A Record Breaking Extratropical Cyclone in 2022: A Sign of Climate Change ?

In October 2022 a cyclone that deepened explosively over the South Pacific - the "Peter I Storm" - acquired a central pressure close to 900hPa, which by some margin is lower than has been seen before (in modern times) for a cyclone in either the northern or southern hemisphere extratropical regions. Whilst observation coverage in the area was sparse, the exceptional central pressure value was supported by operational numerical model output from both ECMWF and GFS models, and also by the ERA5 re-analysis. This talk will examine the evolution of the cyclone, that spanned more than 10 days: from its tropical origins near to Tonga to its final decay on the fringes of Antarctica. We pose the question: what might have contributed to the occurrence of such an extreme? Surface characteristics, such as roughness and sea ice coverage, and large-scale dynamics will be referenced. Operational ECMWF ensemble predictions will also be shown; these suggest that the event was remarkably predictable for lead times of 5 days and less. The ECMWF forecasts will also be compared with some modern data-driven AI-based forecasts, to examine if they were as good. 

The extreme event will then be placed in a climatological context, using ERA5, to reference questions about 'trends in extremes', examining also northern hemisphere behaviour, and comparing the cyclone's life-cycle with that of the northern hemisphere "record holder" from 1986. We will also cross-reference another extreme cyclone case from 1941, to further examine the question of re-analysis integrity in the face of sparse data coverage. It seems that the ERA5 representation of cyclones with extreme central pressures is quite robust. This is probably because such cyclones tend to have a large areal extent. The talk will conclude with contrasting remarks about representativeness for smaller cyclones, that can have particular relevance for high impact weather over Europe.