Spatio-Temporal Variability of the Florida Current: HF Radar Observations in Stream Coordinates

The spatio-temporal variability of the Florida Current is investigated using a 2-year HF radar dataset in the Straits of Florida between 25°N to 26°N. The long-term deployment of HF radar in the region has provided unprecedented spatial and temporal resolution surface velocity measurements with which to study the evolution of this western boundary current. Our HF radar is a phased-array WERA system, operating at 16.045 MHz along the Southeast Florida coastline, with a resolution of 20-min and 1.2-km in time and space, and an average range of 80-km.

We have extended the traditional “stream coordinate” method to work with the 2-D HF radar dataset of surface current velocity. For each snapshot in time, velocity measurements in longitude/latitude are assigned a relative cross-stream position from the jet core, and rotated into a local, downstream coordinate frame. The flow field is reconstructed to form a grid of velocities relative to a new origin – the jet core. This method produces a realistic time average, because it is unaffected by lateral meandering; it is narrower and more intense, akin to the instantaneous flow field. With an improved mean, we improve our calculation of the fluctuations. The approach also provides a direct timeseries of the core’s offshore position (meandering). Temporal variability of jet intensity, width, and lateral shear is quantified for the first time. This method can be applied to any 2-D velocity dataset that encompasses a strong meandering jet.

Understanding the spatio-temporal variability of the Florida Current is important at a range of scales. Locally, frontal instabilities play a major role in the exchange of physical, biological and chemical properties between offshore and coastal waters. At the larger scale, the Florida Current is of great importance to the North Atlantic Sverdrup circulation and global thermohaline flow.