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sebastiaan1973

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    Winter 2023/24 - Discussions & Forecasts

    in Spring Weather Discussion

    Posted

    An extreme cold Central European winter such as 1963 is unlikely but still possible despite climate change

    Abstract. Central European winters have warmed markedly since the mid-20th century. Yet cold winters are still associated with severe societal impacts on energy systems, infrastructure and public health. It is therefore crucial to anticipate storylines of worst-case cold winter conditions, and to understand whether an extremely cold winter, such as the coldest winter in the historical record of Germany in 1963 (−6.3 °C or −3.4σ seasonal DJF temperature anomaly relative to 1981–2010), is still possible in a warming climate. Here, we first show based on multiple attribution methods that a winter of similar circulation conditions to 1963 would still lead to an extreme seasonal cold anomaly of about −4.9 to −4.7 °C (best estimates across methods) under present-day climate. This would rank as second-coldest winter in the last 75 years. Second, we conceive storylines of worst-case cold winter conditions based on two independent rare event sampling methods (climate model boosting and empirical importance sampling): winter as cold as 1963 is still physically possible in Central Europe today, albeit very unlikely. While cold winter hazards become less frequent and less intense in a warming climate overall, it remains crucial to anticipate the possibility of an extreme cold winter to avoid potential maladaptation and increased vulnerability. 

    https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2523/

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    Winter 2023/24 - Discussions & Forecasts

    in Spring Weather Discussion

    Posted

    An intresting read

    Abstract

    In Europe, the increase in temperatures caused by climate change has been particularly fast in the cold season. Although the magnitude of this change is relatively well known, less research has been done on how the increase of temperatures is manifested in different large-scale weather types, called weather regimes. For example, one could expect that the weather patterns in which air is flowing from the rapidly-warming Arctic would have warmed faster than other weather patterns in recent decades. Here we show that such an asymmetric warming actually occurs in the four Euro-Atlantic weather regimes. In northern Europe, the weather regime which is typically associated with cold airmasses from the Arctic (NAO–) has warmed about 25% faster than the cold-season days on average, and about 60% faster than the regime where the air flows from the North Atlantic (NAO+). Consequently, the weather regime that on average brings the coldest weather is warming the fastest in a large part of northern Europe. In contrast, the weather regime that typically brings the warmest weather has warmed the slowest, especially in the continental Europe. Our results provide a new perspective on the reported decrease of sub-seasonal temperature variability.  https://rmets.onlinelibrary.wiley.com/doi/10.1002/asl.1178

    Schermafbeelding 2023-11-15 172048.jpg

    Schermafbeelding 2023-11-15 172035.jpg

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    Winter 2023/24 - Discussions & Forecasts

    in Spring Weather Discussion

    Posted · Edited by sebastiaan1973

    5 hours ago, northwestsnow said:

    Greenland/ Iceland anticyclone works for the Dutch?

     

    No, or the high-pressure area should extend to Scandinavia. Altough I have skate by NW-wind, dewpoint negative, but in this condition there was already ice of 10 cm. 

    A nice read by Simon Lee.

    https://simonleewx.com/2023/11/03/whats-that-coming-over-the-hill-is-it-a-weak-vortex/

    Only a few years ago, ECMWF’s then-twice-weekly 51-member extended-range forecasts were not publicly available — something that is almost hard to comprehend nowadays, as we have daily, 101-member ensemble forecasts available for free on the ECMWF website. The ensemble size is spectacular, and increases forecast reliability. But I’m more interested in what we gain from daily initialisations.

    Moving to daily initialisations means we can better understand the sensitivity of the extended-range forecast to the initial conditions. We should no longer have scenarios where the Thursday forecast said one thing, only for the Monday forecast to jump to something else. A spectacular ‘jump’ occurred between the forecasts issued on 29 January and 1 February 2018, when forecasts that had grown more confident in a strong vortex suddenly flipped to an increased risk of a major SSW (which eventually happened on 12 February). My first PhD paper looked at that forecast evolution.

    In theory, then, a forecast which predicts the same thing in the extended-range when fed a variety of initial conditions over several days or weeks could be on to something. It suggests the model is being attracted to a specific evolution regardless of the initial state (and indeed, regardless of the actual evolution of the weather over that time). Since that’s beyond the medium-range Lorenzian deterministic predictability limit, such behaviour suggests a potential window of opportunity for subseasonal predictability — i.e., an occasion when there is some constraint on the atmospheric evolution 2-6 weeks ahead, in what is otherwise often a predictability desert (not a predictability dessert, which would be tasty).

    That brings us to the reason for this blog: since at least 24 October, ECMWF’s daily 101-member extended-range ensemble forecast has been predicting a weaker-than-normal stratospheric polar vortex to develop by the first week of December. This isn’t a small signal either — the latest forecast, initialised on 2 November (Fig. 1), shows an ensemble mean that’s ~10 m/s weaker than the model’s own climatology by the end of the run. [The model’s own climatology accounts for any mean-state biases in the vortex strength.]

    Figure 1: ECMWF extended-range forecast of the 10 hPa 60°N zonal-mean zonal winds initialised on 2 November 2023. Source: https://charts.ecmwf.int/products/extended-zonal-mean-zonal-wind.

    The weak vortex is more than 4 weeks into the forecast, yet has persisted through at least 10 days of initial conditions (and 10 days of real-world weather evolution). This seems very striking to me. Sufficient that I’m writing this blog instead of working on a manuscript!

    The weak vortex does not appear to be accompanied by a large number of ensemble members showing a major sudden stratospheric warming (SSW; i.e., easterly zonal winds at 10 hPa 60°N). A weak vortex isn’t the same as an SSW, and we wouldn’t expect the ensemble mean to show an SSW until the medium range anyway. That said, SSWs in early December are rare — and so perhaps the relatively small number of members that reverse to easterlies is in fact anomalously large (I don’t have the statistics… ECMWF, can you make an easterly wind probability plot like the C3S ones?).

    So, this looks like a signal worth noting. But why is it there? Much research has found that blocking in the Ural Mountains region [e.g., Kolstad and Charlton-Perez 2011; Peings 2019, White et al. 2019] can enhance upward-propagating planetary wave activity and serve as an SSW precursor. When planetary waves break in the stratosphere, they exert a westward drag on the zonal winds and warm the stratosphere. The stratospheric vortex strength is on average effectively an integrator of the amount of wave activity over the preceding month to six weeks, so this forecast suggests prolonged increased upward wave activity.

    Although I haven’t got the data to explicitly make the link, the same forecasts have also been showing remarkably persistent Ural blocking (Fig. 2) that has also been robust to differences in initial conditions. At least in the 2 November run, there is an ensemble-mean anomalous ridge over the Urals from week 2 to the end of the run! There is also generally an anomalous trough over eastern Siberia/North Pacific. Both of these constructively interfere with the climatological mean stationary wavenumber-1 pattern (cf. Fig. 2 here with Fig. 3 in White et al. 2019).

    Figure 2: Ensemble-mean 500 hPa geopotential height anomalies for 20-27 November 2023, initialised on 2 November 2023. Note the Ural blocking and east Siberian/North Pacific trough. Source: https://charts.ecmwf.int/products/extended-anomaly-z500.

    So, it seems that the model’s weak vortex forecast is consistent with its tropospheric forecast, and that the troposphere keeps evolving the same way, run-to-run. But why?

    The North Pacific trough is consistent with a deepened Aleutian Low, which is a typical response to the tropospheric Rossby wave train induced by El Niño. That’s a key reason why the vortex is — on average — weaker during El Niño winters. Climate models also generally indicate that El Niño winters see the highest SSW frequency, but observations don’t necessarily line up with this, which is probably just an effect of the small and noisy observational sample size.

    But why is the Ural blocking there and so persistent? I’m less sure. One thing that struck me looking at the Z500 anomalies is the presence of persistent troughing near the British Isles and western Europe that is present alongside the Ural blocking. The regime forecasts seem confident in NAO+, even in the ensemble mean. Perhaps this persistent cyclonic flow and associated wave breaking is helping to build the block downstream? One could also ask why there is a persistent cyclonic anomaly in the Atlantic, too. I am running out of steam at this point, but I think this goes to show the multi-scale nature of the problem and how remarkable it is that all these components have persisted run-to-run.

    Now, it’s important to also note that the stratosphere is not a slave to the troposphere. The same tropospheric patterns do not always induce the same stratospheric response, while not every extreme stratospheric event has an extreme tropospheric precursor. The state of the stratosphere matters. In this case, it’s interesting that the vortex becomes extremely strong for the time of year during the medium-range. In fact, it’s likely to set new date-records, and might even qualify as a “strong vortex event” (definitions differ, but broadly when the zonal winds exceed ~40 m/s). You can see that in Fig. 1, where the ECMWF forecast exceeds the 90th percentile (thin red line) of the model climate. After this, the winds begin to weaken, and this evolution has also been persistent run-to-run. Meanwhile, Fig. 3 shows the latest 10-day forecast from NASA’s GEOS model (orange) alongside the MERRA-2 climatology for the whole season, with the GEOS forecast exceeding the date-record maxima.

    I mention all this because wave propagation and breaking in the stratosphere can be enhanced when potential vorticity gradients are steep, and steep PV gradients can manifest as strong zonal winds. This is usually something we think about more in the second half of the winter, when the vortex edge has been sharpened by wave activity through the winter (a bit like peeling an onion), but it could tie in here, especially since these vortex conditions are more typical in midwinter.

    Figure 3: Climatology of the stratospheric polar vortex from MERRA-2, plus GEOS forecast (orange), last winter (blue) and this season so far (red and pink). Source: https://ozonewatch.gsfc.nasa.gov/meteorology/figures/merra2/wind/u60n_10_2023_merra2.pdf.

    Now, I’m not a forecaster, and I’ve certainly seen enough subseasonal forecasts ‘evaporate’ to know that a big and persistent signal doesn’t mean you should bet your house on it. Verification statistics for subseasonal forecasts are low, and often my interest is more centred around understanding why models say what they say, rather than whether the forecast will verify. But if this case does verify… then, given the lead time at which it first appeared, it would be a remarkable case worth investigating further.

    In the shorter term, I’d be on the lookout for the Scandinavia-Greenland pattern. This is a transient (synoptic-scale) pattern that is characterised by cyclones tracking up the east coast of Greenland with an accompanying ridge building over Scandinavia. It is usually associated with anticyclonic wave breaking, and often subsequent development of the Ural high, as well as enhanced upward wave propagation itself. Perhaps I’m biased because I wrote two papers about it (in 2019 and 2020). But it’s been appearing in some medium-range output, such as that shown in Fig. 4 which went on to develop a Ural high and saw a significant amount of upward wave activity inducing a weakened vortex by the end of the run. GFS fantasy land, yes, but it’s the sort of processes that the ECMWF subseasonal run suggests we’d be looking for.

    Figure 4: GFS deterministic forecast for 00Z 16 November, initialised 00Z 2 November. The Scandinavia-Greenland pattern is evident.

    So, potentially interesting times ahead. Or maybe this blog will age terribly and serve as a reminder of the challenges of extended-range forecasting… either way, if you’ve read this far, thank you! It’s always a pleasure to share things like this online, and even more so when people find the time and interest to engage with it. Cheers!

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    Winter 2023/24 - Discussions & Forecasts

    in Spring Weather Discussion

    Posted

    Plain Language Summary

    A robust link exists in the preceding autumn antiphase Tibetan Plateau (TP) and Lake Baikal snow cover anomalies (TBSA) and the winter North Atlantic Oscillation (NAO) during 1979–2021. There are 44% years of antiphase variation in TBSA in autumn, which shows a dipole structure with one positive center over the TP and another negative center over the Baikal. Larger (smaller) snow cover over the TP (Baikal) stimulates a local low (high) pressure system via diabatic cooling (heating). Due to the jet waveguide effect, the antiphase TBSA associated diabatic forcing and perturbation along the subtropical westerly jet favor the atmospheric wave train spanning the TP and North Atlantic. The antiphase TBSA associated transient eddies along the extratropical belt contribute to the atmospheric wave train lying between the eastern Baikal and the North Atlantic owing to the eddy-flow interaction. Along with the seasonal increase in the subtropical westerly jet from autumn to winter, the geopotential height anomalies in the double wave train associated with the antiphase TBSA gradually develop into the winter large-scale NAO circulation through more energy extraction from the stronger basic flow.

    https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL104754

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    Winter 2023/24 - Discussions & Forecasts

    in Spring Weather Discussion

    Posted

    Abstract

    A strong relationship between the quasi-biennial oscillation (QBO) of equatorial stratospheric winds and the amplitude of the Madden-Julian oscillation (MJO) during the boreal winter has recently been uncovered using observational data from the mid-1970s to the present. When the QBO is in its easterly phase in the lower stratosphere, it favors stronger MJO activity during boreal winter, while the MJO tends to be weaker during the westerly phase of the QBO. Here we show using reconstructed indices of the MJO and QBO back to 1905 that the relationship between enhanced boreal winter MJO activity and the easterly phase of the QBO has only emerged since the early 1980s. The emergence of this relationship coincides with the recent cooling trend in the equatorial lower stratosphere and the warming trend in the equatorial upper troposphere, which appears to have sensitized MJO convective activity to QBO-induced changes in static stability near the tropopause. Climate change is thus suggested to have played a role in promoting coupling between the MJO and the QBO. On the emerging relationship between the stratospheric Quasi-Biennial oscillation and the Madden-Julian oscillation - PubMed (nih.gov)

    Seasonal prediction of the boreal winter stratosphere

    Abstract

    The predictability of the Northern Hemisphere stratosphere and its underlying dynamics are investigated in five state-of-the-art seasonal prediction systems from the Copernicus Climate Change Service (C3S) multi-model database. Special attention is devoted to the connection between the stratospheric polar vortex (SPV) and lower-stratosphere wave activity (LSWA). We find that in winter (December to February) dynamical forecasts initialised on the first of November are considerably more skilful than empirical forecasts based on October anomalies. Moreover, the coupling of the SPV with mid-latitude LSWA (i.e., meridional eddy heat flux) is generally well reproduced by the forecast systems, allowing for the identification of a robust link between the predictability of wave activity above the tropopause and the SPV skill. Our results highlight the importance of November-to-February LSWA, in particular in the Eurasian sector, for forecasts of the winter stratosphere. Finally, the role of potential sources of seasonal stratospheric predictability is considered: we find that the C3S multi-model overestimates the stratospheric response to El Niño-Southern Oscillation (ENSO) and underestimates the influence of the Quasi-Biennial Oscillation (QBO).

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