Australian east coast mid‐latitude cyclones in the 20th Century Reanalysis ensemble

Abstract

Extratropical and subtropical cyclones, and their associated fronts, produce the majority of rainfall and extreme weather in the mid‐latitudes around the world. In the Southern Hemisphere in particular, where observations are sparse, we have limited knowledge of their long‐term variability and trends. While the long time period of the 20th Century Reanalysis (20CR) offers potential to assess longer‐term changes, the lack of observational data sets makes it difficult to assess its reliability. We assess the skill of the 20CR at representing mid‐latitude cyclones through a case study on the east coast of Australia. In this region, a comprehensive database has recently been developed of all severe floods between 1871 and 2012, of which >70% are associated with a coastal low‐pressure system. Through use of the full 20CR 56‐member ensemble, we are able to identify the majority of severe Australian east coast lows as early as the late 19th century, as well as recreate the cyclone tracks for some of the most severe events. The analysis shows that the full ensemble can be used for a long‐term assessment of interannual variability in cyclone frequency from at least 1911, despite large inhomogeneities in the ensemble mean pressure fields.

Identifying East Coast Lows with climate hazards on the eastern seaboard

“…Consequently, while pre-1979 reanalyses should always be used with caution, the NCEP1 reanalysis is not expected to induce large discrepancies in the ECL database. This is consistent with results found for midlatitude cyclones using the 20th Century Reanalysis (Wang et al 2013, Pepler et al 2016, with a clear step-change in southern hemisphere cyclones around 1950 but only a small decline in the 1950-2012 period.…”

“…However, it is one of the few reanalyses for which a long period of analysis is available, with known issues with detection of cyclones in the 20th Century Reanalysis ensemble mean (e.g. Wang et al 2013, Pepler et al 2016. The NCEP1 reanalysis is of a coarser resolution than more modern reanalyses, which may result in a failure to detect smaller-scale systems, or underestimation of their intensity and the structure of the inner core.…”

Temperature and precipitation extremes in century‐long gridded observations, reanalyses, and atmospheric model simulations

“…[] highlight consistency in extreme pressure gradients between 20CR and observations over the NE Atlantic, and despite large inhomogeneities in the ensemble mean pressure fields, Pepler et al . [] showed that the full ensemble could be used for a long‐term assessment of midlatitude cyclone frequency on the east coast of Australia. For temperature extremes over North America, there is reasonably good agreement between 20CR and gridded station observations throughout the entire twentieth century [ Donat et al ., ].…”

Synoptic climatology of hybrid cyclones in the Australian region

“…Of special interest in Australia, is the East Coast Low (ECL) region since cyclones developing in this area can cause major damage to the coastline of eastern Australia. The ECL region is bounded by the Australian coast, 161°E, 24°S, and 41°S (Pepler et al () give details). During MJJAS 1979–2010, 61 HCs and 51 NHCs reached their peak intensity in this region.…”

“…That a large fraction of HCs develop in this region raises the question as to how many ECLs are also HCs. Taking the ERA‐Interim‐based objective dataset of Pepler et al (), the cyclone phase space diagnostic is applied to all ECLs. The main finding is that 46% of all ECLs during MJJAS fulfil the HC criteria defined in section 2.2 at least once during their life cycle.…”