Seasonality of Subsurface Shear Instabilities at Tropical Instability Wave Fronts in the Atlantic Ocean in a High-Resolution Simulation

Tropical Instability Waves (TIWs) have been shown to modulate upper ocean mixing. However, previous studies on the modulation of TIW related mixing are based on small numbers of TIWs and have not considered temporal variability, which can lead to discrepancies in the findings. In this study, using a 12-year simulation carried out with a comprehensive, global, high-resolution ocean model, we present evidence of seasonally modulated shear instabilities at TIW fronts in the Atlantic Ocean that reach down to the thermocline depth, potentially inducing mixing below the mixed layer depth. We find that, regardless of whether TIWs are present earlier in the year, frontal instabilities and potential mixing primarily occur in boreal summer, coinciding with a vertical shear maximum between the mean zonal currents. We argue that in the Atlantic Ocean, vertical shear at TIW fronts does usually not suffice to cause frontal instabilities below the mixed layer depth. Instead, the background shear needs to be sufficiently large in addition to TIW shear, to overcome the stability. The background shear in turn varies seasonally and is strongly driven by the variability of the northern branch of the South Equatorial Current (nSEC). As such, the variability of the nSEC strongly contributes to the generation and modulation of instabilities at TIW fronts that reach below the mixed layer depth and have the potential to induce mixing. Our results highlight the importance of seasonal variability when studying TIW impacts and their effect on mixing. Tropical Instability Waves (TIWs) create alternating warm and cold patterns at the ocean's surface, leading to sharp temperature fronts. Past studies have shown that mixing can occur along these fronts, but how this mixing changes over time hasn't been explored. Our research focuses on how instabilities below the surface at TIW fronts vary over time. Studying such instabilities helps to better understand the processes that can impact the regional climate. We analyzed over 10 years of data from a detailed ocean model and found that instabilities at TIW fronts in the Atlantic Ocean can extend down to 100 m and follow a clear seasonal cycle. These instabilities are most common in summer when the difference in zonal current speeds is greatest. We discovered that shear at TIW fronts alone isn't enough to cause these deep instabilities; the overall background shear also needs to be strong. This background shear changes with the seasons, mainly influenced by the northern branch of the South Equatorial Current (nSEC). Thus, the nSEC plays a crucial role in modulating instabilities at TIW fronts and potential ocean mixing. Tropical instability waves exhibit shear instabilities at the front that reach the thermocline depth and follow a distinct seasonal cycle The seasonality of frontal shear instabilities results from a superposition of shear from the instability wave and background current shear Variability in the northern South Equatorial Current dominates the modulation of shear instabilities at Tropical Instability Wave fronts