Essential climate variables (ECVs) encompass physical, chemical, or biological factors that characterize the climate system. The oceanic ECVs, precisely temperature and salinity, are well-defined for the sea surface and bulk water. The sea surface refers to the uppermost 1 mm oceanic boundary layer, also called the skin layer or sea-surface microlayer. However, the in situ skin temperature and salinity measurement presents a formidable challenge. Conventional methods involving direct observation and sampling from ships and buoys are hindered by integrity loss of the skin layer. In this context, we designed and constructed the research catamaran HALOBATES (Figure 1). The catamaran is named after a genus of marine insects, the only truly oceanic insects inhabiting the ocean’s surface and having remarkable capabilities to adapt to their unique habitat.

The autonomous functionality and fully automated sampling capabilities of HALOBATES facilitate high-resolution mapping of the skin layer and the near-surface layer (< 1 m). Sampling of the skin layer relies on a sampling assembly with several partially immersed and continuously rotating glass disks through the sea surface. HALOBATES is equipped with multiple Conductivity, Temperature, and Depth (CTD) sensors in a flow-through system, measuring both skin and bulk temperature and salinity, a total of 7 depths in the upper 1 meter. HALOBATES has the flexibility to integrate additional flow-through sensors and interchangeable payloads, making it adaptable for a wide range of scientific missions. Both atmospheric and oceanic forces influence the characteristics of the skin layer. Consequently, standard mission sensors include a wide range of meteorological sensors, and the Marine Sensor Systems group supports the operation of Acoustic Doppler Current Profilers (ADCPs).

HALOBATES offers two operational modes: manual control using a 2.4 GHz radio controller with a transmission range exceeding 5 km or autonomous operation following pre-programmed waypoints. The pilot team on the mother ship's deck receives real-time data, maintaining complete control through the telemetry system. With an endurance of 30 km at speeds of 2-3 km/h, HALOBATES enables continuous surveys of smaller areas or longer transects, particularly over mesoscale features like fronts and freshwater lenses, as illustrated in Figure 2.

This cutting-edge technology plays a pivotal role in understanding climate-related processes at the boundary layer between the ocean and the atmosphere. HALOBATES is an invaluable data source on Essential Climate Variables (ECVs) and other oceanographic variables within the thin skin and near-surface layer – the layers any vessel or buoy will destroy. Current scientific missions prioritize investigating the air-sea fluxes of freshwater (FreshOcean) and CO₂ (BASS SP1.5), small-scale convection as a link to underlying water layers (BASS – SP2.3), and validating satellite products related to ECVs (NorthSat-X).

Halobates in the German Bight (HE626); Photo: Samuel Mintah Ayim