Evaluating the 20th Century Reanalysis Version 3 with synoptic typing and an East Antarctic ice core accumulation record

Abstract

Weather systems in the southern Indian Ocean influence East Antarctic precipitation variability and surface mass balance. However, the long-term variability in synoptic-scale weather systems in this region is not well understood due to short instrumental records that are mostly limited to the satellite era (post-1979). Ice core records from coastal East Antarctica suggest significant decadal variability in snowfall accumulation, indicating that data from the satellite era alone are not enough to characterise climate variability in the high southern latitudes. It is therefore challenging to contextualise recent precipitation trends and extremes in relation to climate change in this area. We use synoptic typing of daily 500 hPa geopotential height anomalies and the Law Dome ice core (East Antarctica) annual snowfall accumulation record to investigate whether the 20th Century Reanalysis (20CR) project can represent the synoptic conditions associated with precipitation variability at Law Dome prior to the satellite era. We identify 12 synoptic types using self-organising maps (SOMs) based on their dominant pressure anomaly patterns over the southern Indian Ocean, with 4 types associated with above-average daily precipitation at Law Dome. Our results show the 20th Century Reanalysis project represents the meridional synoptic conditions associated with precipitation variability at Law Dome more reliably from 1948, aligning with the increased availability and thus assimilation of consistent surface pressure data from weather stations in the southern Indian Ocean from the late 1940s. This extends the time period available to contextualise recent trends and extremes in precipitation and synoptic weather conditions by up to 3 decades beyond the satellite era. Furthermore, we find a linear combination of the annual frequency in select synoptic types explains a significant amount of the variability in Law Dome snowfall accumulation compared to any individual synoptic type alone. These results will help future research on contextualising East Antarctic surface mass balance variability prior to the satellite era, with implications for improved understanding of the largest source of potential sea level rise, and the atmospheric conditions leading to decadal precipitation variability.