Abstract:
Despite its potential importance in the global climate system, mixing properties of the North Pacific low-latitude western boundary current system (LLWBC) remained unsampled until very recently. We report here on the first measurements of turbulence microstructure associated with these currents, made in the western boundary region of the tropical North Pacific east of the Philippines. The results suggest that thermocline mixing in the North Pacific LLWBC is generally weak with the diapycnal diffusivity κρ∼O(10−6) m2 s−1. This is consistent with predictions from internal wave-wave interaction theory that mixing due to internal wave breaking is significantly reduced at low latitudes. Enhanced mixing is found to be associated with a permanent cyclonic eddy, the Mindanao Eddy, but mainly at its south and north flanks. There, κρ is elevated by an order of magnitude due to eddy-induced geostrophic shear. Mixing in the eddy core is at the background level with no indication of enhancement.
Highlights:
• Thermocline mixing in the North Pacific low-latitude western boundary current system is overall very weak
• Thermocline mixing at the south and north flanks of the Mindanao Eddy was elevated by an order of magnitude due to eddy-induced shear
• The oft-used fine-scale parameterization of turbulence seems to generally overestimate thermocline mixing in the North Pacific LLWBC
Figure. (a) Comparison between microstructure measurements and fine-scale parameterization for the thermoclineaveraged diapycnal diffusivity (< 𝜅𝜌 >therm) at all the observation sites. Agreement within factors of 2 and 10 is designated by the gray bands. The 95% bootstrapped confidence intervals for the estimates from microstructure measurements are represented by bars. (b) Original (gray small symbols) and bin-averaged samples (large open symbols) of the diapycnal diffusivity (𝜅𝜌) versus the gradient Richardson number (Ri) for all the estimates in the thermocline. The bold line shows the analytical approximation discussed in the text with the 95% confidence intervals indicated by gray shading.
Citation: Zhiyu Liu*, Qiang Lian, Fangtao Zhang, Lei Wang, Mingming Li, Xiaolin Bai, Jianing Wang, Fan Wang. Weak Thermocline Mixing in the North Pacific Low-Latitude Western Boundary Current System. Geophysical Research Letters, 2017, DOI: 10.1002/2017GL075210.
More to read (full text): http://onlinelibrary.wiley.com/doi/10.1002/2017GL075210/full
Dr. Zhiyu Liu’s home page: http://mel.xmu.edu.cn/teacherfile.asp?tid=343
