Amplifications and Damping of Diurnal-Inertial Oscillations Observed in the Northern East China Sea from Ieodo Ocean Research Station and Surface Drifters

The Ieodo Ocean Research Station (I-ORS) is an Eulerian ocean observing platform providing long-term time series data of essential oceanic and atmospheric variables. The northern East China Sea, where the I-ORS is located, is characterized by strong tidal dominance but is also affected by surface inertial motions of non-tidal origin. These surface motions can resonate with local diurnal winds, such as the sea-land breeze (SLB), at inertial frequency because of its proximity to the diurnal critical latitude (~30˚N). This study investigates the amplification and damping of diurnal-inertial oscillations in the northern East China Sea by analyzing time series observations at the I-ORS from 15 May to 26 July 2023 and satellite-tracked surface drifters stayed nearby from 6 to 26 July 2023. In addition to the permanent I-ORS CT sensors installed at depths of 3.0, 20.5, and 38.0 m, 32 temperature sensors (thermistor string) were installed on the mooring line; Initial 16 sensors attached within ±1.5 m of the sea surface at an interval of 0.2 m while the other 16 sensors were attached between 3 and 21 m depths at an interval of 1.2m. All the I-ORS sensors including the thermistor string recorded data every minute or 10-minute interval. The location data of surface drifters, initially recorded hourly, were interpolated to 10-minute interval data to calculate zonal and meridional currents, kinetic energy per unit volume, and wind work rate along with wind stress estimated using the I-ORS sea surface wind data (speed and direction). The I-ORS thermistor string observations occasionally show significant diurnal-inertial oscillations in the mixed layer depth and thermocline depth with typical amplitudes of 1.5 m and 2.0 m, respectively, from 10 to 15 July, when strong eastward wind stress reached up to 0.04 N/m² with an enhanced wind work rate of 0.069 W/m². The amplitudes of the diurnal-inertial oscillations in the mixed layer depth and thermocline depth decreased to 0.5m or less from 18 to 22 July, when the wind stress became weaker (< 0.01 N/m²). The surface drifter observations show clockwise-rotating, circular inertial currents with a speed of 35-43 cm/s, separated from the diurnal tidal currents with a comparable speed. The kinetic energy of intermittently amplified, diurnal-inertial surface motions peaked at up to 100 J/m³ on 15 July, which is explained by the peak in wind work rate of 0.069 W/m² considering mixed layer depth of 10 m and duration of 4 h, i.e., 0.069 J/s/m² x 4 h / 3600 s/h / 10 m = 100 J/m³. This study presents the intermittent amplification of wind-induced, resonant diurnal-inertial oscillations at the upper ocean near the diurnal critical latitude based on the Eulerian time-series observations along with surface drifters. Further work is needed to address the generation and decaying dynamics and long-term variability of diurnal-inertial oscillations in this and other regions near the diurnal critical latitude.