Calculation of Vertical Flux and Diffusivity due to Double Diffusion in the Tropical Western Pacific

Turbulence;Double diffusion;Vertical mixing;Tropical western Pacific;난류;이중확산;수직혼합;적도 서태평양;550

자연과학대학 지구환경과학부 ;

University:서울대학교 대학원

关键词: Turbulence;    Double diffusion;    Vertical mixing;    Tropical western Pacific;    난류;    이중확산;    수직혼합;    적도 서태평양;    550;   

Others  :  http://s-space.snu.ac.kr/bitstream/10371/131390/1/000000021046.pdf

美国|英语

来源: Seoul National University Open Repository

【 摘 要 】

This study aims to understand the impacts of double diffusion and turbulence on vertical mixing in thermocline (from 20 m to below 300 m depth) under weak turbulence condition based on 14-day time series observation of fine scale velocity and temperature at 0˚ N, 156˚ E in 2012. The observation allows direct estimates of temperature variance dissipation rate and turbulent kinetic energy dissipation rate. The weak turbulence condition appears to the result from the restricted vertical turbulent mixing due to westward expansion of fresher warm pool which was reported to occur in the western equatorial Pacific during the El Nino period. Vertical diffusivity and vertical flux of heat, salt, and density are calculated using the methods from McDougall and Ruddick [1992]. The Turner angle and the Reynolds number are used to classify observed regimes into neither turbulent nor double diffusive, turbulent, non-turbulent double diffusive, and turbulent double diffusive regimes. The results suggest a considerable contribution of double diffusion to vertical flux of heat and salt larger than that due to turbulence by a factor of 10 – 100.The magnitude of vertical flux of temperature and salinity due to salt fingering is estimated to be about 10-6 – 10-7 K m s-1 and 10-5 – 10-6 psu m s-1. The dominance of the double diffusive vertical flux results in downward total density flux of about 10-7 kg m-2 s-1 rather than upward flux which is the case for the turbulence-dominated vertical flux. Vertical density diffusivity due to double diffusion is 10-5 – 10-7 m2 s-1, which is comparable with that due to turbulence in 10 times greater turbulent dissipation condition. This study highlights the substantial contribution of double diffusion to vertical density flux in the tropical western Pacific under weak turbulence condition. Estimates of the vertical flux based only on turbulence would significantly underestimate the vertical density flux.