Webberweather
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Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
Wow! Thank u so much! This saves me (& I'm sure many others) a ton of time! -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
Is there any way this post can be pinned somewhere near the top of the thread or somewhere on the forum because I constantly have to look for it every time I go to the site?! Thanks! -
As long as some semblance of easterlies are found in the lower stratosphere, and the peak of the WQBO hasn't descended below 20 hPa, we'll still fit into this window, after January I think it's a lost cause. The confounding factors of accelerated WQBO descent during El Nino due to enhanced Kelvin Wave activity pitted against climatological decelerated downward propagation of the easterly QBO regime in the lower stratosphere during northern winter where the mean meridional circulation and thus upwelling at the tropical tropopause is maximized make it difficult to decipher if the QBO will actually advance beyond this window before mid-winter in seeing as how most transitions tend to occur in late boreal winter or spring. Low solar, +ENSO, & a WQBO as a whole isn't a terrible combo for a SSWE, and they typically occur earlier in WQBO Ninos vs EQBO Ninos & the ongoing persistence of the Scandinavian ridge for most of the autumn lends more credence to a potential SSWE. Obviously long-term changes in ozone related to CFCs, and the blocking effect of outgoing radiation by GHGs in the troposphere leading to long-term stratospheric cooling aren't in our favor.
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The closer we get to December the more I'm starting to think your outlook for a seemingly uncharacteristic cold +ENSO December may come to fruition! If this occurs, I'll definitely give a huge hat tip to you because you're one of the few who really discussed this as a legitimate possibility! Obviously, given the fact that we're in a weak El Nino it definitely helps to garner a cooler start and I tend to agree that basing most of the outlook on those canonical relationships (aside from of course the cool February) may not be the best idea here and if anything interannual persistence may in fact bring you closer to the "true" answer!
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Nice write-up! It's definitely the one of the best ones I've seen in quite some time, fantastic job! However the point about the increasing westerly shear stress in concert w/ low solar deterring a sudden stratospheric warming event isn't actually true based on observations. Looking at QBO phase alone isn't enough, it's important to actually know where we are in the overall cycle and where the strongest wind stresses are located. We're actually in the most favorable portion of the QBO cycle for a big SSWE as I showed a week or so ago from Sam Lillo's graphic depicting the peak wind stress and amplitude of the peak wind stress wrt SSW events, which clearly shows easterly to westerly QBO transition being the most conducive for a SSWE, and that's exactly where we are now. I further inquired on why this might be the case in the teleconnection thread. Notice most SSWEs occur when we have peak easterlies at 50-70 hPa.
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Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
Total Atmospheric Angular Momentum (AAM) lags mountain torques by several days to a week or so because the lag is mainly a function of the group speed of the Rossby Wave packets that emanate from source of the mountain torque in addition to the planetary vorticity advection term that implicates the evolution of placement of the cyclones (or anticyclones) involved in the AAM response which move westward and poleward with increasing time, depositing their westerly momentum at least initially in the mid-latitudes, and thus contributing the most to +AAM before migrating to higher latitudes, assuming we begin w/ a positive mountain torque. Another key piece worth mentioning is that the AAM response to Himalayas mountain torque is about 30% stronger than Rockies torque. These 2 figures from Lott, Robertson, & Gill (2003) illustrate the aforementioned discussion nicely. https://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(2004)061<1272%3AMTANHL>2.0.CO%3B2 -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
Any chance of a strong, long-lasting negative NAO will be interfered w/ by unfavorable tropical forcing over the eastern hemisphere for the next week or two, it's the same reason we didn't have a very strong -NAO this week despite the very favorable large-scale mid-latitude signal for one. If the Scandinavian ridge hangs around a little longer thru say about week 3, our fortunes could definitely change once the Pacific jet has extended in response to the positive east Asia mountain torque and emergence of subseasonal tropical forcing into the Pacific. NINO forcing in the 2nd half of November and December usually leads to warmth east of the Rockies (not just December), if we manage to get a -NAO in the meantime, we may end up w/ a mild, yet stormy pattern w/ troughs digging into the western US and sliding thru in the southern stream underneath a blocking ridge over east-central Canada. -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
This is just classic, look how the negative mountain torque anomalies in the mid-latitudes (green/blue first plot), preceded the -AAM at those same latitudes (green/blue 2nd plot) about a week later. Atmospheric Angular Momentum usually lags mountain torque by about a week or so (based on what I've gleaned from literature), this jives w/ that nicely in real-time! -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
The westerly QBO has descended below 30 mb but the easterly regime remains stedfast in the lower stratosphere. As mentioned in an earlier post here, we're in the "goldilocks zone" so to speak for a sudden stratospheric warming event this year in QBO phase space, with the QBO transitioning from easterly to westerly. It should make sense to some why SSWEs are most favored during the easterly to westerly hand-off because this usually means the descending easterlies are closest to the tropical tropopause, providing the largest impact to shear and static stability there, implicating tropical convection and ultimately leading to the most pronounced acceleration of the Brewer Dobson Circulation that leads to the most advanced concomitant warming of the polar stratosphere. The fact we're encroaching on solar minimum and have an El Nino to enhance the standing planetary waves in the western hemisphere and Pacific helps our case for a SSWE this year, while long-term cooling of the stratosphere from CFC release in the mid 20th century and AGW-induced cooling via the aforementioned "blocking effect" of outgoing radiation by increased GHG concentrations will serve as impediments to getting a SSWE this time around. http://www.geo.fu-berlin.de/met/ag/strat/produkte/qbo/qbo.dat- 280 replies
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Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
There's also a component to the cooling in the stratosphere, mesosphere, and thermosphere that's not related to the sun. As the concentrations of greenhouse gases increase in the troposphere due to emissions of fossil fuels, land use changes, and positive feedbacks in response to this from natural sources of GHGs, the temperature decreases above the troposphere because less outgoing longwave radiation makes it out of the troposphere as its selectively absorbed and re-emitted back to the surface by GHGs in the troposphere, contributing to the so-called "blocking" effect. Eventually the temperature would recover after this initial increase in GHGs, but continued accelerated increases in their concentrations will make the negative temperature change in the stratosphere seem quasi-permanent. https://www.earth-syst-dynam.net/7/697/2016/esd-7-697-2016.pdf -
***Winter Countdown Thread 2018-2019***
Webberweather replied to swamplover56's topic in Weather Spotlight
That map also severely underestimates climatological snowfall here in the Carolinas and in the south-central mountains it's pretty atrocious. Greensboro/Winston-Salem average well over 6" for 1981-2010, Raleigh is almost to 6".- 1,177 replies
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***Winter Countdown Thread 2018-2019***
Webberweather replied to swamplover56's topic in Weather Spotlight
We definitely have to be weary of a strong & potentially even a prolonged -NAO regime in/around mid-November. Large, retrograding Scandinavian blocks like this are the main way we get -NAOs at the planetary-scale, & when they're coupled w/ -AAM ~45-60N, red flags should be going up in the minds of LR forecasters that one is likely on the way. It's nice to see NWP is finally catching onto this. Even down here in NC, we're starting to encroach on our legitimate snowfall climo window, we had flurries in the air as recently as 2013 in mid-November; 1968 holds the title for biggest event at this time of the year in my neck of the woods.- 1,177 replies
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Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
One, probably less obvious sign that we're entering into an El Nino regime is the apparent blue-shift in subseasonal variability. Even though there's a pronounced seasonal cycle in MJO phase speeds, with faster propagation occurring during equinoctial seasons, what we're seeing at the moment likely goes beyond the influence of the seasonal cycle alone. You can get an idea of the MJO's periodicity by looking at the near-equatorial low-level wind anomaly hovmollers, note the anomalous easterly winds forecast to reappear in the West Pacific in the next several days, about 3-3.5 weeks (or so) removed from the last easterly trade burst. This return period is too large to be mostly explained by a Kelvin Wave (which usually cross the global tropics in 1-2 weeks) & is probably more attributable to a fast-moving MJO wave. Enhanced eastward advection by the stronger westerly background flow in El Ninos conjunction with less deep, moist convection (in general) over the eastern hemisphere both help accelerate the MJO, the latter does so by increasing static stability in the ascending portion of the MJO wave (Gill (1982)). This may lead to some spectral leakage of the MJO's variance into the Kelvin Wave band. This essentially means that the MJO circumnavigates the globein less time, closer to 30-40 days, in an El Nino event and behaves more like a faster moving, moist Convectively Coupled Kelvin Wave (w/ a higher wavenumber). You could also hypothesize that the zonal elongation of the MJO's active longitudes during El Ninos (because the Pacific east of the dateline is warm enough to readily sustain convection) inherently favors Kelvin Waves which propagate faster eastward than Rossby & Mixed-Rossby Gravity waves propagate west. Another intriguing element that comes into play w/ faster eastward MJO phase speeds (& is discussed in literature linked below) is the upper-level ridge directly tied to convective heating within the MJO's envelope weakens because there's less time for the active convection to deposit heat and -PV into the ridge (which both act to reinforce it) and the upstream wave breaking from the mid-latitudes that often precedes the convection is also considerably weaker. Just some food for thought! Here's a few pieces of literature to get everyone started on what I'm referring to above: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI4230.1 https://journals.ametsoc.org/doi/pdf/10.1175/MWR-D-16-0352.1 https://ams.confex.com/ams/97Annual/webprogram/Paper302240.html -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
The atmosphere has looked like what you'd typically expect to see in a weak El Nino for the last several weeks and multiple subseasonal forcing cycles w/ additional anomalous westerly momentum (in yellow & orange) consistently appearing in the subtropics (indicative of a strong subtropical jet and retracted Hadley Cell) in concert w/ anomalous easterly momentum in the polar regions, it will be interesting to see if this persists later into November or if the positive momentum anomaly continues to propagate poleward going into November. Simply looking at total AAM at denoting how low the total anomaly is in comparison w/ other NINO events doesn't actually explicitly explain how the momentum is distributed, which says something more meaningful about the current regime, EOF analysis could probably be useful in this regard instead of simple averaging. In the context of other weak El Ninos that form after the equinox, there's nothing too out of the ordinary here imo. The eastern Hemisphere monsoon circulations were the main inhibitor of Nino growth during the summer, now that they've become relatively dormant, positive feedbacks and state dependent noise forcing will be able to reinforce this El Nino. A sudden stratospheric warming event or even a period of more modest yet still anomalous wave-driving warming the polar stratosphere could be a wild card that further reinvigorates said forcing by altering the static stability and wind shear near the tropical tropopause, effectively accelerating the Brewer-Dobson Circulation which would allows this NINO to intensify beyond the vernal equinox and persist into 2019-20 or it could completely dampen it, with the eventual response from the ocean-atmosphere system presumably lagging a few months following the SSWE mainly due to the phase speed of an equatorial, oceanic Kelvin Wave. -
Teleconnections: A More Technical Discussion
Webberweather replied to Snowy Hibbo's topic in Weather Spotlight
While there's been some chatter concerning the oncoming WQBO regime and how it could diminish the prospects for a major sudden stratospheric warming event this winter, I think it's worth noting that major sudden stratospheric warming events actually occur most frequently during the phase transition from easterly to westerly QBO. In seeing how the strongest negative zonal wind anomalies associated w/ the easterly QBO are currently around 50 hPa, we're actually optimizing our chances for one this year. Despite the potential for heightened Kelvin Wave activity that hastens westerly QBO descent in El Ninos, downward propagation of the eQBO may decelerate over the coming months as the Brewer Dobson Circulation intensifies upwelling near the tropical tropopause and lower stratosphere (QBO descent is typically fastest in northern summer because the mean meridional circulation is weakest), and I wouldn't be surprised if we don't fully transition to westerlies in the lower stratosphere until we're near the spring equinox (which is climatologically favored btw). I pulled this graphic from Sam Lillo a couple years ago, months are denoted by different colored circles and the size of the circle corresponds to the strength of the SSWE. Note that a majority of the largest ones are found near or just beyond E50 (i.e. wherein the strongest zonal wind anomalies are near 50 hPa and are easterly), same goes for E20, E30, W50, W30, etc.) which is where we currently stand w/ the QBO. Low solar activity and +ENSO could provide some additional support for one by implicating favorable radiative changes in the tropical stratosphere and amplifying the standing mid-latitude planetary waves respectively. We're already beginning to prime the polar vortex for a SSWE thanks to the persistent Scandinavian ridge-Aleutian Low couplet (wave 1 forcing). Recurrence and persistence of this pattern will be crucial in triggering vortex displacement especially when more favorable subseasonal forcing returns (i.e. anomalous equatorial west-central Pacific convection)- 280 replies
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