Near-N oscillations and turbulent mixing in the Equatorial Undercurrents

My current research focuses on small-scale processes in the Pacific Equatorial Undercurrent (EUC). The EUC is a subsurface jet-like eastward-flowing current below a westward surface current driven by the trade wind. Vertical mixing in the EUC is believed to influence the sea surface temperature and the depth of the thermocline, and therefore, can play an important role in the predictability of an El Nino/La Nina event. There have been many observational campaigns focusing on the mixing process, and the data indicates intermittent bursts of turbulence during night time at depth well below the surfaced mixed layer, the so-called deep-cycle turbulence. The dissipation rate inside the bursts can be up to three orders of magnitude larger than the background value. Observational data further indicates that the deep-cycle turbulence is accompanied with the near-N oscillations in the isotherms. A strong correlation between the two phenomena infers that the deep-cycle turbulence results from the breakdown of the oscillations; however, the origin of the oscillations is still debatable among oceanographers. There are two main hypotheses: (1) the oscillations are internal waves that are generated at the base of the mixed layer and propagate downward, or (2) they are signatures of a shear instability local to the EUC.

Our research objective is to understand the mixing processes in which the oscillations lead to the deep-cycle turbulence under different background scenarios. We use numerical models such as DNS and LES to simulate various flow conditions. In one model, we observe a Holmboe shear instability emerges at the base of the surface mixed layer. The growth of the Holmboe instability causes near-N oscillations in the isopycnals in the EUC. Kelvin-Helmholtz billows, formed on the crests of the Holmboe instability, break down and generate significant turbulent mixing. In many occasions, vortices associated with the billows are ejected downward, stretched into horseshoe vortices by the EUC shear, and subsequently create intermittent patches of turbulence in the EUC. Our model results qualitatively agree with recent observational data collected in 2008.

Towed profile Towed profile
Density profile showing the Holmboe instability. Comparison of our computational model with data from Moum et al., JPO (2011).
© 2014 Sutanu Sarkar and the students of the CFD Lab