Horizontal, Vertical and Temporal Structure of Vertical Mixing Modulated by Tropical Instability Waves

Deepak Cherian1, Daniel B Whitt2, Scott D Bachman3, Ren-Chieh Lien4, Ryan Holmes5 and William Large1, (1)NCAR, Boulder, CO, United States, (2)NASA, Mountain View, CA, United States, (3)National Center for Atmospheric Research, Climate and Global Dynamics, Boulder, CO, United States, (4)Applied Physics Laboratory, Seattle, WA, United States, (5)University of New South Wales, Sydney, NSW, Australia
Oceanic heat uptake and its variability in the equatorial Pacific cold tongue play a crucial part in setting the global climate system's mean state as well as influencing modes of variability such as the El Nino/Southern Oscillation.
Vertical mixing within the cold tongue at (0, 140W) has been observed to vary with the strength and phase of passing Tropical Instability Waves (TIWs), one of the dominant modes of kinetic energy variability in the region.
However, process studies in numerical models (e.g. Holmes & Thomas, 2015) show that variations of deep-cycle turbulent mixing with TIW phase can differ in their characteristics from those seen in observations (Inoue et al, 2019).

We use a high resolution (1km horizontal, 2.5m vertical) numerical simulation of the cold tongue (12S-12N, 95W-170W) to study the spatial and temporal modulation of vertical mixing by TIWs.
We find significant vertical and temporal variations of horizontal and vertical shear; stratification as well as parameterized turbulence heat flux with TIW phase by compositing these quantities over at least ten TIW periods at different synthetic TPOS mooring locations along the equator.
Simulated variability between successive TIW periods includes the occasional presence of the little studied yet previously observed "upper core layer": a 20-30m thick zone of mixing above the Equatorial Undercurrent core separated from deep cycle layer by a thin layer of weak shear, weak stratification and little mixing that appears to be meridionally advected to the mooring site.
We will quantify the horizontal structure and variability of vertical mixing under TIW influence and study links to energetic submesoscale variability at TIW fronts with the eventual goal of assessing sampling strategies that might capture this variability.
Our focus on horizontal structure is important given that existing observational datasets of TIW modulated turbulence were collected at the equator even though the TIW variability is stronger north of the equator.