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Tropical Instability Waves (TIWs) at the equatorial Atlantic Ocean lead to SST cooling due to enhanced mixing and heat fluxes above the EUC core. This phenomenon has been studied predominantly at the equator and to the north, where TIWs are most pronounced. However, a recent study has shown the presence of subsurface TIWs in the Atlantic Ocean, which frequently occur to the south of the equator. As TIW induced subsurface mixing leads to SST cooling at the equator, we suspect a similar cooling may occur in the Southern Hemisphere due to the presence of subsurface TIWs. Using one decade of high-resolution ICON ocean simulations, we investigate such effect of subsurface TIWs in the southern hemisphere on SST in the tropical Atlantic Ocean. The analysis of all terms of the mixed layer heat budget allows for the investigation and quantification of the processes involved in subsurface TIW induced SST changes.

Tropical instability waves (TIWs) near the ocean surface are present in all tropical oceans and are known to be important for air-sea interactions and regional climate variability. Recent studies based on observations in the Pacific Ocean found that apart from TIWs at the surface, there also exist subsurface TIWs (subTIWs) which can alter vertical mixing. To date, most studies have focused on TIW related dynamics near the ocean surface. However, to properly assess vertical mixing in the upper ocean, improved understanding of the vertical structure of TIWs and the influence of subTIWs is needed. In this study, we show subTIW presence in the Atlantic Ocean for the first time using mooring observations.Further, we characterize subTIWs in the tropical Atlantic Ocean with a special focus on subTIW spatial and temporal variability and their effect on mixing. For this, data covering almost two decades are used that were generated from a comprehensive, global, high-resolution ocean model forced by the reanalysis ERA5. We find subTIWs between 40 m depth and the thermocline in both model and observations and unlike TIWs, subTIWs are frequently active both north and south of the Equator. The results of our study suggest that subTIWs induce a multi-layer shear structure which has the potential to destabilize the mean flow and thereby cause mixing. These effects are strongest north of the Equator where TIWs and subTIWs act simultaneously, implying possible TIW/subTIW interactions. We conclude that subTIWs are a feature of the tropical Atlantic Ocean with regionally varying implications for vertical mixing and heat fluxes. In addition, subTIWs differ from TIWs in their temporal and regional occurrences Therefore, subTIWs should be considered in future assessments of upper ocean dynamics, particularly in subTIW dominated regions.

The question, whether ocean dynamics are relevant for basin-scale North Atlantic decadal temperature variability is subject of ongoing discussions. Here, we analyze a set of simulations with a single climate model, consisting of a 2000-year pre-industrial control experiment, a 100-member historical ensemble, and a 100-member ensemble forced with an incremental CO2 increase by 1%/year. Compared to previous approaches, our setup offers the following advantages: First, the large ensemble size allows to robustly separate internally and externally forced variability and to robustly detect statistical links between different quantities. Second, the availability of different scenarios allows to investigate the role of the background state for drivers of thevariability. We find strong evidence that ocean dynamics, particularly ocean heat transport variations, form an important contribution to generate the Atlantic Multidecadal Variability (AMV) in the Max Planck Institute Earth System Model (MPI- ESM). Particularly the Northwest North Atlantic is substantially affected by ocean circulation for the historical and pre-industrial simulations. Anomalies of the Labrador Sea deep ocean density precede a change of the Atlantic Meridional Overturning Circulation (AMOC) and heat advection to the region south of Greenland.Under strong CO2 forcing the AMV-SST regression pattern shows crucial changes: SST variability in the north western part of the North Atlantic is strongly reduced, so that the AMV pattern in this scenario is dominated by the low-latitude branch. We found a connection to changes in the deep water formation, that cause a strong reduction of the mean AMOC and its variability. Consequently, ocean heat transport convergence becomes less important for the SST variability south of Greenland.