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.
Highlights
The evolving computational power allows for ever-higher resolutions of Earth system models (ESMs)
In the equatorial Pacific, K-profile parameterization (KPP) reverses the negative bias to a positive wind speed bias
Important for the ocean is the extension of the negative bias over the Southern Ocean down to the surface in both XR simulations, which reduces the zonal wind stress driving the Antarctic Circumpolar Current (ACC) and the upwelling of Circumpolar Deep Water (CDW)
Summary
The evolving computational power allows for ever-higher resolutions of Earth system models (ESMs). Further reductions of atmospheric biases were shown by Hertwig et al (2015), who used ECHAM6.3 with a T255 (∼ 50 km) resolution in the Atmospheric Model Intercomparison Project (AMIP)-type experiments Building on these improvements, we further use a coupled MPI-ESM1.2 version with the T255 atmosphere and the TP04 ocean grid (MPIESM1.2-XR or XR) to investigate the effect of an increased atmospheric resolution on the mean state. We adopt the 0.1◦ (∼ 10 km) tripolar grid (TP6M) of MPIOM, which was already used in an ocean-only simulation forced by NCEP, and in a coupled run with T63 and T255 versions of ECHAM6 – the so-called STORM simulations (von Storch et al, 2012; Stössel et al, 2015, 2018) With this high-resolution, mostly eddy-resolving coupled version (MPI-ESM1.2-ER or ER), we detect noticeable reductions of biases in the ocean and near-surface atmosphere and in the higher atmosphere.
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