Fluctuations of the relationship between ENSO and northeast Australian rainfall

It is well known that during an El Nino Southern Oscillation (ENSO) warm event, drought occurs in regions of northeastern (NE) Australia, leading to anomalously low annual rainfall. The present study explores fluctuations of this ENSO-rainfall relationship. It is found that the relationship tends to weaken when the linearly detrended global mean temperature is rising or particularly high, as in the period of 1931-45 period and since the late 1970s. Prior to a weakening, a correlation pattern of increased rainfall during El Nino events is seen first in northwestern Australia, then in eastern and southeastern Australia, and eventually in NE Australia. The 1931-45 period was particularly intriguing, when in terms of rainfall variability over NE Australia, the interannual ENSO-rainfall relationship went through a process of weakening, reversal, and rapid recovery. Features associated with the reversal are therefore examined and these features are: (1) the global background anomaly pattern (upon which internnal ENSO events operate) is ENSO-like; (2) ENSO sea surface temperature (SST) anomalies in tropical Pacific are weaker compared with those averaged over all ENSO events, whereas SST anomalies in the mid-to-high latitude Pacific (which have opposing polarity to those in tropical Pacific) are larger; (3) there is strong coherence between ENSO and variability in northern mid- to high-latitudes; and (4) the relationship that an El Nino event contributes to a warming anomaly of global mean SST weakens. Possible interrelationship among these features are discussed.