Geostrophic Spirals Generated by the Horizontal Diffusion of Vortex Stretching in the Yellow Sea
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Horizontal velocity spirals with a clockwise rotation (downward looking) rate of 1.7° m−1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation, the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10−4 m2 s−1 on average. An eddy diffusivity of 2870 m2 s−1 is required for dynamic balance by estimating the residual term. This obtained value of 10−4 m2 s−1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m2 s−1, respectively, which supports a closed budget for velocity rotation.
Key wordsgeostrophic spirals horizontal diffusion vortex stretching and surface cooling effect
基于2006年底至2007年初在西, 北黄海的水文观测, 发现海洋的地转流速存在明显的顺时针螺旋形态, 量值为1.7° m−1. 在本文中, 热成风关系和Beta螺旋理论用来研究该螺旋产生的机制问题, 研究发现, 涡拉伸水平耗散是可能导致该地转螺旋的主要诱因. 由于冬季层化很弱, 表层和底层的Ekman泵压辅之以地形导致的上升流速, 无法支撑地转流速的螺旋. 表面的冬季冷却效应和垂向混合被涡拉伸水平耗散所抵消而产生顺时针螺旋. 基于理查德森数估算的垂向混合率平均为10−4 m2 s−1. 为了维持螺旋系统, 需要水平涡耗散率2870 m2 s−1来平衡收支, 该量值与Klocker 和 Abernathey (2014)利用海表面高度计估算的量值极为相近, 支持了螺旋的平衡机制研究.
关键词地转螺旋 涡拉伸 水平耗散 冷却效应
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The authors made equal contributions to this paper. D. WU and F. QIAO chaired the observations in the Yellow Sea and revised the draft. R. X. HUANG and X. SONG analyzed the data and wrote the paper. Mr. Guansuo WANG ran the realistic GCMs and set the sensitivity experiments for the analysis of dynamics. The crews of Science 1 and Dong Fang Hong 2 are acknowledged for their cruises. Special thanks to Dr. Andreas KLOCKER for his generous share of their mesoscale eddy diffusivity estimates, which provided solid support for the rotation budget. Finally, we appreciate that Drs. Shanhong GAO and Yue WANG provided their high-resolution model results of the Yellow Sea. This research is funded by the National Natural Science Foundation of China (Grant Nos. 41306003 and 41430963), the Fundamental Research Funds for Central Universities (Grant Nos. 0905–841313038, 1100–841262028 and 0905–201462003), the China Postdoctoral Science Foundation (Grant No. 2013M531647), and the Natural Science Foundation of Shandong (Grant No. BS2013HZ015). Other projects, including OAFlux+ISCCP, CFSR, MERRA, ERA-Interim, CORE.2, QuikSCAT and AVISO, are also acknowledged for the dynamic calculations in this paper. We appreciate the constructive comments and suggestions from the two anonymous reviewers, which helped improve the quality of the paper.
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