Seasonal variability in the continental shelf waters off southeastern Australia: Fact or fiction?

Abstract

Seasonality is an important timescale driving variability in the waters of many continental shelf regions globally. Along the east coast of Australia, it has been recognised that the East Australian Current (EAC), the Western boundary current (WBC) of the South Pacific gyre, warms and strengthens in the Austral summer. Thus it has been hypothesised that shelf currents also warm and strengthen (poleward) annually. However, the EACs highly dynamic nature results in large variations in the latitude of separation from the coast and eddy shedding. Until recently the lack of long term in-situ observations on the shelf has precluded a study into low frequency (seasonal) variability in shelf circulation. Using at least 3 years of moored in situ temperature and velocity observations we investigate low frequency variability in shelf waters at 2 cross-shelf locations (i) upstream and (ii) downstream of the typical EAC separation latitude. The local winds vary bi-modally upstream and tri-modally downstream varying with the passage of fronts, thus do not drive a seasonal response in the circulation. Harmonic analysis of the velocity and temperature fields shows that upstream of the separation zone, only 6% of the velocity variability occurs on the seasonal timescale, compared to 49% of the temperature variability. Cross shelf temperature gradients and vertical velocity shear increase in summer with an increase in poleward heat advection in the EAC. Downstream of the separation point the influence of episodic eddy encroachments precludes seasonality in the vertical structure of the flow despite an annual cycle in the stratification. The seasonal cycle in temperature moves out of phase with increasing depths, with maxima (minima) in March (September) at 30m compared to maxima (minima) in May (November) at the bottom. This is expected to have a large influence on the timing of nutrient injection onto the shelf, and thus phytoplankton species composition and abundance.