## 2 Data and Methods
### Saildrone  
Saildrones are zero-emission, solar powered, wind propelled marine vehicles produced by Saildrone Inc. with science-grade instrumentation. They have 12 sensors to measure data at the air-sea interface (e.g., SSS, SST, air temperature, 3D winds) and can carry one additional sensor on the keel (either an Acoustic Doppler Current Profiler (ADCP) or scientific echo-sounders). A Global Positioning System (GPS) and an onboard computer enables the vehicles to navigate following prescribed waypoints, while staying within  
Figure 2: The EUREC4A study areas (**A**) located within the “EUREC4A-Circle”, (**B**) within the ‘Tradewind Alley” between the Northwest Tropical Atlantic Station (NTAS) and the Barbados Cloud Observatory (BCO) and (**C**) the North Brazil Current region. The inset map shows the location of the EUREC4A study area within the Atlantic Ocean. The background cloud field is taken from the 5 February 2020 MODIS-Terra overpass. Reprinted from [PERSON] et al., 2021 [23]. a set corridor, taking winds and currents into consideration autonomously. Waypoints can be dynamically updated as environmental conditions change or interesting features develop. Vehicles are controlled and data transferred in near-real-time via two-way Iridium satellite communications. Saildrones can travel approximately 100 km per day, depending on wind speed, and can operate in remote regions where research vessels may be time- and cost-prohibitive. During the EUREC\({}^{4}\)A-OA/ATOMIC cruise, each saildrone vehicle measured salinity via the SeaBird-37-SMP-ODO Microcat and RBR CTD/ODO/Chl-A instruments at 0.5 m depth for 12 s, each minute. This study analyzed saildrone data from the three NASA-funded saildrones (SD1026, SD1060 and SD1061) deployed during the EUREC\({}^{4}\)A-OA/ATOMIC campaign, which operated during the period 17 January-2 March 2020 within the domain 7-13.5\({}^{\circ}\)N and 48-60\({}^{\circ}\)W (Figure 3). The saildrone data files collected at a 1 min temporal resolution over the duration of the measurement period include platform telemetry and near-surface observational data (see [[https://podaac.jpl.nasa.gov/dataset/SAILDRONE_ATOMIC](https://podaac.jpl.nasa.gov/dataset/SAILDRONE_ATOMIC)]([https://podaac.jpl.nasa.gov/dataset/SAILDRONE_ATOMIC](https://podaac.jpl.nasa.gov/dataset/SAILDRONE_ATOMIC)), accessed on 30 April 2021). The utilization of the saildrone platform represents a new and evolving trend in scientific sampling campaigns in which the data collection platform and process are subcontracted to Saildrone Inc. who is responsible for executing the experiment design and assuring the quality of the data. Hence, it is important to independently assess the accuracy of the data provided by the Saildrone operators. Figure 3: The tracks of the three NASA saildrones within the EUREC\({}^{4}\)A-OA study area. The start (17 January 2020) and end (2 March 2020) points of each track are shown by a circle and diamond, respectively. Bathymetry Data from The General Bathymetric Chart of the Oceans (GEBCO) (available at [[https://download.gebco.net/](https://download.gebco.net/)]([https://download.gebco.net/](https://download.gebco.net/)) (accessed on 27 June 2022) is shown behind the saildrone tracks. Preceding the EUREC4A-OA/ATOMIC campaign, saildrone-measured SSS data were validated in polar latitudes during the Innovative Technology for Arctic Exploration (ITAE) by [PERSON] et al. [25] and in the tropics during the Salinity Process in the Upper-ocean Regional Study 2 (SPURS-2) campaign in 2017 [26]. SSS for the saildrones during the polar and tropical campaign had a root mean squared difference (RMSD) of 0.01 and 0.0075 practical salinity unit (psu), respectively. Similar to our current study, SSS and SST data from saildrones were used to validate SMAP satellite derived SSS and SST datasets during a 2018 aja California campaign. Much like the NBC region in the present study, the California current system is dominated by submesoscale and mesoscale variability. [PERSON] et al.