Widespread Intensified Pycnocline Turbulence in the Summer Stratified Yellow Sea

Abstract

AbstractPycnocline mixing lies at the heart of seasonally stratified shelf sea systems, regulating vertical exchange of momentum, mass, heat, and biogeochemical constituents. Based on microstructure measurements in the summer of 2013 and 2017, here we report on the widespread occurrence of intensified pycnocline turbulence in the summer stratified Yellow Sea (YS). Large turbulent kinetic energy dissipation rates ∼ (10−7 W kg−1) and microscale thermal dissipation rates ∼ (10−6 K2 s−1) are found in the pycnocline below the depth of the strongest stratification at many sampling stations. Shipboard velocity measurements suggest that wind‐induced near‐inertial internal waves induced strong velocity shear; however, the calculated velocity shear peaked exactly at (instead of below) the strongest stratification layer and is generally not strong enough to trigger shear instabilities according to the Richardson number criterion. Example results at two repeated sampling stations clearly showed the presence of a low‐salinity water layer (with a thickness of 5–15 m), at the upper boundary of which salt fingering is expected to occur according to the Turner angle. The observed elevated and tended to occur in the salt‐finger‐favorable layers. Although the distribution of and reveals the relation with Turner angle, we further note the possible complexity in the driving mechanisms with the thermohaline‐shear instability potentially playing a role. Given the widespread nature of the intensified pycnocline turbulence, it is expected that this turbulence has important implications for nutrients cycling and thus the maintenance of primary productivity in the summer stratified YS.