Mapping western boundary currents with high frequency (HF) radar

Abstract: 

In this two-part talk, Dr's Amandine Schaeffer and Matthew Archer from the Oceanography Lab (UNSW Mathematics and Statistics) present recent work using HF radar measurements in two western boundary currents (the Florida Current and the East Australian Current)

PART I – Matthew Archer
The Florida Current: Jet structure, meandering and velocity fluctuations observed with HF radar

Within the Straits of Florida, the strongly sheared Florida Current interacts with waves and wind to produce a complex environment that is difficult to forecast, and makes in situ data collection at the ocean surface challenging. For these reasons, HF radar has proven to be a powerful tool for studying the surface currents in this region. We present a description of the Florida Current mean jet structure and meandering, and a case study of a frontal instability along the cyclonic flank of the Florida Current. The operational long-term deployment of HF radar in the Straits has provided unprecedented spatial and temporal resolution surface velocity measurements with which to study the evolution of the Florida Current over time. The HF radar operates at 16.045 MHz along the Southeast Florida coastline, with a resolution of 20 min and 1 km in time and space, and an average range of 80 km. Understanding how the Florida Current is changing in time and space is important at a range of scales. Locally, frontal instabilities along the Current play a major role in the exchange of physical, biological and chemical properties between offshore and coastal waters. At the larger scale, the Current is of great importance to the North Atlantic Sverdrup circulation and global thermohaline flow.

PART II – Amandine Schaeffer
Characterising frontal eddies along the East Australian Current from HF radar observations

The East Australian Current (EAC) is the major feature of the ocean circulation along south-eastern Australia, however, little is known about the submesoscale frontal instabilities of this western boundary current. One year of surface currents measurements from HF radars, in conjunction with mooring and satellite observations, highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the EAC. Eddies were systematically identified using the geometry of the high spatial resolution (~1.5 km) surface currents, and tracked every hour. Cyclonic eddies were observed in average every two weeks, while only a few anticyclonic eddies occurred over a year. Amongst various forms of cyclonic structures, frontal eddies associated with EAC meanders were characterised by poleward advection speeds of 0.3-0.4 m/s, migrating as far as Sydney, 400 km south (based on satellite imagery). Flow field kinematics shows that eddies have high Rossby numbers and a strong impact on horizontal divergence and particle dispersion. Patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift. This is confirmed by subsurface measurements showing temperature uplift as much as 55 m over a day. While frontal eddies appear to be independent of local wind stress, upfront wind can influence their growth and stalling, but also generate large cold core eddies through intense shear. Such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters, influencing physical and biological dynamics, and connectivity over large distances.