Spatial Extent and Variability of Equatorial Deep-Cycle Turbulence in the Pacific Cold Tongue

Deep-cycle turbulence (DCT) is a critical mechanism driving vertical mixing in the equatorial Pacific, playing a pivotal role in modulating heat and nutrient transport within the Pacific Cold Tongue. DCT arises from diurnal variations in stratification and shear, leading to turbulence that extends below the mixed layer. DCT generates significant heat fluxes into the ocean, averaging O(100 W m⁻²) and peaking at ~1000 W m⁻² during nighttime bursts, which contribute to surface cooling and thermocline warming. This process helps maintain cool sea surface temperatures (SSTs) and net heat uptake in the eastern Pacific Cold Tongue, influencing SST dynamics on interannual, seasonal, and subseasonal timescales. These dynamics significantly impact air-sea interactions, as DCT regulates the exchange of heat, momentum, and gases, which play a critical role in shaping tropical weather patterns and global climate variability.

 Despite previous studies elucidating the temporal variability and mechanisms of DCT on the equator, its spatial extent and variability across the equatorial Pacific remain poorly understood due to limited observations.

This study examines the spatial and temporal variability of DCT in the Cold Tongue region using Large Eddy Simulations (LES), which explicitly resolve sub-grid-scale mixing processes. The LES cover a meridional array of seven latitudinal points (1.5°S to 4.5°N) along 140°W and a zonal array spanning the central to eastern Pacific (165°W to 100°W) along the equator during contrasting periods influenced by Tropical Instability Waves (TIWs) and the seasonal cycle. Complementary hourly turbulence outputs from a 20-year MITgcm simulation are utilized to examine parameterized turbulence at these locations, enabling a comparison between sub-grid-resolved turbulence in LES and parameterized turbulence in the MITgcm.

Diurnal composite analyses reveal that parameterized turbulence in the MITgcm overestimates diapycnal heat flux compared to LES-resolved turbulence. The relationship between Richardson number, shear, stratification, and mixing is explored to understand the transition from the marginally stable regime near the equator (0°N, 140°W) to more stable conditions farther from the equator. Preliminary findings illustrate spatial asymmetries in mixing-related variables, with notable differences between the northern and southern hemispheres. These results highlight the need for further exploration of hemispheric asymmetries and their implications for mixing processes.

This study sets the stage for a comprehensive evaluation of mixing representation in the Pacific Cold Tongue region across diurnal to longer timescales, leveraging a hierarchy of model outputs, from LES to regional and global high-resolution simulations.