Direct Measurements of Turbulence in the Upper Western Pacific North Equatorial Current over a 25-h Period.

Wenlong Yang,Hengchang Liu,Chenglong Liu,William Dewar,Juan Liu,Hui Zhou,Yonggang Wang

doi:10.3390/s22031167

Abstract

Measurements of the turbulent kinetic energy dissipation rate (ε) were conducted by a free-fall microstructure profiler in the western Pacific North Equatorial Current (WPNEC) during a continuous period of 25 h, from the sea surface to about 160 m depth. In the mixed layer (ML), ε values were typically on the order of 10−8∼10−7 W kg−1, and an obvious diurnal cycle existed in the upper 40 m of the surface mixing layer. Below the ML, ε was reduced to 10−9∼10−8 W kg−1 with some patches of high ε reaching 10−7.5 W kg−1. The barrier layer was identified in the nighttime with a maximum thickness of 20 m, and it was eroded by the advection of freshwater within the lower part of the isothermal layers associated with an anticyclonic eddy in the afternoon. A simple scaling relevant to shear (S2) instability and stratification (N2) that can predict turbulent dissipation rates in the transition layer, between the well-mixed layer and the thermocline below, was obtained through the scaling . Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.

Highlights

Turbulent mixing plays a key role in the upper layer of the ocean, where active momentum, heat, and gas exchanges between the ocean and the atmosphere exist
The geostrophic velocity anomalies averaged between 8 April (Day 1) and 9 April (Day 2) 2018 based on altimetry data clearly show that the microstructure profile (MSP) station is located in the south part of an anticyclonic eddy (AE) centered around (11.7◦ N, 142.2◦ E) with a diameter of 200 km (Figure 1b)
The time series of the current profile measured by lowered Acoustic Doppler Current Profiler (LADCP) show that the current is northwestward in the upper 80 m during our observations; a northeastward flow occurs in period III (Figure 3a,b)

Summary

Introduction

Turbulent mixing plays a key role in the upper layer of the ocean, where active momentum, heat, and gas exchanges between the ocean and the atmosphere exist. It regulates water properties and drives ocean circulations, which modulate the climate system through affecting the large-scale heat budget [1,2,3,4,5]. The turbulent exchange of momentum and scalars at both the air–sea interface and the ocean mixed layer (ML). Direct measurements of the upper-ocean layer turbulence are very valuable for a better parameterization of mixing and its simulation [14,15]

Methods

Results

Conclusion