Taxwedel, Sven (2004) Zonale Energieflüsse über dem Nordatlantik. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 120 pp.

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Abstract

In this study the mean tropospherical total zonal energy fluxes based on the NCEPNCAR reanalyses data are calculated for the time period 1948-2003. The interannual wintertime variability and the vertical structure of the fluxes are analysed. Since the dominant mode of atmospheric variability, the North Atlantic Oscillation, modulates the mean zonal current over the northern Atlantic region on monthly timescales, the NAO is expected to be the main cause for the interannual variability of energy fluxes. This study aims to analyse the linkage between this variabilty of zonal energy fluxes and the NAO. If the interannual variability of zonal energy transport directed to the european continent is associated with the NAO, the interannual energy flux variability can be referred to the variability of atmospheric circulation. In the first section, a climatology shows the three-dimensional structure of the mean zonal energy fluxes and its interannual seasonal standard deviations. In the second section, the dominant patterns of variability in various cross-sections parallel to the circles of longitude are obtained by EOF-analysis. The two leading EOF-patterns show barotropic structures, which in general agree in all sections. The 1.EOF explains more than 50% of the variance, the 2.EOF 25%. There exists however a significant correlation between the principle components only of the first EOFs of the western and eastern cross sections. The connection between zonal fl ux anomalies and the NAO yields a di polar correlationstructure, which is comparable to the correlation-pattern between the NAO and the zonal wind. The barotropic feature is predominant in each cross-section, moreover the regression-patterns show an eastward increasing tilt of the vertical axis towards north with a clear downward propagation of the northern center of activity from the middle troposphere to the lowest levels over Europe. It is shown that the NAO explains more than 30% of the total variance of interannual variability of the seasonal zonal energy fluxes, over the central Atlantic region even more than 40%, whereby anomalies reach up to 30% of the seasonal mean fluxes during extreme NAO-phases. Subsequently, flux anomalies associated with the Niño3-SST are analysed to clarify the dependence upon ENSO. In western parts of the Atlantic region, the Niño3-SST explains less than 5% of the variability of the zonal energy fluxes. Correlations between them are widely non significant in the analysed area. Nevertheless, strong El Niño events tend to cause stronger flux anomalies. In the last section, vertically integrated energy fluxes through the western and eastern boundary of the Atlantic region (represented as 60°W and 0°W longitudes) are computed. Fluctuations of the total energy flux through these sections are proved to be highly correlated with the NAO. The zonal convergence of flux anomalies between the boundaries show weak linkages to NAO, so that meridional atmospheric energy transport anomalies associated with NAO in the Atlantic region are assumed.

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