The Role of Small‐Scale Topography in Modulating Eddy Scale in the Northern South China Sea

Abstract

The ocean is intrinsically turbulent in nature and as a result of non-linear interactions energy in the ocean can transfer across spatial scales. Geostrophic turbulence theory predicts an inverse energy cascade (from small scales to large scales) of ocean eddy kinetic energy where the eddy length scale increases until it is halted at the Rhines scale by planetary wave propagation (Rhines, 1975;Salmon, 1998). However, satellite observations reveal that the dominate eddy scales in many regions of the oceans are only slightly larger than the local Rossby deformation radius (Stammer, 1997). This indicates that the inverse cascade is often arrested by other physical processes before it reaches the Rhines scale.For example, Arbic and Flierl (2004) found that the bottom friction, or bottom drag, can arrest the inverse cascade and thereby modulate the eddy length scale. Eddies in their model simulations tend to have larger horizontal scales than observations under weak bottom drag. Another possible candidate process that can affect eddy scale is the generation of lee waves over small-scale topography (wave drag), which is believed to be an efficient way to cascade the mesoscale energy to small-scales that are more readily to be dissipated (Nikurashin et al., 2013). The downscale or forward eddy energy cascade (from large scales to small scales) above the small-scale rough topography may contribute to the arrest of inverse cascade and thereby reduce the eddy scales. However, the manner in which wave drag affects the eddy scales may be quite different from that of bottom drag. For example, different from the bottom drag which only occurs in the bottom boundary layer, lee waves can radiate away from the ocean bottom into ocean interior and interact with the mean flows (

Greenhouse Warming Reduces Global Energy Conversion Into Oceanic Lee Waves

“…In view that the global wind work into the surface geostrophic currents is estimated to be around 1 TW (Wunsch, 1999), the generation of lee waves could be an important energy sink for wind‐driven ocean circulations and a source for deep‐ocean turbulent diapycnal mixing (Munk & Wunsch, 1998; Wunsch & Ferrari, 2004). Several studies based on observations (Brearley et al., 2013; Clément et al., 2016; Cusack et al., 2017; Evans et al., 2020; Hu et al., 2020; Meyer et al., 2016) and numerical models (Trossman et al., 2013; Trossman et al., 2016; Melet et al., 2014; Nikurashin et al., 2013; Z. Yang et al., 2021, 2022, 2023b) have highlighted the important role of lee waves in regulating the ocean energetics, powering turbulent diapycnal mixing, which in turn impacts the global climate system. Understanding the response of lee wave generation to the greenhouse warming is thus important for accurately predicting future climate changes.…”