Enhanced Diapycnal Mixing Between Water Masses in the Western Equatorial Pacific

Diapycnal mixing of momentum and heat is crucially important to ocean dynamics, as it affects the state of the ocean and its interactions with the atmosphere. For realistic climate and circulation predictions, the magnitude and distribution of diapycnal mixing must be accurately represented in ocean models. Based on full‐depth microstructure data obtained in the western equatorial Pacific, we demonstrate that there is an enhanced diapycnal diffusivity layer nested below the thermocline of the western equatorial Pacific. Diapycnal diffusivities in this layer range from 5×10−6 to 5× 10−4 m2 s−1, one to two orders of magnitude higher than those predicted by the wave‐wave interaction theory (5 ×10−7−5×10−6 m2 s−1). These enhanced diapycnal mixings are strongly related to the South Pacific Tropical Water intrusion. Stratification is largely weakened by the South Pacific Tropical Water intrusion; thus, the shear can easily trigger shear instabilities and induce strong diapycnal mixing. This enhanced diapycnal diffusivity layer occupies the water column between ~250 and 750 m and spans from the equator to 10°N. Such widespread enhanced diapycnal mixing related to water mass intrusions may play an important role in the dynamics of the equatorial ocean, and it should be taken into account in ocean models. Plain Language Summary The Pacific is the largest ocean in the world and plays an important role in the global climate change. For example, it affects the typhoons, rainfall, and monsoon, which clearly affects human activities. To accurately predict the ocean circulation and climate, it is needed to understand the dynamic mechanism of the ocean, especially the turbulent mixing that changes the properties of sea water and drives the ocean circulation. In this study, we investigated the turbulent mixing between water masses with observation. We found that there is a layer of enhanced turbulent mixing with a thickness of approximately 500 m in the western equatorial Pacific. This strong turbulent mixing layer was strongly related to the water mass intrusion. Understanding the impact of the water mass on the turbulent mixing can provide references for ocean models and improve their prediction of the climate and circulation. Key Points An enhanced diapycnal mixing layer was observed below the thermocline of the western equatorial Pacific The subthermocline‐enhanced diapycnal mixing layer was strongly related to water mass intrusion Water mass intrusion created a weak s