Sensitivity of tropical Atlantic subsurface currents to different model parameter choices in ICON-O

The tropical oceans are home to some of the strongest subsurface current systems of the world. Among these are the Equatorial Undercurrent (EUC) which flows eastward below the thermocline, and the Equatorial Intermediate Current System (EICS) consisting of latitudinally alternating zonal jets between 15S and 15N. Ocean and climate models consistently struggle in correctly representing these subsurface currents, but especially the EUC is quite important for tropical Atlantic climate, e.g. the evolution of the Atlantic cold tongue and the associated Atlantic Niño. We use the ocean component of the ICON model to test how the representation of the Atlantic subsurface current systems reacts to different model parameter choices. In general, the EUC is too weak in our ICON configuration. We can show that the EUC is stronger, i.e. better represented with the TKE vertical mixing scheme than with the KPP scheme. Using the TKE scheme, different parameters are tested and it can be shown that the EUC reacts sensitively to the value of the important tuning parameter c_k, being stronger when c_k is smaller. We also test the sensitivity of the EUC strength in the model to the formulation of the Prandtl number in the TKE mixing scheme, which also includes a changeable constant. Apart from changes in the vertical mixing scheme, we also look at the effect of the vertical resolution of the near-surface ocean. We compare a vertical level thickness of 2m in the upper 20m to 10m in the upper 20m, with the same level distribution below 20m depth for both configurations. We can show that for the thinner surface levels, the EUC is much weaker than for the thicker levels. Intriguingly, also the EICS react to the change in near-surface vertical resolution despite being located at much larger depths. The EICS is, like the EUC, generally too weak in ICON, but becomes stronger when the near-surface levels are thinner.