Subantarctic cyclones identified by 14 tracking methods, and their role for moisture transports into the continent

Extra-tropical cyclones in the subantarctic play a central role in the poleward
transport of heat and moisture into Antarctica, with the latter being a key
component of the mass balance of the Antarctic ice sheet. As the climate in this
region undergoes substantial changes, it is anticipated that the character of these
synoptic features will change. There are a number of different methods used to
identify and track cyclones, which can potentially lead to different conclusions as
to cyclone variability and trends, and mechanisms which drive these features.
Given this, it is timely to assess the level of consensus among 14 state-of-the-art
cyclone identification and tracking methods. We undertake this comparison with
the ERA-Interim dataset for the period 1979-2008 and find large differences in
the number of tracks identified by different methods, but the spatial patterns of
the system density broadly agree. Links between large-scale modes of variability,
such as the Southern Annular Mode (SAM), and subantarctic cyclones as
suggested in the literature are confirmed by our analysis.
Trends in the number of cyclone tracks show a more diverse picture. Robust
trends are identified by almost all methods for austral summer over the region
south to 60°S, mainly due to the strong relation to SAM, whereas in austral
winter the methods disagree in the statistical significance of the trends.
The agreement among the methods is greater when the comparison is confined to
the stronger cyclones. This is confirmed by a moisture flux analysis associated
with these strong synoptic systems. Our results indicate that multiple cyclone
identification and tracking methods should be used to obtain robust conclusions
for trends in cyclone characteristics as well as their relation to the large scale
circulation in the subantarctic region.