Simultaneous Observations of Turbulent Reynolds Stress in the Ocean Surface Boundary Layer and Wind Stress over the Sea Surface

• Published in: Journal of geophysical research. Oceans 2021-02, Vol.126 (2), p.n/a
• Main Authors: Huang, Chuan Jiang, Qiao, Fangli
• Format: Article
• Language: English
• Subjects: eddy covariance; ocean surface boundary layer; turbulent Reynolds stress; wave‐turbulence decomposition; wind stress
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2020JC016839

This study used high‐frequency acoustic instruments mounted on an offshore observation platform to obtain simultaneous in situ measurements of sea surface winds and ocean currents. The acquired data were used to compare quantitatively the turbulent Reynolds stress in the ocean surface boundary layer (OSBL) and the wind stress over the sea surface. During the study period (∼10 d), the sea state was largely dominated by swell and breaking wind waves. The eddy covariance method was used to estimate the two stresses. A combination of the synchrosqueezed wavelet transform (SWT) method and a moving average was employed for wave‐turbulence decomposition of the oceanic velocities. Results showed that the turbulent Reynolds stress in the OSBL is affected greatly by surface gravity waves and that it scales with the wave characteristics. Direct comparison revealed that the turbulent Reynolds stress in the OSBL could be larger than the wind stress in certain circumstances, which differs from the traditional view that the former cannot be greater than the latter. In certain extreme cases, their ratio can be up to one order of magnitude. Therefore, we conclude that surface waves can enhance significantly the turbulent Reynolds stress in the OSBL. Plain Language Summary The traditional view is that turbulent Reynolds stress (or momentum flux) in the ocean surface boundary layer (OSBL) should be equal to or less than the wind stress over the sea surface. Using high‐frequency acoustic instruments installed on an offshore platform, we synchronously measured the winds and oceanic currents near the sea surface. We adopted the synchrosqueezed wavelet transform (SWT) and a moving average to decompose the wave and turbulent fluctuations in the measured oceanic velocities. In some cases, we found that the turbulent Reynolds stress in the OSBL was greater than the wind stress over the sea. Our observations indicated that this stress does not depend on the sea surface winds but increases with the increase of the significant wave height. Therefore, we believe that surface gravity waves could enhance significantly the turbulent Reynolds stress in the OSBL. Key Points Turbulent Reynolds stress in the ocean boundary layer is affected greatly by surface waves Ocean boundary layer turbulent Reynolds stress can be greater than wind stress over the sea surface A combination of the synchrosqueezed wavelet transform and a moving average filter was used for wave‐turbulence decomposition