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Abstract: At the end of May 2008, a massive bloom of macroalgae occurred in the western Yellow Sea off China and lasted for nearly two months, and annual blooms have occurred since then on. During bloom period, the surface-drifting macroalgae have showed an interesting pattern dominated by a banded structure, and the distance between neighboring bands ranged from hundreds of meters to about 6 km with a peak at 1–1.5 km, which is an order of higher than the scale of Langmuir circulation of 50–100 m. In order to explain this new phenomenon, ocean current data obtained from a Doppler current profiler off Qingdao was used to implement stability analysis. By numerically solving the resulting differential Orr-Sommerfeld equation, the secondary circulation induced from the instability of the Emkan current was found to fit well with the observed spatial scale of the surface-drifting macroalgae’s banded structure. As the wind driven Emkan current exist universally in the global ocean, it is reasonable to conclude that the banded structure with kilometers distance between adjoining bands is ubiquitous. We found a new circulation in the upper ocean which is important for exchange of energy, materials and gas between the upper ocean and subsurface layer.
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Key words:
- banded structure /
- stability analysis /
- secondary circulation
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Figure 1. The banded structure of the drifting macroalgae on the sea surface. a. A photograph from a vessel on 6 July 2008 at (36°00′18″N, 120°29′48″E), the two bands of macroalgae are about 400 m apart; b. a photograph taken from an aircraft at 15:37 on 31 May 2008 off Qingdao; c. SAR images of the macroalgae obtained from COSMO-2 on 13 July 2008; and d. subfigure magnified from the box in c (after Qiao et al. (2009)).
Table 1. Values used in numerical experiments
Parameters Values Reynolds number (${{Re} }$) 100, 150, 200, 250, 300, 350, 400, 450 AH/AV ($\text{γ}$) 1:10:1000 Wave number ($a$) 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 Declination angle ($\theta $)/(°) –50, –45, –40, –35, –30, –25, –20, –15, –10, –5, 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 -
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