El Nino/southern oscillation modification to the structure of the monsoon and tropical cyclone activity in the Australasian region

Streamfunction and velocity potential fields, calculated from monthly averaged station data sets, are composited for ENSO and anti‐ENSO extremes in the Australasian region. Four January and four February data sets are used for each of the composites. Gridded sea‐surface temperature (SST) and highly reflective cloud (HRC) data are composited in a similar manner. These data are used to investigate the effect of the phase of ENSO on the Australasian summer monsoon and tropical cyclone activity in the region. Changes in the structure of the monsoon are found to be consistent with composited tropical cyclogenesis and cyclone track data. These effects extend to the north and west of Australia, in addition to the signal previously identified in the Queensland region. During ENSO phases, the summer monsoon trough is weak and displaced equatorward, vertical wind shear is reduced and warmer SSTs are found to the north‐west of Australia and in the central equatorial Pacific. Tropical cyclone activity is reduced to the north‐east of Australia, but increases to the north and north‐west of the continent. Owing to the associated changes in the steering flow, coastal crossings are enhanced in Western Australia and the Northern Territory, but suppressed in Queensland. In the anti‐ENSO composite, the reverse generally is observed. Features specific to the anti‐ENSO are that far fewer storms occur to the north of Australia and storms off the west coast tend to track off‐shore, persisting further south. This southward persistence also is evident on the east coast, but here the likelihood of coastal crossing is enhanced compared with the ENSO composite.