Poleward Propagation of Typhoon-Induced Near-Inertial Waves in the Northern South China Sea

Abstract

One of the main responses of the ocean to typhoons is the generation of near-inertial waves (NIWs), whose intrinsic frequency is close to the local inertial frequency. Based on the mooring observations, we carefully investigated the spatial–temporal variations in NIWs in the northern South China Sea (SCS) after Typhoon “Haima,” which passed through the northern SCS from October 20 to 21, 2016, with its track parallel to the mooring array on its northeast side. Moorings in different locations responded differently to this typhoon, with stronger NIWs found on the continental slope to the shelf break and relatively weak NIWs found on the shallow continental shelf. Strong NIWs appeared twice within a short period and showed different characteristics and frequencies. The first NIWs were generated locally with blue-shifted (super-inertial) frequencies. These initial NIWs were dominated by the first three baroclinic modes and rapidly weakened due to their propagation away and local dissipation. However, the second NIWs mainly consisted of higher baroclinic modes with red-shifted (sub-inertial) frequencies. The analysis of the mean background flow revealed that these red-shifted NIWs were excited at low latitudes, and subsequently advected by the poleward background flow to moorings whose latitudes were higher than their critical latitudes. Higher-mode (n > 3) NIWs were more easily carried away by mean background flow, and, at the same time, high-mode NIWs propagated downward to the subsurface layer, leading to the significant elevation of velocity shear therein. Given these findings, the mean background flow may provide an important route to redistributing the energy of the upper ocean imported from typhoons.

Observed Equatorward Propagation and Chimney Effect of Near‐Inertial Waves in the Midlatitude Ocean

“…Based on turning-point theory and a numerical model, Fu (1981) suggested that the observed local inertial peak over smooth topography could be interpreted in terms of poleward-propagating waves generated at lower latitudes. The poleward propagation of NIWs can also be caused by the background flow (e.g., Huang et al, 2021;Jeon et al, 2019;Tort & Winters, 2018). Tort and Winters (2018) demonstrated a scale selection mechanism by which the super-inertial component of NIWs is able to propagate poleward over long distances in the presence of mesoscale turbulence with horizontal scales considerably smaller than the width of the storm track.…”

Near-inertial waves generated by typhoon MITAG under the influence of anticyclonic eddy east of Taiwan

“…Decompose the near-inertial currents of HYCOM into vertical modes from 0 to 800 m, and perform a depth integration for different modes (Figures 10,11). Examples of applying this method to the range from the surface to the maximum depth that NIWs can reach were common in previous work (Chen et al, 2013;Hou et al, 2019;Huang et al, 2021), so the range of 0-800 m selected for this study is fairly reasonable.…”

“…The subtropical northwest Pacific has complex multiscale oceanic dynamic processes and is also characterized by an abundance of TCs (Hu et al, 2020). Due to the difficulty of implementing marine observations, most previous studies have focused on the South China Sea (Huang et al, 2021;Yang et al, 2021), and research on NIWs east of Taiwan, where mesoscale processes are highly active, remains limited. In this paper, by comparing observations with HYCOM data, we confirmed that HYCOM data can effectively simulate the typhoon-generated NIWs east of Taiwan during Typhoon MITAG.…”

Observed equatorward propagation and chimney effect of near-inertial waves in the mid-latitude ocean

“…Based on turning-point theory and a numerical model, Fu (1981) suggested that the observed local inertial peak over smooth topography could be interpreted in terms of poleward-propagating waves generated at lower latitudes. The poleward propagation of NIWs can also be caused by the background flow (e.g., Tort & Winters, 2018;Jeon et al, 2019;Huang et al, 2021). Tort and Winters (2018) demonstrated a scale selection mechanism by which the super-inertial component of NIWs is able to propagate poleward over long distances in the presence of mesoscale turbulence with horizontal scales considerably smaller than the width of the storm track.…”