Stratosfeer herfst 2017

[Karel]

eQBO houdt nog even stand. Troposferische poolwervel voorlopig verplaatst naar Noord Canada.

[Sebastiaan]

Op de vorige pagina had ik aangegeven een sterke MJO te verwachten: https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017GL072832

Een recente studie nuanceert een en ander:
Activities of the Madden‐Julian Oscillation (MJO) in boreal winter has recently been found to be stronger in easterly phases of the stratospheric quasi‐biennial oscillation (QBO) than its westerly phases. This QBO‐MJO connection was investigated in this study using a method that identifies individual MJO events by tracking their eastward propagating signals in precipitation. Stronger MJO activities in QBO easterly phases are a consequence of more MJO days, not larger amplitudes of individual MJO events as previously thought. More MJO days come from more MJO events initiated over the Indian Ocean and their longer duration because of a weaker barrier effect of the Maritime Continent on MJO propagation. Zonal heterogeneity exists in the connection between QBO, MJO, and tropical total precipitation in general. This poses a challenge to our current understanding of the MJO dynamics, which has yet to fully include upper‐tropospheric and stratospheric processes.

http://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017GL072832

http://twitter.com/WorldClimateSvc/status/1044327541077610497
CFSv2 is strongly indicating a high-amplitude MJO phase 1 in conjunction with a weak stratospheric polar vortex in early October. The only good analog for the time of year appears to be 2011 - interestingly a La Niña year [sebastiaan: herfst 2011 negatieve QBO].

[Sebastiaan]

https://twitter.com/WorldClimateSvc/status/1044327541077610497


Niet erg verrassend op basis van:
https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017GL072832

Enige bijstelling is te vinden in:

https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2017JD028171

[Sebastiaan]

https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.3280

Stratosphere–troposphere coupling is often viewed from the perspective of the annular modes and their dynamics. Despite the obvious benefits of this approach, recent work has emphasised the greater tropospheric sensitivity to stratospheric variability in the Atlantic basin than in the Pacific basin. In this study, a new approach to understanding stratosphere–troposphere coupling is proposed, with a focus on the influence of the stratospheric state on North Atlantic weather regimes (during extended winter, November to March). The influence of the strength of the lower‐stratospheric vortex on four commonly used tropospheric weather regimes is quantified. The negative phase of the North Atlantic Oscillation is most sensitive to the stratospheric state, occurring on 33% of days following weak vortex conditions but on only 5% of days following strong vortex conditions. An opposite and slightly weaker sensitivity is found for the positive phase of the North Atlantic Oscillation and the Atlantic Ridge regime. For the North Atlantic Oscillation regimes, stratospheric conditions change both the probability of remaining in each regime and the probability of transitioning to that regime from others. A logistic regression model is developed to further quantify the sensitivity of tropospheric weather regimes to the lower stratospheric state. The logistic regression model predicts an increase of 40–60% in the probability of transition to the negative phase of the North Atlantic Oscillation for a one standard deviation reduction in the strength of the stratospheric vortex. Similarly it predicts a 10–30% increase in the probability of transition to the positive phase of the North Atlantic Oscillation for a one standard deviation increase in the strength of the stratospheric vortex. The stratosphere–troposphere coupling in the European Centre for Medium‐range Weather Forecasts Integrated Forecasting System model is found to be consistent with the re‐analysis data by fitting the same logistic regression model.

[Sebastiaan]

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009GL038776
Een oudje maar m.i. nog steeds relevant.
The primary causes for the onset of major, midwinter, stratospheric sudden warming events remain unclear. In this paper, we report that 25 of the 27 events objectively identified in the ERA‐40 dataset for the period 1957–2001 are preceded by blocking patterns in the troposphere. The spatial characteristics of tropospheric blocks prior to sudden warming events are strongly correlated with the type of sudden warming event that follows. Vortex displacement events are nearly always preceded by blocking over the Atlantic basin only, whereas vortex splitting events are preceded by blocking events occurring in the Pacific basin or in both basins contemporaneously. The differences in the geographical blocking distribution prior to sudden warming events are mirrored in the patterns of planetary waves that are responsible for producing events of either type. The evidence presented here, suggests that tropospheric blocking plays an important role in determining the onset and the type of warmings.

[Karel]

Ja Sebastiaan nog steeds zeer relevant: de invloed van de extratropische circulatie.

Blocking is op het Noordelijk Halfrond in tegenstelling tot op het Zuidelijk Halfrond een betrekkelijk veel voorkomend verschijnsel. SSWarmings ook. Er is er deze eeuw op het Zuidelijk Halfrond maar 1 SSW bekend in de winter daar.

Maar Blocking op het Noordelijk Halfrond wordt ook vaak niet gevolgd door SSWarmings.

Opvallend dat juist blockings op de Pacific en op de Atlantic er zo bij betrokken zijn en onderscheidend zijn in Vortex displacement events (wave1) (Atlantic) en vortex splitting events (wave 2)(Pacific eventueel samen met de Atlantic).

De computermodellen zijn tegenwoordig steeds beter in het berekenen van blocking op de langere termijn (10- 15 dagen). Afwachten maar!

[Sebastiaan]

It should be fairly evident, unless you have your head firmly entrenched within the deepest recesses of a jack o'lantern, that this composite portrays a fairly harsh winter for at least the northeast. And potentially the mid atlantic, as well. However note that the word "composite" is emphasized, as there are voices of dissent among our analog set. The winter of 2006-2007 is perhaps the most prominent outlier of this set, so it was discussed at some length what variable(s) may have contributed to that particular outcome. The fact that the modoki characteristic of that warm ENSO event begin to decay very early in its life cycle, about at this time of year, was cited as one prominent reason why this particular analog is beginning to diverge a bit from the current season.

We also began to discuss the Quasi Biennial Oscillation (QBO) and the solar cycle as two contributing factors to the degree of blocking experienced in a given season. Please refer back to the last update for a deeper explanation of the QBO, but the simple explanation is that a negative, or easterly QBO favors blocking because it renders the polar vortex more diffuse and prone to disruption, and a positive, or westerly QBO is less favorable for this to occur. The mean QBO value in August 2006 was +9.10, and ascending, as compared to the August 2018 value of -20.41. This is one notable disparity between the two seasons, which could imply that 2018 is more likely to feature substantial high latitude blocking. However we also implied last week that this year's QBO value is in a state flux, and this is because it is rising, and transitioning with time to a westerly direction. This renders any prognostication a bit more nebulous. The trend of the QBO and its pace can be every bit as important as its absolute value at any point in time, which is an important consideration regarding analogs. That being said, here are the respective analog years and corresponding August QBO values and trends.

August 1953: -1.21 and growing more negative. No match
August 1958: -15.59 and slowly falling. Trending Opposite direction. Decent match
August 1969: +9.78. No match.
August 1976: -4.89 and trending more easterly. No match.
August 1977: -11.24 and fairly rapidly becoming westerly/positive in time for winter. MATCH
August 1979: -22.24 and peaking, but remained negative for winter. Decent match.
August 2004: +8.74 and trending negatively. No Match.
August 2006: +9.10 and very slowly trending easterly. No match.
August 2014: -21.64 and trending negative. Matches absolute value, but wrong trend. Decent match.

Clearly, based solely upon the QBO data, 1977 is the strongest analog of the weak el nino comparisons. However both QBO and the solar cycle need to be considered within the context of assessing the probability of sustained periods of blocking during the cold season.
Solar Minimum Looms
The number of spots on the sun is utilized as a rough proxy for how much solar energy is being emitted at any given point in time. And like everything here on plant earth, this waxes and wanes on a cyclical basis. This solar consideration is important because low solar output has been linked to sustained blocking, and thus harsher winters. For a better idea of whether or not this is true, lets assess the data.
The following is a list of solar cycles since 1950, which runs back to solar cycle 19, along with dates of accompanying solar minimum and solar maximums, as well as average length of cycle.

The curent model consensus concurs that the minimum may arrive by mid 2019. https://easternmassweather.blogspot.com/2018/

[Sebastiaan]

Glosea5 correlatie:


Zoals we zien schommelt de correlatie tussen een 0,8 bij de tropen tot geen significante correlatie in de witte gebieden.
Zowel ter breedte van de Azoren als in het bij IJsland en Groenland is de correlatie tussen de 0,6 en 0,8. Wellicht scoren deze gebieden vrij hoog omdat de Azoor als het IJslandlaag vrij standaard zijn.

[Sebastiaan]

In deze studie (https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.3280) bekijken ze vier regimes en stratosfeer op 100 hPa. Hieronder de vier regimes met percentage van hoeveel ze voorkomen van november tm maart.



De frequentie van de regimes bij de bijbehorende stratosferische toestand (weak-neutral-strong).

De tekst die hierbij hoort:
Frequency of occurrence of North Atlantic weather regimes immediately following weak stratospheric vortex conditions (left, red bar), neutral stratospheric vortex conditions (central, white bar) and strong stratospheric vortex conditions (blue, right bar). The 95% confidence interval (see text for details) is shown at the end of each bar.

Hun conclusie bij deze grafiek is:
For neutral conditions, BL is the most frequently observed regime, followed by NAO+. Stratospheric vortex conditions significantly
shift weather regime frequency as expected. Following weak stratospheric vortex conditions, NAO- occurs almost twice as frequently
as during neutral stratospheric vortex conditions with consequent reductions in the occurrence frequency of both AR and NAO+.
Following strong stratospheric vortex conditions, the opposite sensitivity is found, large reductions in NAO- frequency and increases
in both AR and NAO+. Changes to BL frequency are smaller and not significant. Previous studies have highlighted the links between
Greenland Blocking and stratospheric vortex conditions (Woollings et al. 2010a; Davini et al. 2014), reflected here in the sensitivity of
the NAO- regime.

[Sebastiaan]

September -9.91, augustus was -20,41 https://www.esrl.noaa.gov/psd/data/correlation/qbo.data