What the EL?

| posted on July 22, 2010

By Chris Dixon Correspondent

On Friday, with the latest round of biblical rainfall bearing down on Southern California and Hawaii, I rang up Dan Cayan, director of the Climate Research Division of the Scripps Institution in San Diego. The question? What the EL’s going on with the weather? Dr. Cayan has one of the better grasps of Pacific climatology than anyone on the planet. Yet even he admitted to being puzzled about the intensity of the storms that have lately been soaking Hawaii and California. Why is the weather so bad? Why is it thundering in Laguna Beach as I type? And what’s to come? The good doctor has some interesting answers…

Chris Dixon: I think our readers would be interested to hear your perspective on the stormy weather we’ve been having on the west coast for the last month. Specifically, it seems like most of us knew we were looking at a weak to moderate El Nio this year, but it seems like stormwise, this year thus far has been a repeat of, say, ’97.

Dan Cayan: In terms of precipitation, yes, but in terms of the kinds of storms we’re having, its not at all the same as 97-98. These storms have been systems that have been confined to the far eastern North Pacific. They’ve generally dropped out of the Gulf of Alaska and really haven’t had the broad North Pacific extent of storms that we’ve seen in other El Nio years. Those years, we have a real strong stream of westerly winds, or the jet stream that makes its way all the way across the Pacific. In this case, the Jet Stream is much more loopy and in the central Pacific it’s making a real northward loop and then it loops down onto the West Coast. Within that West Coast trough, we’ve had this persistent, current storminess.

We actually saw the first spell of that in October, and then of course, we’ve had interludes of storms. This storm we’re having now is another of the same type.

CD: So these storms are sort of coming out of the Gulf of Alaska and then diving southward, rather than taking a line straight across the Pacific.

DC: Right and I believe that has an implication on the kind of waves that are seen in California beaches.

CD: It seems that way. As stormy as the weather has been, we haven’t had much giant surf. We have had a couple of good swells, and there was one that really lit up Hawaii and California in mid December. It seems like we’ve had the weather of a big El Nio, but not necessarily the persistent big waves.

DC: Yeah. Well, that’s attributed to the fact that you don’t have the really long fetch of winds that impart their energy into the really large swells. The waves that we are getting are a bit shorter period and maybe not quite as energetic as the ones that occurred in 97-98.

CD: There were storms that literally spanned the entire Pacific in 97-98.

DC: Right. this winter so far has really not had that really deep Aleutian low kind of character that we’ve seen in El Nios in 97-98 and 82-83 and a number of others.
This is a bit of an odd case. There just really does not seem to be the really strong tropical influence on the North Pacific circulation. (he chuckles) I don’t exactly know why we’re getting so much storminess on the West Coast, which is another question. And even though we’re getting storms in the South, if you look at the North — Washington and the interior Northwest, they have not built up the snowpack and water supply this winter that we have from essentially California southward. In that respect we’re similar to the precipitation pattern that one expects out of a more typical El Nio, but when you look at the storms that cause this precipitation activity, they just do not follow the El Nio kind of profile.

CD: Hawaii just got slammed by a big storm and an extended period of really heavy rain and they’re expecting another. I’m wondering if this is tied into what I read was a large pool of warm water above Hawaii.

DC: I’d say that whatever is happening is happening because of larger scale influences. In this case there is some kind of global pattern that’s been setting up this trough off the west coast — and the peculiar form of this El Nio, where the warmer water along the tropics is actually stationed way far west from about probably 150 degrees west across the dateline almost to New Guinea. The more sort of classical El Nio has warm water from about 150 degrees west to the South American Coast — eastward. So there are patches of kind of modest warm water right along the tropics, but nothing massive at all. We’re seeing something that has resemblance to an El Nio, but in many ways it’s different.

CD: I’ve always read that more sea surface heat gives more fuel to storms. Are we looking at higher sea surface temperatures almost maybe across the whole north Pacific?

DC: Well, if you looked at everything with sort of a coarse lens, it’s on the warm side. The Atlantic is warm, and there are patches across the north Pacific right around Japan. Then there’s this major area to the northeast of Hawaii. So it’s probably a symptom of what the atmosphere has been doing. There may be some subtle feedbacks where it is helping the atmosphere find this favorable spot for a trough. And one of the real mysteries is, each winter seems to have kind of its own identity. In this case, we’re getting kind of these similar storm patterns where this western North America, East Pacific trough keeps setting up. It keeps popping back up. Even after lulls. Right before Christmas you know, we had this beautiful Santa Ana weather.

CD: And that coincided with a good swell.

DC: Right. It looked like it was going to be dry for awhile, and then all the sudden, around the 27th or 28th, this low developed again and it developed even stronger than the forecast was portraying. That’s hung in for the last week or so. It looks like it’s going to be here through Tuesday or so, if I read the present weather charts correctly.

So this pattern seems to be kind of a fingerprint of this particular winter. If you look back at October, we had kind of a similar spell where we had that early rain. It was quite remarkable in fact, and was one of the wetter Octobers we’ve had if you look at the historical record.

CD: When that October storminess happened did you think it was just an anomaly?

DC: Well, usually the very early part of the winter doesn’t connect too well with the conditions of January to February. February is a period climatologically where the westerlies are strongest and extended most to the south. It’s conceivable that we’ll get some breakthroughs of the westerly wind systems and you could get some really massive North Pacific swells that are coming across quite an expanse of longitudes. But so far what we’ve seen has been this sort of odd pattern where everything is sort of confined to the easter quarter of the North Pacific.

CD: In terms of the crystal ball. What have been the discussions among you and your colleagues about how the rest of this winter might play out weather wise?

DC: Well, it’s a weak El Nio, and it’s an odd one. I think the pronouncements by and large have not been nearly as resolved, and unfortunately, they lack consensus. It’s kind of a crapshoot as to what happens the rest of the winter I’m sorry to say. Because of the weakness in the tropical Pacific forcing, it’s harder to say what’ s going to happen.

CD: And that goes for the surf too?

DC: Climatologically, February is the period when the westerlies are the strongest across the North Pacific. It means that the winds across the surface across the middle latitudes of the North Pacific that would generate large swell, the strongest likelihood will be between now and the end of February.

CD: Will the storms last past February?

DC: Well, we’re still in the core of the storm season, but El Nio’s tend to increase length of the storm season across the North Pacific.

CD: I remember in 97, there was new snow on Big Bear in May. Speaking of that, what about the ski season? I hear that Mammoth and Tahoe are looking at over a hundred inches just out of this storm.

DC: I would be surprised if we see this flurry of storms continuing. Again, this has just really been unusual – the sort of one after another impulses that are coming in off this activated Gulf of Alaska trough. I think the signs are pretty good for snow. I’d be optimistic that this will continue to be a good winter for skiing. It’s not been at all good in the northwest, but in the latitudes from Northern California southward, the snow courses are showing good snow amounts.

CD: How hard is in this day of supercomputers and modeling and all that to really say, this is what the oceans and the weather are going to do in the next week or so?

DC: I’d rather be in California than Colorado forecasting. Colorado has so many different ingredients that result in storms. Subtle changes in wind direction can mean the difference between upslope and not. They can get sources of moisture from the east and west. It’s very complicated. The West Coast is actually not so bad. There’s pretty good intelligence across the North Pacific, and while models tend to fall apart within three to five days, they’re not so bad. So the forecasts even though there are certainly foibles, but I’d say forecast skills in the far west are pretty good.

CD: So would the advice for surfers during the next few weeks be to head for the hills?

DC: Well, you know it was interesting. Scipps Pier the day before yesterday looked outstanding. The waves weren’t huge but they were really well shaped. So we may not be getting the really large storms but at least there’s been storm activity out there. I suspect we’re going to see good interludes — so I wouldn’t get too depressed yet.