Description: Western Boundary Currents, such as the Gulf Stream, are regions of vivid air-sea interaction. Mesoscale features of these currents play a fundamental role in global ocean heat transport and exchange with the atmosphere. Related processes and their interactions across scales have gained increasing attention in the last years, since high-resolution, mesoscale-resolving modeling became computationally feasible on climate time scales. Here, we show the impact of explicitly resolving the oceanic mesoscale in the coupled global climate model FOCI on North Atlantic and European climate. For this purpose, we use the ocean nesting capability in FOCI, which facilitates regional ocean grid refinement. We explore and compare pre-industrial simulations each extending over at least 150 years: a reference run without any grid refinement and an experiment with a nest in the North Atlantic. Technically, the regional ocean nest maintains frequent two-way exchange with the global host grid, which in turn is fully coupled to the atmosphere model. The ocean model NEMO has a global resolution of 1/2˚ model with 46 vertical levels and 1/10˚ refinement in the nest region, while the atmosphere model ECHAM6 has a 1.8˚ horizontal resolution (T63) and 95 vertical levels, including the strato- and mesosphere. Within the nest region, the increased resolution leads to a more eddy-rich simulation and an improved mean state. The North Atlantic Current is considerably better represented, which reduces the typical North Atlantic cold bias from -8˚C in the reference run without nest to -2˚C. Beyond local bias correction of the mean state, we will also discuss the impact of explicitly modeling ocean mesoscale dynamics on atmospheric variability on different time scales, such as the North Atlantic Oscillation or the Atlantic Multidecadal Variability.

Description: The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N ‐ a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.