Australian Rainfall Anomalies in 2018–2019 Linked to Indo‐Pacific Driver Indices Using ERA5 Reanalyses

Abstract

AbstractThe 2019 and 2018–2019 periods had record low All‐Australia rainfall in both observations and the ECMWF's Reanalysis 5, or ERA5, data set over 1979–2019. An analysis of the relationships between interannual variability of rainfall and atmospheric circulation, vertically integrated moisture flux, and temperature anomalies using ERA5 alone is undertaken. Both standard driver indices and those that combine the Pacific and Indian Ocean influences are used. Regression fields for low annual Australian rainfall show a widespread negative rainfall anomaly extending into the east Indian Ocean or IND region and a positive anomaly in the western Pacific PAC region. A moisture flux anomaly takes moisture eastwards toward PAC. This pattern largely persists in the four seasons. In March–May 2019 it can be partially linked to a positive Niño 3.4 anomaly and in June–August to a positive Indian Ocean Dipole. For the annual case, the detrended Niño 4 index explains 24% of the 2019 ERA5 Australian rainfall deficit. This rises to 38% for the 2018–2019 deficit (46% for observations), for the Pacific‐Indian Dipole index that combines the PAC and IND regions, whose sea surface temperature anomalies have opposite effects. The correlation of the index with 2‐year rainfall is −0.7. There is much variability, in the Australian monsoon rainfall especially, that is not linked to simple indices, although rainfall is well matched with moisture flux convergence. This extends to the equatorial zone where the climate of the Maritime Continent can be better examined with ERA5's 0.25° data.