Observations and modeling of symmetric instability in the ocean interior in the Northwestern Equatorial Pacific

Hui Zhou,Wenlong Yang,Chenglong Liu,Rui Li,Xu Chen,Hengchang Liu,Ganesh Gopalakrishnan,William Dewar

doi:10.1038/s43247-022-00362-4

Abstract

Symmetric instability is a mechanism that can transfer geostrophic kinetic energy to overturning and dissipation. To date, symmetric instability has only been recognized to occur at the ocean surface or near topographic boundary layers. Analyses of direct microstructure measurements reveal enhanced dissipation caused by symmetric instability in the northwestern equatorial Pacific thermocline, which provides the first observational evidence of subsurface symmetric instability away from boundaries. Enhanced subsurface cross-equatorial exchange provides the negative potential vorticity needed to drive the symmetric instability, which is well reproduced by numerical modeling. These results suggest a new route to energy dissipation for large scale currents, and hence a new ocean turbulent mixing process in the ocean interior. Given the importance of vertical mixing in the evolution of equatorial thermocline, models may need to account for this mechanism to produce more reliable climate projections.

Highlights

Symmetric instability is a mechanism that can transfer geostrophic kinetic energy to overturning and dissipation
We suggest that the enhanced subsurface cross-equatorial exchange during the late fall in the 2017 La Niña promotes the negative potential vorticity (PV) needed for the onset of SI
A series of two to three consecutive microstructure profiler (MSP) casts were launched at each site to obtain profiling measurements of temperature, conductivity, and microscale velocity shear from the sea surface down to nominal depths of 500 m (Supplementary Data 1), depending on weather and oceanographic conditions

Summary

Introduction

Symmetric instability is a mechanism that can transfer geostrophic kinetic energy to overturning and dissipation. The turbulent kinetic energy dissipation rate, ε, in the surface mixed layer is relatively high with the maximum amplitude exceeding 10−7 W kg−1 (Fig. 2a). These patches are consistent with those of high ε and strong vertical shears at these depths (Fig. 2a, d, e) and exhibit low gradient Richardson numbers (Ri 1⁄4 N2=S2 with S2 being the shear squared) as calculated from the SADCP measurements (Fig. 2f).

Results

Conclusion