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- ItemTropospheric forcing of the boreal polar vortex splitting in January 2003(München : European Geopyhsical Union, 2010) Peters, D.H.W.; Vargin, P.; Gabriel, A.; Tsvetkova, N.; Yushkov, V.e dynamical evolution of the relatively warm stratospheric winter season 2002–2003 in the Northern Hemisphere was studied and compared with the cold winter 2004–2005 based on NCEP-Reanalyses. Record low temperatures were observed in the lower and middle stratosphere over the Arctic region only at the beginning of the 2002–2003 winter. Six sudden stratospheric warming events, including the major warming event with a splitting of the polar vortex in mid-January 2003, have been identified. This led to a very high vacillation of the zonal mean circulation and a weakening of the stratospheric polar vortex over the whole winter season. An estimate of the mean chemical ozone destruction inside the polar vortex showed a total ozone loss of about 45 DU in winter 2002–2003; that is about 2.5 times smaller than in winter 2004–2005. Embedded in a winter with high wave activity, we found two subtropical Rossby wave trains in the troposphere before the major sudden stratospheric warming event in January 2003. These Rossby waves propagated north-eastwards and maintained two upper tropospheric anticyclones. At the same time, the amplification of an upward propagating planetary wave 2 in the upper troposphere and lower stratosphere was observed, which could be caused primarily by those two wave trains. Furthermore, two extratropical Rossby wave trains over the North Pacific Ocean and North America were identified a couple of days later, which contribute mainly to the vertical planetary wave activity flux just before and during the major warming event. It is shown that these different tropospheric forcing processes caused the major warming event and contributed to the splitting of the polar vortex.
- ItemLongitude-dependent decadal ozone changes and ozone trends in boreal winter months during 1960-2000(Göttingen : Copernicus, 2008) Peters, D.H.W.; Gabriel, A.; Entzian, G.This study examines the longitude-dependent decadal changes and trends of ozone for the boreal winter months during the period of 1960–2000. These changes are caused primarily by changes in the planetary wave structure in the upper troposphere and lower stratosphere. The decadal changes and trends over 4 decades of geopotential perturbations, defined as a deviation from the zonal mean, are estimated by linear regression with time. The decadal changes in longitude-dependent ozone were calculated with a simple transport model of ozone based on the known planetary wave structure changes and prescribed zonal mean ozone gradients. For December of the 1960s and 1980s a statistically significant Rossby wave track appeared over the North Atlantic and Europe with an anticyclonic disturbance over the Eastern North Atlantic and Western Europe, flanked by cyclonic disturbances. In the 1970s and 1990s statistically significant cyclonic disturbances appeared over the Eastern North Atlantic and Europe, surrounded by anticyclonic anomalies over Northern Africa, Central Asia and Greenland. Similar patterns have been found for January. The Rossby wave track over the North Atlantic and Europe is stronger in the 1980s than in the 1960s. For February, the variability of the regression patterns is higher. For January we found a strong alteration in the modelled decadal changes in total ozone over Central and Northern Europe, showing a decrease of about 15 DU in the 1960s and 1980s and an increase of about 10 DU in the 1970s and 1990s. Over Central Europe the positive geopotential height trend (increase of 2.3 m/yr) over 40 years is of the same order (about 100 m) as the increase in the 1980s alone. This is important to recognize because it implies a total ozone decrease over Europe of the order of 14 DU for the 1960–2000 period, for January, if we use the standard change regression relation that about a 10-m geopotential height increase at 300 hPa is related to about a 1.4-DU total ozone decrease.
- ItemUtility of Hovmöller diagrams to diagnose Rossby wave trains(Abingdon : Taylor & Francis, 2011) Glatt, I.; Dörnbrack, A.; Jones, S.; Keller, J.; Martius, O.; Müller, A.; Peters, D.H.W.; Wirth, V.The study investigates and compares various methods that aim to diagnose Rossby wave trains with the help of Hovmöller diagrams. Three groups of methods are distinguished: The first group contains trough-and-ridge Hovmöller diagrams of the meridional wind; they provide full phase information, but differ in the method for latitudinal averaging or weighting. The second group aims to identify Rossby wave trains as a whole, discounting individual troughs and ridges. The third group contains diagnostics which focus on physical mechanisms during the different phases of a Rossby wave train life cycle; they include the analysis of eddy kinetic energy and methods for quantifying Rossby wave breaking. The different methods are analysed and systematically compared with each other in the framework of a two-month period in fall 2008. Each method more or less serves its designed purpose, but they all have their own strengths and weaknesses. Notable differences between the individual methods render an objective identification of a Rossby wave train somewhat elusive. Nevertheless, the combination of several techniques provides a rather comprehensive picture of the Rossby wave train life cycle, being broadly consistent with the expected behaviour from previous theoretical analysis.