Hydroclimate extremes in a north Australian drought reconstruction asymmetrically linked with Central Pacific Sea surface temperatures

摘要

An understanding of tropical hydroclimate variability, the associated drivers and how it is likely to change is a major scientific and societal challenge that is acutely hampered by short instrumental records. We present a 246year tree-ring drought reconstruction of the Standardised Precipitation Evaporation Index (SPEI) for monsoonal northern Australia for the end of the wet season (March-May; MAM). This reconstruction extends the instrumental record back by 150 years. Around one third of total annual rainfall falls during MAM, making it a crucial component of the monsoonal cycle. MAM is also the season most impacted by the differential decay process of Central Pacific (as opposed to Western Pacific) El Nino events that are linked with dry conditions over northern and northwestern Australia more generally. Our reconstruction therefore provides an opportunity to consider how central Pacific variability has modulated MAM hydroclimate in Australia's monsoonal north over the past two and a half centuries. We found that MAM hydroclimate extremes in the region have a strong, but asymmetric relationship with central Pacific sea surface temperatures (SSTs) and ENSO indices. Extremely wet MAMs in monsoonal north Australia were associated with cooler SSTs, above average rainfall across much of Australia, and often coincided with La Nina events. The spatial relationship between dry extremes and Pacific SSTs during dry events was generally, but weakly, consistent with the SST signature of central Pacific El Nino events. The association between reconstructed dry extremes in the monsoonal north and dry conditions across the rest of Australia is also less extensive and weaker than for wet events. Results suggest that more extreme wet events in the Australian monsoonal north likely reflect cool central Pacific SSTs and later termination of the Australian monsoon. Consecutive years with extremely dry MAMs became more frequent over the latter part of the 20th Century while the probability of an extreme dry MAM followed by an extreme wet MAM the next year peaked in the mid 20th Century and has since declined.