Characterising the variability and change of the North Atlantic storm-track
PublisherUniversity of Reading
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The aim of this thesis is to identify recent inter-annual patterns of variability and long- term changes in the North Atlantic storm-track using ERA-40 reanalysis data. These results are also compared with variability in simulations by the newly developed Met Office Hadley Centre global coupled-model HadGEMl. The storm-track and storm activity is represented by both a variance and a feature- tracking analysis. The former involves (2-6 day) bandpass filtering the wind fields u',v' to calculate Transient Eddy Kinetic Energy. Feature-tracking involves the identification of positive vorticity (ε) extrema from a background field. These methods are found to be complementary. The analysis indicates that the leading inter-annual North Atlantic storm-track pattern (PI) over the ERA-40 study period involves meridional shifts of the storm-track exit region. This pattern is prominent in both the upper- and lower-troposphere and is found to predominantly be associated with shifts in the typical paths taken by medium and strong storms. PI appears to be unrelated to any key mean-flow teleconnection patterns such as the North Atlantic Oscillation (NAO). PI is strongly associated with local mean- flow changes to geopotential and temperature that support more intense storm activity, which indicates that this pattern is self-maintaining. Two less important patterns are identified in the TEKE teleconnection analysis which represent storm-track intensity in the west (P2) and east (P3) mid-latitude North Atlantic. There is clear evidence of a northward shift in medium and strong storm paths at lower-levels in ERA-40, whilst in the upper-troposphere the strongest storms have not shifted northwards, whilst medium storms have. The HadGEMI storm-track was found to have large decadal variability comparing two forty year periods, which in turn suggests that the ERA-40 results are perhaps just a 'snapshot' of North Atlantic storm-track behaviour. The inclusion of historic greenhouse gas forcings does not influence storm-track variability greatly in comparison with the internal variability of HadGEMl.