how many impacts we've had already ― heck, even way back in May and early June we had some remnant moisture from Hurricane Blanca as well ― it shudders me to think of possible impacts later in this season, including possible repeats of the 1939 and/or 1858 events, given that 2015 accumulated cyclone energy is already ahead of 1997 levels. Although, I for one would definitely take a direct hit from a tropical cyclone as an added bonus on top of already extreme winter El Niño impacts over this drought any day… catch-22, I guess. These are definitely exciting times indeed.

06 July, 2015

July 2015 ENSO update: Equatorial anomalies, WWB's continue to ramp up

If I haven't been posting much to this blog in recent weeks/months, I apologize. Part of the reason has been my exceptionally high Twitter activity… ah, and activity there tends to be a distraction. Anyhow, I've been using a myriad of tools to track this pending El Niño event – everything from retweets, to WWB time-lon forecasts, to surface current anomalies, to observed SST anomalies, to SST anomaly forecasts, and all of them are beyond impressive.

SST anomalies: Exceptionally impressive to say the least

My last update (in May) showed a marginally warm strip along the equator. Now, however, it's July. What do we have here? Well…

Compare that to May, and clearly it's a sign that this event is, hands-down, the strongest since 1997. Do SST anomalies alone tell the whole story? Of course not, but it goes to show just how impressive this event is, with more WWB's and downwelling Kelvin waves (next paragraphs) on the way. What makes this map clearly differ from 2014 (especially) is the Banda Sea cold pool: it forces high pressure over Indonesia, thus keeping the atmospheric response locked in place.

Westerly trades: Cross-equatorial tropical cyclones, redux

You may recall that what initially kickstarted this event was a pair of tropical cyclones on both sides of the equator at the same longitude back in March: Cyclone Pam (yes, that's right, that monster, the one that ended up being a direct hit on Vanuatu, completely obliterating heavily populated portions of the island) on one side of the equator, and Tropical Storm Bavi (which never made it to typhoon status) on the other. Fast-forward to July 1 Australian time (technically late June 30 in California) and that exact same thing happened again: TS Chan-hom on one side of the equator, Cyclone Raquel (also a TS when the Saffir-Simpson Scale is applied) on the other. Although Cyclone Raquel was clearly weaker than Pam, it was still paired with another cyclone on the opposite side of the equator. When this occurs, it's like a WWB pitching machine: winds rotate counterclockwise north of the equator, clockwise south of it, and between the two, winds have only one way to blow: from W. Here:

As you can clearly see, what we're looking at is easily the most powerful westerly wind burst since March, and moreover, when Raquel dissipated, the Southern Hemisphere Booster followed right behind. Now, there's a pressure gradient of high in W, low in E, which can keep that WWB progressing further E. In ~5 days, this westerly wind burst could reach the far E Pacific, where more hurricanes (starting with Dolores) should form. For a review: the word "typhoon" is only used W of the date line; E of it, they're still hurricanes.

Kelvin waves: 3 and counting

You may recall that the April/May Kelvin wave was set off by the westerly wind burst induced by the Pam/Bavi cross-equatorial pair. However, the May westerly wind burst set off a second downwelling Kelvin wave. While the Kelvin wave in April only contained small patches of +6°C anomalies at depth, this one brought with it anomalies at depth of +6°C across the board, with patchy +7°C T-Depth anomalies. Then, Chan-hom and Raquel pitched in, and the result was a third Kelvin wave. Although it doesn't look too impressive at the moment, it's very fast-moving: in just a matter of, like, 3 days, it's gone from 165°E to the date line, and the WWB that spawned it continues to move east as well. On top of that, there's now a strong MJO superimposed on top of the Niño signal, adding to those westerly anomalies, and as mentioned above, there's also anomalous cooling of the Banda Sea helping to lock that signal in place.


So, we've got everything coupled… it's just a waiting game now. Let's see how strong this event gets, shall we? It would definitely mean the world to us in CA, especially in conjunction with cooling AMO, since cool Atlantic in general tends to want to shift the storm track south, and with the Hudson Bay now also heating up with warm anomalies, blocking should reposition over Canada… everything looks to be coming together. Everyone, this is going to be a wild ride.