Over the ocean, direct measurement of the turbulent fluxes is very difficult due to platform motion, flow distortion, and the effects of sea-spray. Instead, marine meteorologists and oceanographers have long relied on flux-profile relationships that relate the turbulence fluxes of momentum, heat and moisture (or mass) to their respective profiles of velocity, temperature, and water vapor (or other gases). These flux-profile relationships are required in indirect methods such as the bulk aerodynamic, profile, and inertial dissipation methods that estimate the fluxes from mean, profile, and high frequency spectral measurements, respectively. The flux-profile or flux-gradient relationships are also used extensively in numerical models to provide lower boundary conditions and to "close" the model by approximating higher order terms from low order variables.

The most commonly used flux-profile relationships are based on Monin-Obukhov (MO) similarity theory. MO similarity has been validated by a number of overland experiments including the landmark Kansas, Minnesota, and ITCE experiments in the 1970s. These and other experiments have generated a number of similar semi-empirical functions that are used in the indirect methods over the ocean. However, the use of overland measurements to infer surface fluxes over the ocean is questionable, particularly close to the ocean surface where wave-induced forcing can affect the flow. Therefore, the universality of these relationships to all surface layers is a current topic of intense debate.

Direct measurement of the atmospheric fluxes along with profiles of water vapor and temperature were made during the 2001 GASEX experiment in the equatorial Pacific. The measurements were made from the R/V Brown at the end of a boom that placed the sensors 10-m upwind of the bow. Turbulent fluxes of momentum, heat, and water vapor were made by sonic anemometers/thermometers and infrared hygrometers. A mast at the end of the 10-m boom supported a profiling system that moved a suite of sensors between 3 and 12 meters above the mean sea level. The moving sensors were referenced against a fixed suite of sensors to remove naturally occurring variability during the profiling periods. Preliminary results show good agreement with commonly used parameterizations based on overland measurements. This indicates that the MO similarity functions are applicable over the ocean in conditions where the theory is applicable, i.e., in surface layers where the structure of the turbulence is dominated by the relative importance of mechanical (i.e., wind shear) versus thermal forcing.