Observations of strong near-inertial currents induced by the reflection of tropical cyclone-induced near-inertial waves on the continental slope of the northern South China Sea

Abstract

Near-inertial waves (NIWs) play an important role in determining oceanic energy and are highly related to oceanic dynamics, including tropical cyclones (TCs). However, most studies have focused on the features of NIWs in the upper layer, and the characteristics of NIWs near the seabed remain poorly understood. Based on current meter records from two moorings, we examined the characteristics of NIWs during the passage of four TCs, namely, Fengshen, Kammuri, Nuri and Hagupit, in the northern South China Sea (SCS). The two moorings were deployed on the continental slope, with one located in relatively shallow water, i.e., the top of the slope, and the other in relatively deeper water, i.e., in the middle of the slope. The data from the shallower mooring revealed that the near-inertial kinetic energy (NIKE) near the seabed was significantly enhanced during the passage of TCs Nuri and Hagupit. During the passage of these TCs, an elongated near-inertial current (NIC) ellipse was observed near the seabed with the major axis aligned perpendicular to the isobaths; these NICs were different from the slightly elliptical NICs observed at the deeper mooring. Considering the topographic slope around the shallower mooring, the NICs with different properties were caused by the reflection of NIWs near the seabed. The Richardson number decreased to below 0.25 during TC Hagupit, indicating that shear instability tended to occur near the seabed at the shallower mooring when enhanced NICs were superimposed on subinertial currents. A scaling analysis indicated that the downward penetration of TC-generated NIWs was partly blocked by the shear of subinertial currents during TC Fengshen. This partly explains why most of the NIKE was concentrated in the upper layer during TC Fengshen. This study, based on observations, highlights that the elongated NIC ellipse and strong shears of NIWs that occurred near the seabed were caused by NIW reflection. Therefore, part of the TC-generated NIKE was reflected by the seabed, and part of the NIKE might be dissipated due to shear instability.