An advanced towed CTD chain for high resolution physical-biological in situ measurements in the upper ocean

Eddies, fronts, and filaments of varying scales populate the upper ocean and are particularly important in coastal regions. These features play a vital role on biogeochemical and mixing processes as well as in the energy budget. To capture their high spatial variability, it is desirable to simultaneously resolve the horizontal and vertical gradients of hydrographic properties on scales from O(10) m to O(100) km. We present an improved towed CTD chain for rapid quasi-synoptic in situ measurements of submesoscale oceanographic features to fill this observational gap. The advanced towed CTD chain is robust, lighter and scientifically more useful than previous versions. Added flexibility in terms of freely adaptable chain and sensor setup enables tailor-made surveys for a variety of research questions. The advanced towed CTD chain collects data at a very high horizontal resolution in O(1) m with a vertical resolution between 1 to 10 m, depending on CTD probe count and spacing. Individual CTD probes used within the chain are self-contained instruments equipped with temperature, conductivity, pressure and either fast response dissolved oxygen or fluorescence sensors placed at multiple depths enabling simultaneous hydrographic and biogeochemical studies at high resolution. With the flexible probe hardware it is possible to collect data either with real-time data visualisation for adaptive sampling missions or - in a much simpler and lighter setup - log data internally for offline evaluation. Together with the towed CTD chain a set of software tools and techniques for processing CTD chain data has been developed to provide an easy-to-use and complete system. Data examples collected in various areas like the Amazonas river plume and Cape Verde Island wake highlight the advanced CTD chains robustness, flexibility and scientific capabilities.