June 2, 2011

Ringing In the 2011 Hurricane Season

Filed under: Tropical — Erin @ 11:36 pm

I think it’s safe to say that I am joined by a substantial part of the Southeast and Midwest in bidding a very loud “GOOD RIDDANCE” to the 2011 spring severe weather season, even as a student meteorologist. This season took a toll on me in a way that I honestly did not expect. It is painful to watch such atmospheric carnage unfold when the career you have embarked upon, whether as a forecaster or a research scientist, is intended to minimize such tragedies. Even when it is fairly universally recognized that the disaster was not in any way the fault of meteorologists, that only leaves a sense of helplessness. So yes, I am quite ready to say “good riddance” to the tornadoes, at least in this particular corner of the world, for a few months. The Southeast, along with much of the rest of the East, is embroiled in a heat wave at the moment, a fairly sure sign that a summer pattern has taken hold. On that cue, enter hurricane season. Hurricanes were my first atmospheric “love” and even the season of 2005 did not change this for me. It is with a sense of excitement that I start opening up my tropical web browser bookmarks regularly again.

I don’t see a lot of point in making a specific numerical forecast for this year’s hurricane season. Suffice it to say that my best guesses that I formed in winter are unchanged, and that I expect an active season with a higher-than-average chance of American strikes, unlike last season. I am not expecting a transition to El Niño, which would tamp down activity in the Atlantic, but I am not entirely sold on the expectation that the ENSO state will remain neutral throughout the season.  I think there are close to even odds that it will begin to return to La Niña conditions again by autumn, albeit milder than those of last winter.  However, either pattern will promote tropical activity.

Did I mention checking tropics-related web bookmarks once more?  Well, it turns out that the Atlantic basin is following the “official” calendar right on schedule, so I have reason to look at the tropics regularly already.  There is a disturbance in the Caribbean Sea that is the first really interesting possibility for tropical development.

Here is an image of how this disturbance looked at 11:00 PM Thursday night:

For reference, here is a true visible satellite image of the same disturbance a few hours earlier, which shows the mid- to low-level circulation of this system better:

There are two “blobs” in the Caribbean, but the first image makes it clear that the one to watch is the one closer to Central America. That is the one that, according to loops of visible satellite images, has visible rotation occurring, and it is analyzed as a low pressure center in official maps:

The area of convection to its east is associated with a tropical wave that is expected to merge with the low, adding energy and moisture to the brew.

This system is interesting for so early in the season because it has some atmospheric variables in its favor despite the calendar. The low has been developing low-level convergence (winds drawing together) and upper-level divergence, both of which are conducive for tropical cyclogenesis, though these areas of convergence and divergence need to become better aligned with each other:

As the images indicate, the area of convergence is the chief culprit in the misalignment. The divergence is occurring above the area of convection, indicating that the low is developing a system for ventilating itself.

An analysis of vorticity shows that the system has positive vorticity at the 850 and 700 mb levels and that the two levels are basically aligned, which in a tropical cyclone (or proto-cyclone) is positive for development:


850 mb vorticity

700 mb vorticity

Shear is all right above the system but not especially favorable in the surrounding environment:

This is no surprise for this early in the season, but the National Hurricane Center expects the environment to become more favorable for this system in the next couple of days. An examination of the GFS model indicates that they are probably correct in this expectation; though shear is expected to be prohibitive of any tropical activity in the Gulf of Mexico, it is supposed to lighten up around the low pressure center.

Incidentally, the GFS doesn’t seem to do much with this cyclone other than letting it churn in place. Don’t expect a hurricane out of this! At best, I’d say it might rate a tropical depression. It is primarily an interesting feature to watch for so early in the season, a harmless storm that we tornado-weary weather folk can observe without anxiety. Tropical cyclogenesis is a fascinating, somewhat mysterious, and awe-inspiring phenomenon, and instances like this that are not the classic “Cape Verde wave in September starts to spin in the middle of the Atlantic” pattern are particularly interesting because the process of genesis for them is not cut-and-dry like the well-known central Atlantic tropical wave process is. This system may very well be a harbinger, but that remains to be seen. For now, it’s a neat feature to watch.

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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!

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July 2, 2009

Global Warming Now Messing with El Nino?

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

Possibly.

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.

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