Two-dimensional mapping and tracking of a coastal upwelling front by an autonomous underwater vehicle

Coastal upwelling is a wind-driven ocean process. It brings cooler, saltier, and usually nutrient-rich deep water upward to the surface. The boundary between the upwelling water and the normally stratified water is called the “upwelling front”. Upwelling fronts support enriched phytoplankton and zooplankton populations, thus having great influences on ocean ecosystems. In our prior work, we developed and field demonstrated a method of using an autonomous underwater vehicle (AUV) to autonomously identify an upwelling front, map the vertical structure across the front, and track the front´s movement on a fixed latitude (i.e., one-dimensional tracking). In this paper we present an extension of the method for mapping and tracking an upwelling front on both latitudinal and longitudinal dimensions (i.e., two-dimensional) using an AUV. Each time the AUV crosses and detects the meandering front, the vehicle makes a turn at an oblique angle to recross the front, thus zigzagging through the front to map it. The AUV´s zigzag tracks alternate in northward and southward sweeps, so as to track the front as it moves over time. From 29 May to 4 June 2013, the Tethys long-range AUV ran the algorithm to autonomously detect and track an upwelling front in Monterey Bay, CA. The AUV repeatedly mapped the frontal zone over an area of about 200 km² in more than five days.