Impacts of ENSO on Australian rainfall: what not to expect

In eastern Australia we expect to experience wet conditions during La Nina and dry during El Nino events. We explore how well these expectations match historical outcomes by assessing, for spring, how much rain fell during past La Nina and El Nino events. We use a tercile framing and find that for rainfall averaged across eastern Australia, La Nina approximately doubles the chance of spring rainfall being in the wet tercile whereas El Nino approximately doubles the chance of a dry spring. Also of note is that during La Nina, the dry tercile is mostly vacant and during El Nino, the wet tercile is mostly vacant, indicating that one should not expect dry conditions in La Nina or wet in El Nino for eastern Australia as a whole. At individual locations across Australia, the results vary, and in some cases, including the eastern seaboard, La Nina or El Nino events do not change the odds of wet and dry springs significantly beyond chance expectations. For example, in the Sydney region, the normal chance of experiencing a wet tercile spring is 33% and this increases only slightly in a La Nina to 38%, suggesting that La Nina is not a strong indicator for wet conditions in this region. These outcomes may help to manage our expectations for the likely rainfall outcomes during future El Nino-Southern Oscillation (ENSO) events.