Intra-annual variability of the North West Shelf of Australia and its impact on the Holloway Current: Excitement and propagation of coastally trapped waves

Abstract

Shelf circulation on the North West Shelf (NWS) of Australia is dominated by seasonal variations of the alongshore Holloway Current forced by the monsoonal winds. However, the intra-annual variability of the current remains poorly understood. In this study, a combination of model outputs and two years of mooring time series data were used to analyse the intra-annual variability of the Holloway Current. The dominant intra-annual variability of the alongshore current on the NWS were at intraseasonal and semiannual frequencies, as revealed by sea level data. Whereas the interannual variability in the region is mostly forced remotely by tropical Pacific processes, the intraseasonal and semiannual signals are mostly driven by variations of regional winds off the northern coast of Australia. Phase analysis of sea level anomalies indicated that the excitation and propagation of Coastally Trapped Waves (CTWs) are forced by Madden-Julian Oscillations (MJO; intraseasonal) and semiannual wind anomalies. Intraseasonal CTWs are excited over the eastern parts of the Kimberley shelf, whereas semiannual waves are generated further to the east, over the northern Australian coast. The phase speed of intraseasonal CTWs is ~5.5 ms−1, characteristic of the first CTW mode, and 2.5 ms−1for semiannual CTWs, characteristic of the second CTW mode. The fluctuations in the velocity field over the NWS translates into significant alongshore current transport anomalies, ranging on average from ~0.4 Sv to ~0.6 Sv, for intraseasonal and semiannual frequency, respectively. Southwestward (northeastward) intraseasonal transport anomalies peak during Phase 1 (Phase 5) of the MJO cycle (out of the 8 phases of MJO) on the Kimberley shelf, whereas the corresponding transport anomalies on the Pilbara shelf peak during Phase 3 (Phase 7). Semiannual southwestward (northeastward) transport anomalies on the Kimberley shelf peak in April and October (January and July), preceding those on the Pilbara shelf by one month.