Underway Sampling

Underway sampling is continuous measurement of surface seawater and marine-atmosphere properties from a ship while it steams, drawing water through a hull intake to a flow-through analysis system. A thermosalinograph logs sea-surface temperature and salinity; added instruments measure partial pressure of CO2 (pCO2), fluorescence-based chlorophyll, oxygen, and nutrients, all geo-tagged by GPS. The Surface Ocean CO2 Atlas (SOCAT) is built largely from underway pCO2 lines on research and volunteer observing ships. Because it runs without stopping, underway sampling maps surface fronts and gradients at high along-track resolution that discrete station work cannot match. Intake warming and bubble contamination are the main error sources.