Intraseasonal Variability of Cross-Slope Flow in the Northern South China Sea

Abstract

Cross-slope flow plays an important role in the exchange of material, heat, and momentum between the continental shelf and the open sea. In the northern South China Sea (SCS), long-period observations confirm that there is significant cross-slope flow. The variability of this flow is dominated by the intraseasonal component (i.e., the 10–90-day period band) that contributes 74.6% of the total standard deviation. The 10–90-day bandpassed cross-slope flow exhibits almost the same direction vertically in the observed layers, and its first empirical orthogonal function, whose direction is also not changed, contributes 86.7% to its total variance. The strong 10–90-day bandpassed cross-slope flow is phase locked to the boreal winter half year. The intraseasonal variability of cross-slope flow is mainly associated with mesoscale eddies west to the Luzon Strait. The contrasting baroclinic instability growth rates, strong in winter and weak in summer, result in a seasonal cycle of mesoscale eddy kinetic energy, that is, vigorous in winter and weak in summer, which explains the winter phase lock. The interannual variability of baroclinic instability growth rate is mainly determined by the vertical shear of velocity. The strongest vertical shear of velocity from 2014 to 2016 occurred in the winter of 2016/17 and induced the most rapid baroclinic instability growth rate and consequently the largest mesoscale eddy kinetic energy, which resulted in the strongest intraseasonal variability of cross-slope flow. The vertical shear of velocity in the northern SCS is mainly determined by the Luzon Strait transport.