Variations of Equatorial Shear, Stratification, and Turbulence Within a Tropical Instability Wave Cycle

Equatorial Internal Wave Experiment observations at 0 degrees, 140 degrees W from October 2008 to February 2009 captured modulations of shear, stratification, and turbulence above the Equatorial Undercurrent by a series of tropical instability waves (TIWs). Analyzing these observations in terms of a four-phase TIW cycle, we found that shear and stratification within the deep-cycle layer being weakest in the middle of the N-S phase (transition from northward to southward flow) and strongest in the late S phase (southward flow) and the early S-N phase (transition from southward to northward flow). Turbulence was modulated but showed less dependence on the TIW cycle. The vertical diffusivity (K-T) was largest during the N (northward flow) and N-S phases, when stratification was weak, despite weak shear, and was smallest from the late S phase to the S-N phase, when stratification was strong, despite strong shear. This tendency was less clear in turbulent heat flux because vertical temperature gradients were small at times when K-T was large, and large when K-T was small. We investigated the dynamics of shear and stratification variations within the TIW cycle by using an ocean general circulation model forced with observed winds. The model successfully reproduced the observed strong shear and stratification in the S phase, except for a small phase difference. The strong shear is explained by vortex stretching by TIWs. The strong stratification is explained by meridional and vertical advection. Plain Language Summary Moored observations in the central equatorial Pacific during 2008 La Nina captured a series of tropical instability waves (TIWs). Vertical shear of horizontal velocity, stratification, and turbulent mixing above the Equatorial Undercurrent varied strongly in different phases of each TIW cycle. The dynamics of modulations of stratification and shear by the TIW cycle were identified using results from an oceanic model.