A comparison of low pressure system statistics derived from a high resolution NWP output and three re-analysis products over the Southern Ocean

Low pressure system statistics derived from the Antarctic Mesoscale Prediction System (AMPS) and NCEP, NCEP2 and JRA25 re-analysis products are compared by using an automatic cyclone tracking scheme. Since AMPS model spatial and temporal resolutions are much higher than the ones of the re-analyses, this study provides a valuable insight of the ability of numerical models to simulate low pressure systems with an increasing resolution. Results based on AMPS data agree relatively well with re-analyses based results when looking at the seasonal variability and spatial patterns of low pressure system properties over large scales. Results differ systematically close to the Antarctic ice sheet, where the horizontal resolution is important in resolving the continental topography. This appears as lower AMPS system densities and smaller system sizes close to the Antarctic coast. Results differ most during winter when the correct parametrization of surface energy balance is crucial over the sea-ice covered ocean. AMPS data show more systems at around and south of 60◦S in the Antarctic Circumpolar Trough, where re-analyses display systems of larger size. Earlier studies of cyclonic systems over the Southern Ocean show that there is a spectrum of atmospheric systems, where small synoptic systems merge into meso-scale lows. Accordingly a high resolution model setup with appropriate physics parametrization, like AMPS, is required to generate small systems. This study highlights that when new data become available it is important to update low pressure system statistics to gain a better understanding of high latitude processes over intermediate scales.