Intermittent Intense Thermocline Shear Associated With Wind‐Forced Near‐Inertial Internal Waves in a Summer Stratified Temperate Shelf Sea

Abstract

Thermocline shear (i.e., velocity shear in the thermocline) lies at the heart of stratified shelf sea systems, regulating the vertical mixing and transport of mass and biogeochemical constituents. Based on year‐long moored ADCP measurements in the southern Yellow Sea (YS), the baroclinic kinetic energy and thermocline shear are investigated in this study. The rotary spectra analysis of the velocity measurements and numerical simulations with the slab model demonstrate that the wind‐forced near‐inertial internal waves (NIWs) contribute most of the baroclinic kinetic energy (50%) and thermocline shear (55%) in this stratified temperate shelf sea. The NIWs are mostly in the first two modes and are frequently generated during the warm season, associated with the presence of strong stratification and disturbances from extratropical cyclones. Most interestingly, the observed thermocline shear shows intermittent enhanced near‐inertial shear in the YS for the first time. Further analysis reveals that the thermocline shear is in the form of a clockwise rotating vector with the shear maxima (hereafter the shear spike) occurring at an angle of ∼90° to the right of the wind vector. These shear spikes, which prevail in warm seasons, are further demonstrated to be related to the wind‐shear‐alignment mechanism through comparing with a theoretical shear production model. Given the prevalence of shear spikes in warm seasons, this process is believed to have important implications in inducing thermocline mixing, which may further modulate nutrients cycling and help the maintenance of primary productivity in seasonally stratified shelf seas.

Widespread Intensified Pycnocline Turbulence in the Summer Stratified Yellow Sea

“…In the previous work of Yang et al. (2021), based on year‐long (August 2012–July 2013) moored ADCP measurements in the southern YS, it has been demonstrated that the wind‐induced NIWs can be a quite prevailing phenomenon in the YS during the warm season as a response to frequent wind pluses. Moreover, Yang et al.…”

“…Moreover, Yang et al. (2021) identified strong near‐inertial responses at the mooring station (121.7°E, 35.8°N, ∼180 km to Stn. C05) during the same period of the 2013 cruise in the present study (14–19 July 2013, see their Figure 9).…”

“…Due to the absence of simultaneous ADCP measurements during the cruise of 2013, we cannot identify the properties of shear and baroclinic velocity. In the previous work of Yang et al (2021), based on year-long (August 2012-July 2013) moored ADCP measurements in the southern YS, it has been demonstrated that the wind-induced NIWs can be a quite prevailing phenomenon in the YS during the warm season as a response to frequent wind pluses. Moreover, Yang et al ( 2021) identified strong near-inertial responses at the mooring station 9).…”

“…In contrast to the well-defined boundary layer turbulence (Liu & Wei, 2007;Lozovatsky et al, 2008), the characteristics and associated driving mechanisms of the pycnocline turbulence in the YS remain elusive. Observations and numerical simulations have shown that the YS hosts different types of internal waves, including ITs, NIWs, and ISWs (Liu et al, 2009(Liu et al, , 2019Song et al, 2021;Yang et al, 2021). Available observations have revealed the impacts of ISWs and ITs on pycnocline turbulence in the YS (Liu et al, 2009;Xu et al, 2020).…”

Spatial and seasonal variations of near-inertial kinetic energy in the upper Ross Sea and the controlling factors

“…The slab model used in our study is a linear damped slab model originally proposed by Pollard and Millard (1970). It has been widely used to examine surface near-inertial responses (Pollard, 1980;Martini et al, 2014;Yang et al, 2021;Yang et al, 2023). The governing equation is given by…”