Tropical Impact on the Boreal Winter Troposphere

In this thesis, different aspects of the tropical impact on the extratropical troposphere during boreal winter are analysed, using both reanalysis data and relaxation experiments in which selected regions of an atmospheric model are drawn towards reanalysis data. The first part analyses the development and possible origins of the anomalous atmospheric circulation during the severely cold European winter of 1962/63. It is found that, while the winter weather over Europe started getting severe by the end of December 1962, the tropical atmosphere was favourable for a tropospheric Rossby wave train that led to the negative regime of the North Atlantic Oscillation (NAO). It is suggested that this Rossby wave train was associated with a suppression of convective activity over the maritime continent, reflected by easterly anomalies in the zonal mean zonal wind in the upper troposphere along the equator (U150). Also, the Quasi-Biennial Oscillation (QBO) in the equatorial stratosphere was in its easterly phase, favourable for the sudden stratospheric warming (SSW) that was observed by the end of January, allowing the cold weather to persist until the beginning of March 1963. A possible influence towards a negative NAO by North Atlantic sea surface temperature (SST) is noted. In the second part of this thesis, variability of U150 and its influence on the extratropics are investigated. It is found that variability in U150 represents a leading mode of the zonal mean zonal wind in the tropical troposphere accounting for about 25% of the variability. U150 is found to be westerly when the Madden-Julian Oscillation (MJO) is active, especially when there is anomalous convection over the maritime continent and the western tropical Pacific. Easterly U150 occurs when the MJO is suppressed or the intertropical convergence zone is anomalously shifted to the south. Furthermore using both reanalysis data and ensemble mean data of an experiment with tropical relaxation towards reanalysis data, a northward shift of the Aleutian low is found to be associated with westerly U150 as well as with a wave train across the North Atlantic, originating in the subtropical eastern Pacific, resembling the wave train observed in January 1963 for the easterly phase of U150. Subtropical Rossby wave source anomalies near North America are identified as drivers for the Rossby wave train. The third part of this thesis investigates midlatitude blocking frequency variability in the northern hemisphere winter on interannual to decadal timescales. It is found that realistic tropical variability improves both climatology and variability of blocking frequency in the context of the forecast model of the European Centre for Medium-Range Weather Forecasts. Extratropical SST and sea-ice are found to further enhance the fraction of decadal blocking variability that can be accounted for. Stratospheric variability is not found to impact tropospheric blocking variability. Finally, the influence of several climate modes on midlatitude blocking is discussed. Overall in this thesis, the tropics are shown to exert strong influence on the extratropics, on weekly to decadal timescales