Near-inertial oscillations in seasonal highly stratified shallow water

Abstract

The Yellow Sea (YS) is highly stratified seasonally due to the presence of the bottom cold water (BCW), also called the YS Cold Water Mass (YSCWM). Intermittent measurements from June 2009 to August 2014 show that the near-inertial oscillation (NIO) can be excited by synoptic weather systems during the presence of the YSCWM between March and October. The magnitude of the NIOs depends on both the wind work and the water stratification. Thus, extreme wind events, such as tropical cyclones, destratify the shallow water and are unfavorable for NIOs. In general, the NIO induced by individual synoptic system is weak, but the event occurred from 6 to 22 July 2009 excited by four extratropical cyclones can result in a near-inertial current speed of 0.3 ms−1. The average wind work during this event was 1.32 × 10−3 Wm−2, and estimates of 42.67 MW energy transferred from wind into the YS during this event. However, only 6.1%~8.4% of the wind work can be transited into the BCW interior. Budget analysis indicates most energy is dissipated to mix the highly stratified water rather than damped by the bottom friction. The NIO frequency blueshift in the vertical also indicates higher energy dissipation between different layers. The decline of the YSCWM weakens the vertical stratification and thus the growth of NIOs. The calculation of wind work depends on the computation of wind drag coefficient and the conversion from wind measured height to 10 m above surface. In general, there is about 10% difference on wind work by using different methods.