Coupled atmosphere-ocean simulations of contemporary and future South Pacific tropical cyclones

Abstract. Tropical cyclones – TCs – affecting the South Pacific region are studied using coupled atmosphere-ocean earth system models and (offline) storm tracking software which tracks the position of simulated pressure lows through time. The models used are the United Kingdom Earth System Model, version 1 – UKESM1 – and the related New Zealand Earth System Model, the NZESM. The model pair considered here differ only in their treatment of the ocean and the NZESM has a nominal resolution of 0.2° in the region surrounding New Zealand and 1° elsewhere; UKESM1 has a a uniform 1° resolution everywhere. After validating the storm tracking algorithm against the track of cyclone Giselle from 1968 and cyclone Gabrielle from 2023 we use the Saffir-Simpson scale to split the tracked systems into categories based on their severity. For systems formed in the vicinity of New Zealand (and globally) the overall number is overestimated but stronger (category 2 and 3) storms are underestimated. We also see a general decrease in the total number of storms as radiative forcing, F, increases although there is some evidence of a small increase at extreme levels of warming. In the metrics studied here we find no difference between the ensembles of UKESM1 and NZESM simulations and going forward use the UKESM1, which has larger available ensembles. The power dissipation index, PDI, gives a first order measure of TC strength and we find that the average PDI per storm increases with F by up to 26 % under a 'fossil-fuelled development' scenario. Although the physical mechanisms behind the increase in average PDI with F are relatively simple to understand, those governing the frequency of occurrence are not. In the results shown here, vertical wind shear increases with F which tends to reduce TC numbers but the effect of the tropospheric relative humidity is much less clear. The increase in the area of the tropics bounded by the 26.5° isotherm should, on its own, increase the number of TCs, in opposition to the general behaviour observed, except perhaps at extreme levels of future warming.