Spatial and temporal variability of heat, water vapor, carbon dioxide, and momentum air-sea exchange in a coastal environment

The spatial and temporal variability of heat, moisture, momentum, and CO2 turbulent fluxes in a coastal environment were assessed using simultaneous eddy correlation measurements from a tower, a boat, and an aircraft platform. The flux measurements were made using new instrument systems, including new CO2 and H2O sensors, low-flow distortion packages, and careful sensor motion correction systems. The flux tower was operated on the windward beach of Florida's Bahia Honda Key, while the boat was stationed upwind between 1 and 15 km offshore. The airplane flew transects 10 to 20 m above the ocean surface, along flight paths extending from the tower to 40 km offshore. Dissolved CO2 in the coastal waters and atmospheric CO2 concentrations were continuously measured throughout the experiment. The results indicate good agreement among the different sensing systems and demonstrate that air-to-sea trace gas, momentum, and energy flux density measurements are achievable from both a boat and an aircraft. Further, the observations emphasize the complex temporal and spatial trends possible in a coastal region. Due to the nonequilibrated boundary layer, generated by spatial trends, nearshore measurements are not representative of either the entire coastal region or of the open ocean. The observed 10 W m−2 sensible heat flux was time-invariant but did vary spatially with surface temperature, which was strongly correlated with ocean depth. The 100 to 200 W m−2 evaporative moisture flux dominated energy exchange and varied both in space and in time. No consistent diurnal variation was observed, but the spatial trend also followed surface temperature. As expected, momentum flux scaled with wind speed. CO2 exchange showed large spatial and temporal variance. Spatially, the surface temperature warmed as the shore was approached and CO2 was apparently driven off. Temporally, CO2 exchange varied greatly, as did ΔpCO2 which ranged from +50 to −100 ppm. This large variability in ΔpCO2 is thought to illustrate the importance of biological and physical processes and to explain the large CO2 fluxes observed (not always in the same direction) in this and other flux studies in nearshore conditions. Ten kilometers from shore, where the spatial and temporal variability was less, CO2 transfer velocities were of the order of 0.1 cm s−1.

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