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  1. The key role of background sea surface temperature over the cold tongue in asymmetric responses of the Arctic stratosphere to El Niño–Southern Oscillation Authors: Fei Xie, Xin Zhou, Jianping Li, Cheng Sun, Juan Feng and Xuan Ma Published: Nov 2018 Abstract: The response of the Arctic stratosphere to El Niño activity is strong but the response to La Niña activity is relatively weak. The asymmetric responses of Arctic stratosphere to El Niño and La Niña events are thought to be caused by asymmetric El Niño–Southern Oscillation (ENSO) teleconnections. Here, we suggest that the background sea surface temperature (SST) over cold tongue of tropical eastern Pacific may be an important contributor to the asymmetric ENSO teleconnections. The atmosphere is very sensitive to tropical SST variations in the range of 26 °C–30 °C. During El Niño events, the background SST over cold tongue plus El Niño SST anomalies typically falls into the range. Under these conditions, the atmospheric response to El Niño SST anomalies is strong. During La Niña events, the background SST plus La Niña SST anomalies is typically below the range, which leads to a weak response of the atmosphere to SST anomalies. The proposed mechanism is well supported by simulations. Link to full paper: http://iopscience.iop.org/article/10.1088/1748-9326/aae79b/meta
  2. Blessed Weather

    Teleconnections: A More Technical Discussion

    Michael Ventrice Nov 5th:
  3. Sudden Stratospheric Warmings – developing a new classification based on vertical depth, applying theory to a SSW in 2018, and assessing predictability of a cold air outbreak following this SSW Author: L. van Galen Published: August 2018 Abstract: n this study a new classification has been developed to classify Sudden Stratospheric Warmings (SSWs) based on their vertical depth. This new classification was developed because the depth of a SSW has been found to be important for the magnitude of tropospheric impact (Gerber et al., 2009; Palmeiro et al., 2015), and the official SSW classification does not tell anything about the vertical extent of a SSW. The new classification adapted from a previously developed classification of Kramer (2016; hereafter referred to as K16)). It prescribes that the zonal mean zonal wind between 60N and 70N should reverse over a depth of at least 80 hPa between 10 and 100 hPa for at least two days in a 5-day period. This classification was termed a ‘Deep Stratospheric Warming’ (DSW). In the stratosphere, the new DSW classification was compared to both the SSW-classification and the K16-classification. However, the K16-classification included events that were too weak to be considered important for tropospheric impact. Coupled to the fact that K-16-events did not contain any information about the vertical depth of SSWs, this classification was not taken into account in subsequent analyses. Compared to the official SSW classification, it appeared that DSWs were less frequent; whereas SSWs occurred about 6 times per decade, DSWs were found to occur only 4 times per decade. Furthermore, the SSW and DSW classifications were connected: most DSWs occurred a few days to weeks after the SSW date. This was explained by the behavior of SSWs and DSWs: first a warming at 10 hPa took place (the SSW date). After that, the wind reversal extended downward in an irregular fashion, during which after some SSWs at some point the wind reversal extended sufficiently far to the lower stratosphere to cause a DSW to be classified. Connected to the differences in classification date, the DSWs were rarely located close to the moment of rapid warming in the stratosphere. Thus, unlike SSWs, the DSWs did not show the ‘sudden warming’ behavior. Rather, (most of) the DSWs seemed to be a result warm anomalies in the mid-stratosphere that developed sometime during or after the SSW and subsequently downwelled on a timescale of about a week. In terms of upper tropospheric impact, the DSW classification resulted in a similar response compared to the SSW classification: anomalously high temperatures near the pole and a southward displaced jet stream. Furthermore, the jet stream was found to become slightly more meridionally oriented in the midlatitudes and slightly more zonally oriented in the subtropics, but this signal was fairly weak. The main difference between the DSW and SSW was that the DSW effects were mostly stronger in magnitude than the SSW effects. Thus, a DSW results in stronger upper tropospheric impacts compared to a SSW. A similar story was found in terms of surface impacts. Both the SSW and DSW classifications resulted in positive pressure anomalies over the pole and generally negative pressure anomalies in the midlatitudes (indicative of a negative Arctic Oscillation); but the anomalies were more pronounced after a DSW. The surface temperature response after both a SSW and DSW was chaotic, though, showing that the DSW classification did not result in a stronger or more structured surface temperature response. Finally, a thermodynamic perspective was presented to explain why the DSW classification resulted in a stronger tropospheric response compared to the SSW classification method (only in terms of a -AO and a weaker jet stream in the midlatitudes). The cause was speculated to be that the warming accompanied by a DSW extended further to the upper troposphere. This is important for two reasons, being: 1) A deeper warming causes the pressure surfaces to be less tilted from the midlatitudes to the pole, resulting in a more pronounced weakening of midlatitude westerlies, including the polar jet stream. 2) A warming that extends more towards the troposphere exerts more pressure on the underlying air, because the density of air closer to the troposphere is higher. This results in a higher surface pressure. ` In short, the new DSW classification enables to better assess the zonal mean timing and intensity of tropospheric impact compared to the SSW classification. However, no specific conclusions could be drawn about the zonally asymmetric effects of DSWs. Link to full paper: http://bibliotheek.knmi.nl/knmipubIR/IR2018-05.pdf Credit goes to @sebastiaan1973 for finding this presentation - thank you.
  4. Blessed Weather

    Stratospheric Discussion and Forecasting

    Hi Guys - fyi: "In the top panel image of Figure 6.14, we see the single high-low structure that exists between the Gulf of Alaska and Northern Europe centered roughly along the 60°N latitude. The thick black line highlights the wave structure. Atmospheric scientists refer to this single high-low structure as a planetary wave-1 pattern, since a single wave (one ridge and one trough) straddles the entire planet at that latitude. At 60°N, the wavelength of this wave-1 is 20,000 kilometers. In the bottom panel image, we see two highs and two lows in a double high-low structure extending around the northern high latitudes. It is again centered roughly on 60°N. We refer to this as a planetary wave-2, with a wavelength of 10,000 kilometers at 60°N." Source: http://www.ccpo.odu.edu/SEES/ozone/class/Chap_6/index.htm
  5. Blessed Weather

    Stratospheric Discussion and Forecasting

    Hi sebastiaan. Welcome to this side of the 'pond'. That is indeed a very interesting paper you've summarised in your post. I'm not sure if you've taken a look around the Research Portal on 33andrain (follow the 33 University tab on the navigation bar) but it would be great to add this one to the library. Would you mind if I did that on your behalf? Malcolm.
  6. The changing impact of El Niño on US winter temperatures Authors: Jin‐Yi Yu, Yuhao Zou, Seon Tae Kim, Tong Lee Published: Aug 2012 Abstract: In this study, evidence is presented from statistical analyses, numerical model experiments, and case studies to show that the impact on US winter temperatures is different for the different types of El Niño. While the conventional Eastern‐Pacific El Niño affects winter temperatures primarily over the Great Lakes, Northeast, and Southwest US, the largest impact from Central‐Pacific El Niño is on temperatures in the northwestern and southeastern US. The recent shift to a greater frequency of occurrence of the Central‐Pacific type has made the Northwest and Southeast regions of the US most influenced by El Niño. It is shown that the different impacts result from differing wave train responses in the atmosphere to the sea surface temperature anomalies associated with the two types of El Niño. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2012GL052483
  7. Snow-atmosphere coupling and its impact on temperature variability and extremes over North America Authors: G. T. Diro, L. Sushama, O. Huziy Published: July 2017 Abstract: The impact of snow-atmosphere coupling on climate variability and extremes over North America is investigated using modeling experiments with the fifth generation Canadian Regional Climate Model (CRCM5). To this end, two CRCM5 simulations driven by ERA-Interim reanalysis for the 1981–2010 period are performed, where snow cover and depth are prescribed (uncoupled) in one simulation while they evolve interactively (coupled) during model integration in the second one. Results indicate systematic influence of snow cover and snow depth variability on the inter-annual variability of soil and air temperatures during winter and spring seasons. Inter-annual variability of air temperature is larger in the coupled simulation, with snow cover and depth variability accounting for 40–60% of winter temperature variability over the Mid-west, Northern Great Plains and over the Canadian Prairies. The contribution of snow variability reaches even more than 70% during spring and the regions of high snow-temperature coupling extend north of the boreal forests. The dominant process contributing to the snow-atmosphere coupling is the albedo effect in winter, while the hydrological effect controls the coupling in spring. Snow cover/depth variability at different locations is also found to affect extremes. For instance, variability of cold-spell characteristics is sensitive to snow cover/depth variation over the Mid-west and Northern Great Plains, whereas, warm-spell variability is sensitive to snow variation primarily in regions with climatologically extensive snow cover such as northeast Canada and the Rockies. Furthermore, snow-atmosphere interactions appear to have contributed to enhancing the number of cold spell days during the 2002 spring, which is the coldest recorded during the study period, by over 50%, over western North America. Additional results also provide useful information on the importance of the interactions of snow with large-scale mode of variability in modulating temperature extreme characteristics. Link to full paper: https://link.springer.com/article/10.1007/s00382-017-3788-5
  8. Snow–Atmosphere Coupling Strength. Part I: Effect of Model Biases Authors: Li Xu, Paul Dirmeyer Published: April 2013 Abstract: Snow–atmosphere coupling strength, the degree to which the atmosphere (temperature and precipitation) responds to underlying snow anomalies, is investigated using the Community Climate System Model (CCSM) with realistic snow information obtained from satellite and data assimilation. The coupling strength is quantified using seasonal simulations initialized in late boreal winter with realistic initial snow states or forced with realistic large-scale snow anomalies, including both snow cover fraction observed by remote sensing and snow water equivalent from land data assimilation. Errors due to deficiencies in the land model snow scheme and precipitation biases in the atmospheric model are mitigated by prescribing realistic snow states. The spatial and temporal distributions of strong snow–atmosphere coupling in this model are revealed to track the continental snow cover edge poleward during the ablation period in spring, with secondary maxima after snowmelt. Compared with prescribed “perfect” snow simulations, the free-running CCSM captures major regions of strong snow–atmosphere coupling strength, with only minor departures in magnitude, but showing uneven biases over the Northern Hemisphere. Signals of strong coupling to air temperature are found to propagate vertically into the troposphere, at least up to 500 hPa over the coupling “cold spots.” The main mechanism for this vertical propagation is found to be longwave radiation and condensation heating. Link to full paper: https://journals.ametsoc.org/doi/full/10.1175/JHM-D-11-0102.1
  9. Blessed Weather

    Stratospheric Discussion and Forecasting

    Lots of Twitter activity these past few days on the potential (or not) of some disruption to the Strat PV (SPV) as we move into November. A couple to add to those above: Simon Lee 29 Oct: Jason Furtado 30 Oct: The stratosphere and troposphere are currently decoupled and this morning's Berlin output suggests that in the forecast period it's not the stratosphere's zonal westerly winds descending, but rather the influence of the troposphere pattern pushing up into the lower stratosphere: The ECM forecast blocking pattern is looking favourable for Wave 1 disruption to the SPV with suggestions of elongation and displacement off the Pole towards the Siberian side late in the forecast. Charts for ECM 500hPa, 100hPa and 10hPa for 8th/9th Nov: Wave activity and amplitude forecast (30th Oct GFS 06z run): Wave charts: http://http://weatheriscool.com/index.php/stratospheric-forecast-wave-series/ Further info in the paper Blocking precursors to stratospheric sudden warming events. Illustrative charts: Displacement v Split blocking patterns: Displacement v Split Wave activity: Link to paper in Research Portal: http://https://www.33andrain.com/topic/959-blocking-precursors-to-stratospheric-sudden-warming-events/
  10. Snow–atmosphere coupling in the Northern Hemisphere Authors: Gina R. Henderson, Yannick Peings, Jason C. Furtado & Paul J. Kushner Published: Oct 2018 Abstract: Local and remote impacts of seasonal snow cover on atmospheric circulation have been explored extensively, with observational and modelling efforts focusing on how Eurasian autumn snow-cover variability potentially drives Northern Hemisphere atmospheric circulation via the generation of deep, planetary-scale atmospheric waves. Despite climate modelling advances, models remain challenged to reproduce the proposed sequence of processes by which snow cover can influence the atmosphere, calling into question the robustness of this coupling. Here, we summarize the current level of understanding of snow–atmosphere coupling, and the implications of this interaction under future climate change. Projected patterns of snow-cover variability and altered stratospheric conditions suggest a need for new model experiments to isolate the effect of projected changes in snow on the atmosphere. Link to paper: (This paper is currently behind a paywall. Please use 'reply' below if you know of a free-to-view copy). https://www.nature.com/articles/s41558-018-0295-6 Alternatively a full copy is available for temporary download from here: 10.1038@s41558-018-0295-6.pdf
  11. Blessed Weather

    Teleconnections: A More Technical Discussion

    Many thanks griteater. I didn't include the following research paper in my post, but very relevant to your chart: Westerly Wind Events in the Tropical Pacific and their Influence on the Coupled Ocean‐Atmosphere System: A Review "Based on the examination of 10 years of 10-meter winds from the ECMWF analyses, Haarten proposed a subjective classification based on large-scale aspects of the circulation associated with periods of WWEs. According to her classification, nine typical patterns can represent the near-surface flow during 90% of the synoptic westerly wind variability. A single cyclone or a series of cyclones and several different types of cross-equatorial flow are the major components of the patterns." Link to paper: https://www.33andrain.com/topic/1460-westerly-wind-events-in-the-tropical-pacific-and-their-influence-on-the-coupled-ocean‐atmosphere-system-a-review/
  12. Blessed Weather

    Teleconnections: A More Technical Discussion

    Belatedly following up on your extensive WWB post David, I’d like use the papers we’ve both placed in the Research Portal to zoom in a bit more on the causes of WWBs (also known as Westerly Wind Events WWEs). Then I would like to consider this research when looking at what’s currently happening with regard the ongoing WWB in the western tropical Pacific and the developing El Nino by looking at some latest charts. First of all, using one of the papers you reviewed, a reminder of the importance of WWBs to the formation of ENSO events (in this post, click on the paper title to go to the Research Portal for an onward link to the full paper): The impact of westerly wind bursts and ocean initial state on the development, and diversity of El Niño events “Westerly wind bursts (WWBs) that occur in the western tropical Pacific are believed to play an important role in the development of El Niño events.” The research used the coupled general circulation model simulate the response of the climate system to an observed wind burst and found that: “….when the WWB is imposed, the situation dramatically changes: the recharged state slides into an El Niño with a maximum warming in the eastern Pacific.” Then this paper, that clearly shows the relationship between WWBs and the MJO and Rossby Waves: Modulation of equatorial Pacific westerly/easterly wind events by the MJO and convectively‑coupled Rossby waves “We unambiguously demonstrate that the Madden–Julian Oscillation (MJO) and Convectively-coupled Rossby Waves (CRW) modulate both WWEs and EWEs occurrence probability. 86 % of WWEs occur within convective MJO and/or CRW phases and 83 % of EWEs occur within the suppressed phase of MJO and/or CRW. 41 % of WWEs and 26 % of EWEs are in particular associated with the combined occurrence of a CRW/MJO, far more than what would be expected from a random distribution (3 %). Wind events embedded within MJO phases also have a stronger impact on the ocean, due to a tendency to have a larger amplitude, zonal extent and longer duration.” So how frequently do WWBs occur? The paper Synoptic settings of westerly wind bursts, albeit rather old, looked at all the WWBs between 1980 and 1989 and found there were 131 events, but that inter-annual variability was high, ranging from 1 to 29 events a year. It was found that this was influenced by the phases of the Southern Oscillation Index (SOI), with high WWB activity in negative SOI and low WWB activity in positive SOI years. Coincidently, the SOI has recently moved into negative territory, although whether that is (or will be) influencing the current situation (shown in later charts) is unclear: Source: https://stateoftheocean.osmc.noaa.gov/atm/soi.php Other research shows that the occurrence of a WWB event is not solely associated with the MJO. The following paper looks at what happened in the build-up to the 2015-2016 El Nino: Genesis of westerly wind bursts over the equatorial western Pacific during the onset of the strong 2015–2016 El Niño “The strong 2015–2016 El Niño was initiated by several strong westerly wind bursts over the equatorial western Pacific in March and May 2015. These westerly wind bursts trigger eastward propagating warm Kelvin waves and lead to large sea surface temperature (SST) warming in the equatorial eastern Pacific. The first burst of westerly winds in early March was mainly induced by the Arctic Oscillation (AO) event. These westerly wind anomalies were enhanced subsequently due to the Madden‐Julian Oscillation activity and northerly cold surges from East Asia‐western Pacific in mid‐March. Another westerly wind burst in May, induced by anomalous southerly winds from the Australian continent, further increased the SST anomaly in the equatorial eastern Pacific. This study provides an evidence of the AO influence on this strong El Niño‐Southern Oscillation (ENSO) event and demonstrates the complexity in the genesis of westerly wind bursts during the El Niño outbreak, which may help improve the prediction of ENSO.” So bearing in mind the above research, what is the current situation with regard WWBs and the slowly developing El Nino? Before looking at the relevant charts, this extract from an informative blog by Bob Tisdale back in 2015 describes what we are looking for: “One of the factors that kept the 2014/15 El Niño from strengthening last year was the absence of westerly wind bursts after the initial ones early in the year. See Figure 2. It includes Hovmoller diagrams of surface zonal wind stress along the equator for 2014 on the left, 2015-to-date in the center, and 1997 as a reference for a powerful El Niño on the right. In 1997, there were numerous westerly wind bursts over the course of the year, helping to strengthen it into a “super El Niño”. Link to Bob Tisdale blog: https://bobtisdale.wordpress.com/2015/05/18/the-recent-westerly-wind-burst-in-the-western-equatorial-pacific-could-help-to-strengthen-the-201516-el-nino/ Looking at the latest charts we can see a WWB event currently occurring in the West Pacific, circled on this NOAA 850hPa wind anomaly chart for Oct 23rd: The above event is also showing on the Michael Ventrice 850hPa Zonal Wind anomaly chart below: So clearly a WWB event is in progress, but what is driving this? Above research shows that 86 % of WWEs occur within convective MJO and/or CRW phases and 83 % of EWEs occur within the suppressed phase of MJO and/or CRW. The area of westerly winds is currently developing in the suppressed phase of MJO, as shown in the latest NOAA MJO update on Oct 22nd: Again, using the above research, another possible reason is “northerly cold surges originated from East Asian land may induce westerly wind anomalies over the tropical western Pacific via modulating the local atmospheric convection.” The following 10m wind chart from BOM shows a Low pressure around the Philippines feeding a northerly flow down towards the WWB location. On this occasion is this what’s the major contributory factor to the westerly burst? Concluding remarks: WWBs are clearly important in the development of ENSO phases and there is an event ongoing. Research shows this should assist in progressing the current developing El Nino. However, with my level of expertise I am uncertain about the cause of this specific WWB event and invite further comments and contributions.
  13. Blessed Weather

    Teleconnections: A More Technical Discussion

    Many thanks Geoff. Goodness knows how I managed to miss that Tom had already started a Strat thread back last November. That would have been a much better place for the posts that I and others placed in the teleconns thread. Anyway, now we know I'm sure that Alistair @Catacol, James @Singularity and myself will be regular posters there over the coming months. Malcolm.
  14. Blessed Weather

    Teleconnections: A More Technical Discussion

    We've been discussing strat vortex matters in this thread, including an analysis of the 2018 event back on the early pages. I think most of us on here consider it a teleconnection. A couple of us have pondered whether to start a separate strat thread; I'll check with some of the others but my feel is we should, so I will likely do that over the next couple of days. The Hannah Attard website has an archive of daily 10, 50 and 100hPa charts starting back in Aug 2014. http://www.atmos.albany.edu/student/hattard/archive.php And the NOAA website has various annual charts back to 1979. http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/
  15. The Downward Influence of Sudden Stratospheric Warmings: Association with Tropospheric Precursors Author: Ian White Published: Oct 2018 Abstract: Tropospheric features preceding Sudden Stratospheric Warming events (SSWs) are identified using a large compendium of events obtained from a chemistry-climate model. In agreement with recent observational studies, it is found that approximately one third of SSWs are preceded by extreme episodes of wave activity in the lower troposphere. The relationship becomes stronger in the lower stratosphere, where ∼60% of SSWs are preceded by extreme wave activity at 100 hPa. Additional analysis characterises events that do or do not appear to subsequently impact the troposphere, referred to as downward and non-downward propagating SSWs, respectively. On average, tropospheric wave activity is larger preceding downward-propagating SSWs compared to non-downward propagating events, and associated in particular with a doubly-strengthened Siberian High. Of the SSWs that were preceded by extreme lower-tropospheric wave activity, ∼2/3 propagated down to the troposphere, and hence the presence of extreme lower-tropospheric wave activity can only be used probablistically to predict a slight increase or decrease at the onset, of the likelihood of tropospheric impacts to follow. However, a large number of downward and non-downward propagating SSWs must be considered (> 35), before the difference becomes statistically significant. The precursors are also robust upon comparison with composites consisting of randomly-selected tropospheric NAM events. The downward influence and precursors to split and displacement events are also examined. It is found that anomalous upward wave-1 fluxes precede both cases. Splits exhibit a near instantaneous, barotropic response in the stratosphere and troposphere, while displacements have a stronger long-term influence. Link to full paper: https://cims.nyu.edu/~gerber/pages/documents/white_etal-JC-submitted.pdf
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