Satellite observations of the small-scale cyclonic eddies in the western South China Sea

F Liu,S Tang,C Chen

doi:10.5194/bg-12-299-2015

Abstract

Abstract. High-resolution ocean color observations offer an opportunity to investigate the oceanic small-scale processes. In this study, the Medium Resolution Imaging Spectrometer (MERIS) daily 300 m data were used to study small-scale processes in the western South China Sea. It is indicated that the cyclonic eddies with horizontal scales of 10 km are frequently observed during the upwelling season of each year over the 2004–2009 period. These small-scale eddies were generated in the vicinity of the southern front of the cold tongue, and then propagated eastward with a speed of approximately 12 cm s−1. This propagation speed was consistent with the velocity of the western boundary current. As a result, the small-scale eddies kept the high levels of phytoplankton rotating away from the coastal areas, resulting in the accumulation of phytoplankton in the interior of the eddies. The generation of the small-scale eddies may be associated with strengthening of the relative movement between the rotation speed of the anticyclonic mesoscale eddies and the offshore transport. With the increases of the normalized rotation speed of the anticyclonic mesoscale eddies relative to the offshore transport, the offshore current became a meander under the impacts of the anticyclonic mesoscale eddies. The meandered cold tongue and instability front may stimulate the generation of the small-scale eddies. Unidirectional uniform wind along the cold tongue may also contribute to the formation of the small-scale eddies.

Highlights

90 % of the kinetic energy of ocean circulation is contained in small-scale features, and 50 % of the vertical exchange of water mass properties between the upper and the deep ocean may occur at the submesoscale and mesoscale (Bouffard et al, 2012)
A series of small cyclonic phytoplankton tendrils at the southern edge of the phytoplankton filament were found during June and October of each year over the 2004–2009 period (Fig. 2)
The phytoplankton tendrils had a mean diameter of 25 km and obviously rotated out of the filament as the concentration variability of the phytoplankton tendril seemed consistent with the phytoplankton filament concentration variability

Summary

Introduction

90 % of the kinetic energy of ocean circulation is contained in small-scale features, and 50 % of the vertical exchange of water mass properties between the upper and the deep ocean may occur at the submesoscale and mesoscale (Bouffard et al, 2012). F. Liu et al.: Satellite observations of the small-scale cyclonic eddies processes requires higher space–time resolution of ocean color observation. It was reported that a phytoplankton filament in the western SCS is consistent with the mesoscale eddies transportation and Ekman upwelling (Tang et al, 2004; Xie et al, 2003; Xiu and Chai, 2011).

Results

Conclusion