Enhanced vertical mixing by internal tides in the northern East China Sea in summer

Abstract

The East China Sea (ECS), a marginal sea of the North Pacific Ocean, is one of the most important regions for internal wave generation and associated turbulence mixing. Here we present observed evidence of intense turbulence dissipation near the thermocline and in the bottom boundary layer (BBL) in the northern ECS during summer, based on microstructure profile measurements in Augusts of 2005 and 2006. The 25-hour long timeseries measurement of the microstructure in the northern ECS in summer reveals that the enhanced turbulence dissipation rate along the thermocline occurs at almost every six hours, indicating influence of internal tide stirred by semi-diurnal tide (M2) that is dominant in the observed area. The intrusions of enhanced near-bottom turbulence into the upper layers are also observed. This intrusion, together with oscillation of the BBL height, strongly suggests influence of the semidiurnal tide (M2) on strong vertical mixing in the BBL. The elevated vertical mixing by the internal tide would enhance nutrients supply into the thermocline from the deep waters, and thus support elevated chlorophyll-a (Chl-a) that has been frequently observed within the thermocline in the northern ECS during summer. Our findings suggest that internal-tide induced vertical mixing may play an important role in nutrient budget and thus Chl-a concentration in the continental shelf in the northern ECS.r the thermocline and in the bottom boundary layer (BBL) in the northern ECS during summer, based on microstructure profile measurements in Augusts of 2005 and 2006. The 25-hour long timeseries measurement of the microstructure in the northern ECS in summer reveals that the enhanced turbulence dissipation rate along the thermocline occurs at almost every six hours, indicating influence of internal tide stirred by semi-diurnal tide (M2) that is dominant in the observed area. The intrusions of enhanced near-bottom turbulence into the upper layers are also observed. This intrusion, together with oscillation of the BBL height, strongly suggests influence of the semidiurnal tide (M2) on strong vertical mixing in the BBL. The elevated vertical mixing by the internal tide would enhance nutrients supply into the thermocline from the deep waters, and thus support elevated chlorophyll-a (Chl-a) that has been frequently observed within the thermocline in the northern ECS during summer. Our findings suggest that internal-tide induced vertical mixing may play an important role in nutrient budget and thus Chl-a concentration in the continental shelf in the northern ECS.