Sydney Harbour: Seiches, tides and mean circulation

Abstract

Beginning with observations from 1901, sea level elevations and currents from Sydney Harbour are examined across a broad frequency range. The mean currents, annual components of sea level, tides and seiches, mainly in the South Arm, are the focus. Tidal and mean currents are ~0.01 m s-1. The general circulation is estuarine-like with a thin, near-surface outflow layer and a thicker, deeper inflow. The distribution of contaminants in bottom sediments suggests the circulation, though weak, plays a retentive role in the Arm, transporting sediments towards its head. Analysis of 11-years of sea level data indicates a strong annual cycle, more energetic during winter than from late spring to early fall. The increased energy occurred at all frequencies except for tides. Seiches, with periods of ~0.5 to 2 h, emerge as a strong contributor to sea level and currents. The distributions of elevation and flow amplitudes associated with seiches were derived. With maximum observed values of 0.74 m and 0.24 m s-1, seiche displacements and currents can exceed those associated with tides and the mean circulation. While earlier studies identified only the dominant fundamental seiche mode, recent sea level data sampled at 1-minute show that modes 2-4 occur. Keywords: Sydney Harbour, sea level, mean circulation, tides, seiches