Abstract
Dramatic climate changes have occurred in recent decades over the Arctic region, and very noticeably in near-surface warming and reductions in sea ice extent. In a climatological sense, Arctic cyclone behaviour is linked to the distributions of lower troposphere temperature and sea ice, and hence the monitoring of storms can be seen as an important component of the analysis of Arctic climate. The analysis of cyclone behaviour, however, is not without ambiguity, and different cyclone identification algorithms can lead to divergent conclusions. Here we analyse a subset of Arctic cyclones with 10 state-of-the-art cyclone identification schemes applied to the ERA-Interim reanalysis. The subset is comprised of the five most intense (defined in terms of central pressure) Arctic cyclones for each of the 12 calendar months over the 30-yr period from 1 January 1979 to 31 March 2009. There is a considerable difference between the central pressures diagnosed by the algorithms of typically 5–10 hPa. By contrast, there is substantial agreement as to the location of the centre of these extreme storms. The cyclone tracking algorithms also display some differences in the evolution and life cycle of these storms, while overall finding them to be quite long-lived. For all but six of the 60 storms an intense tropopause polar vortex is identified within 555 km of the surface system. The results presented here highlight some significant differences between the outputs of the algorithms, and hence point to the value using multiple identification schemes in the study of cyclone behaviour. Overall, however, the algorithms reached a very robust consensus on most aspects of the behaviour of these very extreme cyclones in the Arctic basin.
Highlights
Extratropical and high latitude cyclones play a key role in the weather and climate experienced over much of the globe, and baroclinic eddies are central factors in balancing the global energy and momentum budgets
These identification algorithms make different decisions, associated with the physics and numerics of finding and tracking of cyclones. It follows that the results of an analysis of a case study or a climatology may depend on the specific cyclone scheme that was used in the investigations
This analysis was undertaken for the same period and reanalysis as the surface cyclone identification, and the tests for vertical associations was undertaken with the technique described by Lim and Simmonds (2007)
Summary
Extratropical and high latitude cyclones play a key role in the weather and climate experienced over much of the globe, and baroclinic eddies are central factors in balancing the global energy and momentum budgets. We took as our extreme cyclone the one which was ranked 1 in the greatest number of schemes This consensus approach worked well, and there were only eight instances of a cyclone in a scheme being forced to be regarded as the deepest cyclone (i.e. it was not ranked most intense in all schemes) via the consensus and all of those never ranked lower than third (see the details to be discussed in Table 2.) The time when the selected cyclone for a given month reached its minimum central pressure varied slightly between the tracking algorithms, so the most frequent time was taken as the ‘actual’ time of cyclone minimum.
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