Response of wind stress and surface wave to the gusts under swell and wind-wave conditions

Wind gusts play an important role in the modulation of wind stress and have remarkable influence on surface waves. The response of wind stress and surface wave to wind gusts is investigated under wind-wave and swell dominated conditions through direct flux measurements on two coastal towers in the Bohai Sea and South China Sea. Observations show that wind stress significantly increases under gusty conditions compared to non-gusty conditions whether dominated by wind-waves or swells. The stronger the gusts, the greater the increase in wind stress. However, surface waves show different responses to the gusts under wind-wave and swell dominated conditions. When wind-waves dominate, the stronger the gust, the higher the significant wave height, while when swells dominate, the relatively lower the significant wave height. The separation of wind-wave and swell spectra indicates gusts can increase the wind-wave energy and decrease the swell energy. The weakening of swell and the strengthening of wind-wave may lead to an increase in wind stress.