November 20, 2010

From La Nina to What?

Filed under: Oscillations — Erin @ 9:03 pm

We are currently in a La Nina episode of moderate strength. La Nina is the stage of the El Nino-Southern Oscillation in which the equatorial Pacific is cooler than average:

La Nina

This episode has likely played a large part in the 2010 Atlantic hurricane season’s intensity and length. It probably also affected large-scale weather patterns during the intense drought over the American South (which still continues, despite rain, because have a significant rain deficit to make up.) La Nina is expected to continue through the winter. La Nina winter conditions for the Southeast are, in my opinion, wretched: Warmer and drier than normal. If we get a snow event, it’ll be in spite of the ENSO pattern. However, by no means should snow be ruled out; winter precipitation is determined by short-term synoptic-scale weather systems more than anything else. Indeed, our first freeze this year happened earlier than last year, which was an El Nino winter (El Nino winters for the Southeast are cooler and wetter than usual).

This La Nina event is following a pattern and timeline of strengthening most similar to the 1998/1999 La Nina event, as the official list of ENSO statuses indicates. That year was not known as a “wintry” winter for the South. There was a devastating ice storm in some areas, but the storm’s description makes it quite clear that this was an aberration that effectively defied the La Nina conditions that had been controlling the winter previously:

Much of the fall and early winter season of late 1998 had been very mild for southerners. But on Tuesday December 22nd, a strong surge of shallow Arctic air drove southward and spelled a quick end to the unseasonable warmth and helped set the stage for one of winter’s nasty tricks. A moist southwest flow above the cold air brought several upper level disturbances. This combination of cold air and moisture held together for several days and brought periods of freezing rain and sleet to the three state region, beginning Tuesday night and not ending until Christmas morning. Up to 2 inches of ice accumulated on power lines and much of the area experienced long power outages, nearly seven days in some cases. Hardest hit was an area from northeast Louisiana to north central Mississippi where tree and power line damage was moderate to severe. The prior warmth had left most trees with a little more foliage than usual and greater potential for ice accumulation.

(Please note that I am NOT forecasting an ice storm for the Southeast this winter! It is not possible to forecast an event like this far in advance.)

I have a growing suspicion that the most profound impact, or influence perhaps, of this year’s La Nina will actually be felt next year. There is a link between intense tornado seasons and a La Nina the previous winter. Some notorious examples are 2008 (the Super Tuesday tornado outbreak, among others), 1999 (the record F5 tornado in Oklahoma), 1974 (the unprecedented and historically unique Super Outbreak, which had either 6 or 7 F5 tornadoes), and 1971 (the most recent F5 tornado in Mississippi). The mechanics of this phenomenon are not well understood yet, and the statistical link is not as highly correlated as it might be. In fact, there may even be a lesser link between El Nino winters and an active tornado season. But there is a growing consensus in meteorology that La Nina winters tend to precede active springtime tornado seasons.

The long-range ENSO computer models are suggesting that this La Nina episode may go away by next year, leaving us in a slightly negative neutral state:

ENSO models

If this plays out, we should consider another long-term implication, the effect of the assumed demise of La Nina on next year’s hurricane season. From this far ahead in time, we have no skill at forecasting any other factor except the Atlantic multi-decadal oscillation that entered a warm phase in 1995, so the status of ENSO would play a large part in long-range hurricane season forecasts. From the current warm phase of the AMO, hurricane seasons in which a La Nina episode from the previous winter met its demise include 2008, 2001, 2000, and 1996. From the previous warm AMO phase, such years include 1968, 1963, 1957, and 1951. However, I do not think that these years from the 50s and 60s are that applicable to this year, because this year’s La Nina is already stronger than any of those were. A couple of them were quite shallow, in fact. And three of the years saw the development of an El Nino following the La Nina, an occurrence that right now seems unlikely.

Given the strength of our current La Nina already, I actually would not be that surprised if the computer models shifted to predict the continuation of La Nina through next winter. Some of them do, in fact. In any case, whether we shift to neutral by next summer or remain in a (weakened but reintensifying) La Nina, it seems that next year’s hurricane season is likely to be active just as this year’s was. 2011 has the potential of being quite a headline-generating weather year!

July 2, 2009

Global Warming Now Messing with El Nino?

Filed under: Climate change,Oscillations — Erin @ 1:37 pm


Scientists have recently discovered a positive relationship between an alternative type of El Nino, called El Nino Modoki, and Atlantic hurricane activity. The connection had largely slipped under the radar because the scientists in Japan and Korea who knew the most about Modoki did not connect it with Atlantic hurricanes. (It would just figure. This has happened before with respect to ENSO and hurricanes; the ENSO specialists didn’t work with the hurricane specialists, and so that connection wasn’t known for years either.) This type of El Nino involves warming in the central and/or western Pacific rather than the Pacific coast of South America, and it is associated with increased Atlantic hurricane activity and increased landfalls—unlike the traditional El Nino, which tends to suppress activity in this basin.

2004 (of Ivan infamy) was an El Nino Modoki year. 2006 was a traditional El Nino year. 2009… may be a split:

“We spent all last week trying to figure that out,” [Peter] Webster [meteorologist of Georgia Tech] said. ‘It looks like it might be a hybrid,” with warming starting in the east and them moving west, possibly meaning more hurricanes late in the season.

Webster speculates that the Modoki phenomenon may be caused by global warming. Then again, it may not be. I’m rather interested, in fact, in the year 1969, which was an El Nino year for most months but still had a highly active hurricane season, including record Hurricane Camille. General consensus is that we are just now seeing the effects of climate change in our weather, so 1969 may (or may not) be out of the window of opportunity for global warming to have had an effect on El Nino.

As far as that “increased late-season hurricane activity” is concerned, though… that’s about what I figured. I’ve been seeing parallels between the oceanic setup of this year and 2004, and that rather reinforces my belief.

January 8, 2007

The New, Improved, Super-charged El Niño

Filed under: Climate change,Oscillations — Erin @ 9:42 am

Also, African Rain Moves Westward?

UPDATED, 1/10/2007:
2006 was the warmest year on record, it was reported today, ahead of the previous title holder, 1998. 1998 was influenced heavily by the El Niño that began in 1997. And, sure enough, scientists have concluded that this year’s Niño was, effectively, super-charged by global warming. From the right-leaning Chicago Tribune:

In 1998, record high temperatures were driven by an unusually powerful El Nino current that disrupted weather patterns worldwide. The current El Nino, a periodic warming current that took shape last summer, is far weaker and has had only a moderate effect on global climate, several experts said.

“What we are seeing is much more than El Nino,” said climate analyst Kevin Trenberth at the National Center for Atmospheric Research in Boulder, Colo. “The overall pattern is consistent with our concepts of global warming.”

The original blog entry:
Part 1: El Niño 2006-07: Overachiever or Cheat?

As most people are aware, an El Niño formed in autumn of 2006 and has continued to this date. This event was basically single-handedly responsible for cutting off the 2006 hurricane season at the knees. El Niños do that — the last time we had one, it developed in late autumn of 2004 and ended a two-month streak of damaging hurricanes. And that one wasn’t even very strong. It was weak enough, in fact, that most articles in the popular press that talk about this year’s event don’t even refer to 2004; they say that the last El Niño was in 2002. The one this year is a moderate Niño.

This NOAA site shows the Oceanic Nino Index, an indicator of the temperature departure from the average for various three-month periods, going back to 1950. Positive values indicate El Niño-like conditions and negative values indicate La Niña. The site isn’t updated to reflect the new values, but they have increased past the levels of the 2004 Niño event.

The bottom line, though, is that this year’s event is not even close to the strength of the notorious 1997-1998 event. It is moderate. Mild.

Yet, during the most recent period of warmth for the Eastern United States, heat records were set in numerous eastern cities. The most common date for the old records?

Yep, 1997. There was a winter “heat wave” during the same time period during that year’s Niño as well. (Read more…)

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