Max Planck Institute Earth System Model (MPI-ESM1.2) for the High-Resolution Model Intercomparison Project (HighResMIP)

TLDR

The study compares coupled simulations of MPI‑ESM1.2 under the HighResMIP protocol to improve the climate mean state of the atmosphere and ocean. The simulations analyze the separate effects of increasing horizontal resolution of the ocean (0.4 to 0.1°) and atmosphere (T127 to T255) submodels, and of replacing the Pacanowski‑Philander vertical ocean mixing scheme with the K‑profile parameterization. High‑resolution ocean removes biases and majorly impacts the mean state; high‑resolution atmosphere reduces wind stress, improves mixed‑layer depths, and slows the Antarctic Circumpolar Current, while in the North Atlantic reduced wind weakens the subpolar gyre and slows AMOC with the PP scheme, but the KPP scheme strengthens convection and sustains a stronger AMOC even with T255 atmosphere. Abstract.

Abstract

Abstract. As a contribution towards improving the climate mean state of the atmosphere and the ocean in Earth system models (ESMs), we compare several coupled simulations conducted with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM1.2) following the HighResMIP protocol. Our simulations allow to analyse the separate effects of increasing the horizontal resolution of the ocean (0.4 to 0.1∘) and atmosphere (T127 to T255) submodels, and the effects of substituting the Pacanowski and Philander (PP) vertical ocean mixing scheme with the K-profile parameterization (KPP). The results show clearly distinguishable effects from all three factors. The high resolution in the ocean removes biases in the ocean interior and in the atmosphere. This leads to the important conclusion that a high-resolution ocean has a major impact on the mean state of the ocean and the atmosphere. The T255 atmosphere reduces the surface wind stress and improves ocean mixed layer depths in both hemispheres. The reduced wind forcing, in turn, slows the Antarctic Circumpolar Current (ACC), reducing it to observed values. In the North Atlantic, however, the reduced surface wind causes a weakening of the subpolar gyre and thus a slowing down of the Atlantic meridional overturning circulation (AMOC), when the PP scheme is used. The KPP scheme, on the other hand, causes stronger open-ocean convection which spins up the subpolar gyres, ultimately leading to a stronger and stable AMOC, even when coupled to the T255 atmosphere, thus retaining all the positive effects of a higher-resolved atmosphere.