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Recent observations in the Pacific Ocean suggest that, apart from tropical instability waves (TIWs) at the surface, there also exist subsurface tropical instability waves (subTIWs), which can alter vertical mixing. However, the extent to which subTIWs impact mixing and heat flux is still unknown. Moreover, studies on subTIWs have been conducted exclusively in the Pacific. Here, we show the presence of subTIWs in the Atlantic for the first time, using mooring observations. Analyzing 16 years of simulations of a comprehensive, global, high-resolution ocean model, we characterize subTIWs in the Atlantic with regard to their spatial and temporal variability and investigate their influence on vertical mixing. We find subTIWs in the Atlantic between 40 and 90 m depth in both the model and observations. Furthermore, the model results reveal that unlike TIWs, subTIWs frequently also occur south of the Equator in the Atlantic Ocean. We show that subTIWs induce an oscillating multilayer shear structure, suggesting subTIWs to destabilize the mean flow and thereby induce mixing. This is strongest north of the Equator, where both TIWs and subTIWs act simultaneously. We conclude that despite similar characteristics, TIWs and subTIWs are distinct waves which both impact mixing and heat flux within the thermocline. Therefore, future studies of thermocline dynamics in the tropical oceans should not only consider TIWs but also take into account the effect of subTIWs, particularly in the subTIW dominated region south of the Equator. © 2021 The Authors
