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Thread: What happened to LA Nina?

  1. #1
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    Default What happened to LA Nina?

    I guess you can never tell how the the winter will turn out here in Oklahoma but what happened to our LA NINA pattern I thought we would not see a whole lot of arctic cold snaps here much less all this snow here lately. What is going on with our weather in the winters the last two years? Makes me wonder if the weather patterns are totally changing. I mean this snow may be on the ground for weeks. And a possible major winter storm next week? How much more can we take? When will it end? Any thoughts?

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    Administrator Team Jeff Snyder's Avatar
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    Brad -- if it means anything, only this week seems to have been a deviation from the norm. Before Tuesday, we had seen very little precip in the southern plains (severe drought in parts of central Oklahoma), which, AFAIK, tends to be a characteristic of La Nina winters. A week or two of winter precip and cold temps in Oklahoma, even during a La Nina, isn't terribly surprising, though. The global circulation is very complex, and there are bound to be times when the local weather to deviates significantly from the ENSO-indicated means (e.g. southern plains weather differing from the average experienced during a La Nina). In other words, ENSO events tend to favor a global circulation (e.g. orientation of the jet streams, etc.) that makes certain "types of weather" more or less likely in various parts of the world, but there's no guarantee that these conditions will occur without break. This is all to say that I wouldn't read much into it.
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    Stormtrack supporter Wes Carter's Avatar
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    I'm hesitant to throw my two cents in on this one because I realize I'm out of my league for the most part, but if it spurns further discussion Brad and I might learn a bit from it.

    Earlier this winter I had a discussion with Randy Bowers and Clarence Bennett about taking into account the Arctic Oscillation along with ENSO. Since then I have been watching it and the AO Index has been negative most of the winter, going neutral right at New Year's time frame before going back negative until last week, when it went positive. I might be reading too much into this because it is anecdotal evidence at best, but the arctic oscillation going neutral coincides with the severe weather outbreak on New Year's Day, and the trend towards positive last week preceded the latest rounds of snowfall. I realize that just like ENSO it is just one piece of a larger puzzle, but it looks to me like it does play a fairly large role along with ENSO.

    Last edited by Wes Carter; 02-04-2011 at 06:58 PM. Reason: Add chart on AO

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    Stormtrack supporter Wes Carter's Avatar
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    I did some searching on the internet and found this article from NASA's Earth Observatory.

    Also, compare the reports of severe weather this fall and winter in the charts below to the Arctic Oscillation chart above.

    October:

    November:

    December:

    January:

    February:

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    It is interesting that as soon as the AO went positive, the trajectory of artic air masses suddenly shifted west.... It's been unusually warm and dry all winter in Denver, but as soon as it switched we got some of our coldest temps in over a decade. We even broke our record low maximum temp on Feb 1! Perhaps this deserves more looking into. Other things to consider: Does the AO have different effects in ENSO+ and ENSO- years? (ie, is the effect more obvious in one or the other? Or does it behave differently altogether?) It seems like the forecasts have a decent track record this year based on that graph. Could the AO forecast be used to improve long range storm forecasts (timing and location probs)?

    Perhaps most importantly: What does it mean for chase season?

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    Stormtrack supporter Wes Carter's Avatar
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    I have some of the same questions Zach. I'm wondering if this is something that's already well known among meteorologists and atmospheric types but us chasers (especially newbs like me) might not be as aware of. Google searches do give returns about a negative AO causing a colder than normal winter in the Eastern US, but I'm not finding much about the relationship to anything else. Granted, we aren't going to have severe thunderstorms with super cold temps, but I'm wanting to find out if the AO has as much of an effect in the early spring season in a La Nina year, especially in Dixie Alley and the Southern Plains when La Nina is supposed to favor these areas. This evening when I get time I'm going to peruse AMS articles to see if I can find out more, as well as look at other La Nina winters and springs.

    Brad, I hope I haven't hijacked your thread too much. But maybe we will get some more knowledgeable people from Stormtrack in on this discussion that can answer your question rather than pose another one.
    Last edited by Wes Carter; 02-05-2011 at 10:47 AM. Reason: Clarification of something I know little about

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    Administrator Team Patrick Marsh's Avatar
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    Hi Wes,

    Your analysis of the graphic you posted does not jive with the graphic. In fact, the the AO was extremely negative for most of December and January. It didn't return to neutral until mid January, not the first of the year.

    The AO is essentially a measure of the near-sea-level (1000mb) pressure in the arctic (poleward of 20N). When the AO is positive, the pressure in the arctic is low and when the AO is negative, the pressure is high. When the arctic pressure is low (AO+), this results in a stronger gradient between the pole and the mid-latitudes, resulting in a stronger, more zonally oriented polar jet, which from a thermal wind argument tends to lock in the cold air across the arctic. When the arctic pressure is high (AO-), the gradient between the pole and the mid-latitudes is less, resulting in a jet stream that is apt to meandering/buckling and thus cold air intrusions into the mid-latitudes.

    With its relation to severe convective weather, it's a lot like the Pacific-North American Pattern (PNA). A PNA+ scenario is one where there is a west-coast ridge and and eastern tough. A PNA- scenario is one where there is a west coast trough, and an east coast ridge. Based on that, one might conclude that from a chasing stand point we would want a PNA-, right? Not exactly. We need to have a strong temperature gradient somewhere near the plains in order to have a strong mid- and upper-level jet, which is needed for severe convective weather. If we examine the PNA impacts on severe weather completely in isolation (meaning this is a theoretical argument), it is best achieved when the PNA is in the process of transitioning from one phase to the other. In other words, the really big outbreaks (severe convective or severe winter weather) tend to happen when the PNA is transitioning from PNA- to PNA+. In this scenario you have a west coast trough that is ejecting east into the plains, and being replaced by a west coast ridge. The west coast ridge tends to dislodge cold air from western Canada southward into the US, giving you a classic severe weather setup of cold air north and west, warm air south and east, and a strong, dynamic system in the gradient.

    I mention the PNA scenario to tell you that if we examine the AO (again in isolation) neither the negative phase or the positive phase are extremely great for severe weather. Your best bet is when the AO is in the process of rapidly transitioning from one phase to the other. (As with the PNA, all things equal, the faster the transition, the stronger the gradients.) I'd tend to prefer a AO+ to AO- transition, since the AO- would have led to arctic air intrusions somewhere into the middle-latitudes, probably shunting the rich moisture equatorward. Not to mention that as the transition from AO- to AO+ takes place the polar jet would be lifting north. In the reverse scenario (transition from AO+ to AO-) the mid-latitudes would be relatively warm (and hopefully moist) so that when the arctic air surges south, there would be the possibility of severe weather along the boundary between the two airmasses. As with most things in meteorology, the really "fun" stuff occurs in the gradients...not the maximums (minimums).

    Tying this back into the original question at hand, La Nina (or more precisely, ENSO) is only one source of influence on the overall large scale pattern. In this post I've identified two others that can have just as strong (or stronger) influence on the hemispheric flow -- and there are many others (QBO, NAO, MJO, etc). Meteorologists and Climatologists are just beginning to understand these teleconnections between the state of these large scale oscillations and the observed weather patterns. Most, if not all, of our understanding of ENSO comes from observations post 1950 (and, really, post 1970). Since the 1970s the AO has favored a more positive state, meaning that most of our understanding of ENSO's impacts are based on a singular phase of the AO -- AO+. Lately the AO has favored the negative phase, which means we're still learning what the appreciable impacts are when combined with ENSO states.

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    I took a look at the more recent 00z and 12z GFS 500mb HEIGHT/ANOM ensemble mean forecast and the general trend appears to be heading toward a more typical La Nina pattern with more persistent ridging developing in the eastern U.S. I think it will be interesting to see if our number and likely hood of severe thunderstorm threats increase in future model runs across various regions. BTW, the most recent CFS ensemble mean prediction insist by definition that we remain in a La Nina ENSO state through the severe weather season.

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    Stormtrack supporter Wes Carter's Avatar
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    Quote Originally Posted by Patrick Marsh View Post
    Hi Wes,

    Your analysis of the graphic you posted does not jive with the graphic. In fact, the the AO was extremely negative for most of December and January. It didn't return to neutral until mid January, not the first of the year.

    The AO is essentially a measure of the near-sea-level (1000mb) pressure in the arctic (poleward of 20N). When the AO is positive, the pressure in the arctic is low and when the AO is negative, the pressure is high. When the arctic pressure is low (AO+), this results in a stronger gradient between the pole and the mid-latitudes, resulting in a stronger, more zonally oriented polar jet, which from a thermal wind argument tends to lock in the cold air across the arctic. When the arctic pressure is high (AO-), the gradient between the pole and the mid-latitudes is less, resulting in a jet stream that is apt to meandering/buckling and thus cold air intrusions into the mid-latitudes.

    With its relation to severe convective weather, it's a lot like the Pacific-North American Pattern (PNA). A PNA+ scenario is one where there is a west-coast ridge and and eastern tough. A PNA- scenario is one where there is a west coast trough, and an east coast ridge. Based on that, one might conclude that from a chasing stand point we would want a PNA-, right? Not exactly. We need to have a strong temperature gradient somewhere near the plains in order to have a strong mid- and upper-level jet, which is needed for severe convective weather. If we examine the PNA impacts on severe weather completely in isolation (meaning this is a theoretical argument), it is best achieved when the PNA is in the process of transitioning from one phase to the other. In other words, the really big outbreaks (severe convective or severe winter weather) tend to happen when the PNA is transitioning from PNA- to PNA+. In this scenario you have a west coast trough that is ejecting east into the plains, and being replaced by a west coast ridge. The west coast ridge tends to dislodge cold air from western Canada southward into the US, giving you a classic severe weather setup of cold air north and west, warm air south and east, and a strong, dynamic system in the gradient.

    I mention the PNA scenario to tell you that if we examine the AO (again in isolation) neither the negative phase or the positive phase are extremely great for severe weather. Your best bet is when the AO is in the process of rapidly transitioning from one phase to the other. (As with the PNA, all things equal, the faster the transition, the stronger the gradients.) I'd tend to prefer a AO+ to AO- transition, since the AO- would have led to arctic air intrusions somewhere into the middle-latitudes, probably shunting the rich moisture equatorward. Not to mention that as the transition from AO- to AO+ takes place the polar jet would be lifting north. In the reverse scenario (transition from AO+ to AO-) the mid-latitudes would be relatively warm (and hopefully moist) so that when the arctic air surges south, there would be the possibility of severe weather along the boundary between the two airmasses. As with most things in meteorology, the really "fun" stuff occurs in the gradients...not the maximums (minimums).

    Tying this back into the original question at hand, La Nina (or more precisely, ENSO) is only one source of influence on the overall large scale pattern. In this post I've identified two others that can have just as strong (or stronger) influence on the hemispheric flow -- and there are many others (QBO, NAO, MJO, etc). Meteorologists and Climatologists are just beginning to understand these teleconnections between the state of these large scale oscillations and the observed weather patterns. Most, if not all, of our understanding of ENSO comes from observations post 1950 (and, really, post 1970). Since the 1970s the AO has favored a more positive state, meaning that most of our understanding of ENSO's impacts are based on a singular phase of the AO -- AO+. Lately the AO has favored the negative phase, which means we're still learning what the appreciable impacts are when combined with ENSO states.
    Patrick, thanks for taking the time to explain that. I know you have been busy with your dissertation as well as analyzing data from slight, moderate, and high risk scenarios so I do appreciate your response.

    I did goof looking at the chart for the AO in the New Year's time frame. I don't know how I botched that, unless I scrolled down and looked at the 14 day forecast when I was scrolling up and down comparing the two. I was looking for a trend rather than a one to one relationship, but still it was a pretty blatant oversight.

    The PNA pattern is something I have not looked into yet, but is now on my list of things to read about. But your explanation about the change in phase between PNA+ and PNA- as well as AO+ and AO- and the resulting shift in jet streams and temperature gradients tied a lot of things together for me and gave me some direction in where to look further. Sometimes the challenge is not finding the information as much as it is knowing what to look for in the first place. Google has not created an app to help us figure out what questions we need to ask yet.

    I am looking forward to reading some of the articles about the research into AO- as it becomes better understood how it affects things. When I was in school I studied some heat transfer and fluid dynamics but that was 20+ years ago and in my course of study it was more in closed systems. Trying to learn more about the atmosphere in a system that is pretty much an open system with so many more variables and influences is quite overwhelming. The more I learn the more I realize how a person could spend their whole life studying this beast and still have many questions still unanswered.

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    We had a very cold December here in the UK, when the analogues for La Nina type winters favoured a slightly milder than average winter. The problem? The strongly -ve NAO. Indeed, we've had over 12 months of -ve NAO, a record in observational history. Theories abound, of course - however, one is very interesting. That suggests that low sunspot activity favours northern blocking. History of the UK's temperature suggests that there is a pretty good correlation between cold winters over here and low sunspot numbers.
    Paul Knightley.
    http://ukstormchaser.blogspot.com/
    TORRO Director of Severe Weather Forecasting Division.

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