An Observational Study of the Impact of Gustiness on Air-Sea Momentum Fluxes and Waves Using a Novel Gustiness Metric

Abstract

As an integral component of the Earth system, the exchange of momentum at air-sea interface plays an important role in connecting the ocean and atmosphere. Thus, the parameterization for air-sea fluxes is essential for accurate global ocean circulation, wave growth, and climate and weather (long/short scale) models. It will be shown that gustiness (the fluctuation of wind speed and direction) enhances the momentum flux and waves. Hence, we will capture the characteristics of gustiness and quantify its impacts on momentum flux which is essential for the development of robust climate and weather models. Two unique, high quality observational data sets are used to explore the spatial and temporal variability of gustiness, to characterize its form, and quantify its impact on the air-sea boundary. The first data set was collected during the Impact of Typhoons on the Ocean in the Pacific (ITOP) experiment, which includes concurrent measurements of high-frequency wind vector, waves, and bulk met-ocean parameters. We define a new gustiness index which considers the influences of changing wind speed and direction both, and show that gusty runs have significant anisotropic characteristics, which lead to strengthened turbulence and alter the spectral properties of turbulence. The second data set was collected during the Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) experiment. Here, an array of anemometers between 3 and 16 m is used to study the vertical variability of gustiness. We demonstrate that vertical component of wind speed has more energy under gusty conditions which increases with height. We also show that the dynamical mechanism of gustiness is related to an increase in energy. This energy originates in low frequencies and cascades to the wave scales and contributes to the momentum flux. Additional flux is shown to come from a higher height, which is consumed rapidly by sea friction.

Finally, using both ITOP and CASPER data, the impact of gustiness on waves is explored. We separate the wave data into different wind speed and gustiness bins and calculate the enhancement factor of theoretical wave spectra within each bin. A linear relationship between enhancement factor and gustiness is found. We find that gustiness increases the energy of waves at the peak frequency and causes a wider distribution of directional wave energy. With gustiness increased, waves become more developed, and the wave spectrum converges upon the theoretical spectrum. Sea surface roughness is found to have a linear relationship with gustiness, which could explain how gustiness can increase the development of state of waves.