Naohisa Takagaki

Investigations of air–water momentum and heat transport at extremely high wind speeds are crucial. Three different types of wind wave tanks are used to develop a method for investigating transport in laboratories using a tank with removable solid or net bottom walls to suppress wind wave development. The wind profile and water–level fluctuations are measured by Pitot tubes, differential manometers, and wave gauges. The air-side friction velocities are estimated using the profile method. The friction velocities are damped in the cases with the removable solid or net bottom walls, because the wind wave suppression due to the bottom wall provides a small form drag that acts on the wind waves. Through the rearrangement of the previous wind and wave values measured in a Russian tank, the usefulness of the presented wind wave suppression method is demonstrated for future investigations of the air–water momentum and heat transport at extremely high wind speeds. Moreover, the method can be applied to clarify the effects of the fetch on the air–water momentum and heat transfer at extremely high wind speeds.

Drag coefficient on the ocean surface is determined by various studies based on different mechanisms, such as turbulence and wave breaking, closely related to wind speed. The global ocean datasets of wind speed are distributed by various temporal resolutions based on reanalysis, assimilation, and satellite data. Recently, the wind speed data with higher temporal resolution have been provided. Using 6-hourly and hourly wind datasets, the air-sea momentum fluxes were estimated by several drag coefficient models proposed by Large & Yeager (2009), Andreas et al. (2012), Takagaki et al. (2012), & Hwang (2018). The globally averaged annual mean air-sea momentum fluxes were derived from the different drag coefficient models. The maximum difference of the annual mean values among the models reaches approximately 30% of annual mean values. The meridional structure of zonally averaged annual mean air-sea momentum flux has double peak at relatively higher latitudes from 40°S/N to 60°N/S. At those peaks maximum difference among the models reaches more than 30% of the zonally averaged annual mean. In terms of differences in temporal resolution on the wind speed datasets on each grid, the differences between hourly and 6-hourly wind data became larger with decreasing average period. The maximum difference of 66.7% was recognized on daily mean. The large difference was remarkable in higher wind speed regions, such as typhoon’s paths in the western Pacific. The effects of wind variability on different temporal resolution datasets are significant for estimating the air-sea momentum flux.