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About sebastiaan1973

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  1. sebastiaan1973

    Teleconnections: A More Technical Discussion

    Good thinking. My compliments. So, what do you think are the implications in Western Europe?
  2. sebastiaan1973

    Teleconnections: A More Technical Discussion Wintertime high-latitude blocking is associated with persistent changes in temperature and precipitation over much of the Northern Hemisphere. Studies have shown that the Madden–Julian oscillation (MJO), the primary form of intraseasonal tropical variability, significantly modulates the frequency of high-latitude blocking through large-scale Rossby waves that alter the global circulation. However, the characteristics of MJO teleconnections are altered by El Niño–Southern Oscillation (ENSO), which modifies the global flow on interannual time scales, suggesting that the MJO influence on blocking may depend on the ENSO phase. The characteristics of MJO Rossby waves and blocking during ENSO events are examined using composite analysis and a nonlinear baroclinic model. The ENSO phase-dependent teleconnection patterns are found to significantly impact Pacific and Atlantic high-latitude blocking. During El Niño, a significant persistent increase in Pacific and Atlantic blocking follows the real-time multivariate MJO (RMM) phase 7, characterized by anomalous enhanced tropical convection over the East Indian Ocean and suppressed west Pacific convection. The maximum Atlantic blocking increase is triple the climatological winter mean. Results suggest that the MJO provides the initial dipole anomaly associated with the Atlantic blocking increase, and transient eddy activity aids in its persistence. However, during La Niña significant blocking anomalies are primarily observed during the first half of an MJO event. Significant suppression of Pacific and Atlantic blocking follows RMM phase 3, when east Indian Ocean MJO convection is suppressed and west Pacific convection is enhanced. The physical basis for these results is explained.
  3. sebastiaan1973

    ***Winter Countdown Thread 2018-2019***

    Well it's quite a contradiction to Glosea5, which has an good reputation for forecasting the NAO. Altough we have to wait another week for the final winter NAO-forecast. E.g.
  4. sebastiaan1973


  5. sebastiaan1973

    Stratospheric Discussion and Forecasting

    That's okay.
  6. sebastiaan1973

    Stratospheric Discussion and Forecasting

    Recently KNMI released this report Quite an intresting read. Summary In 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 34 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.