Abstract
Abstract. For the first time, we compare the effects of four different ocean vertical mixing schemes on the mean state of the ocean and atmosphere in the Max Planck Institute Earth System Model (MPI-ESM1.2). These four schemes are namely the default Pacanowski and Philander (1981) (PP) scheme, the K-profile parameterization (KPP) from the Community Vertical Mixing (CVMix) library, a recently implemented scheme based on turbulent kinetic energy (TKE), and a recently developed prognostic scheme for internal wave dissipation, energy, and mixing (IDEMIX) to replace the often assumed constant background diffusivity in the ocean interior. In this study, the IDEMIX scheme is combined with the TKE scheme (collectively called the TKE+IDEMIX scheme) to provide an energetically more consistent framework for mixing, as it does not rely on the unwanted effect of creating spurious energy for mixing. Energetic consistency can have implications on the climate. Therefore, we focus on the effects of TKE+IDEMIX on the climate mean state and compare them with the first three schemes that are commonly used in other models but are not energetically consistent. We find warmer sea surface temperatures (SSTs) in the North Atlantic and Nordic Seas using KPP or TKE(+IDEMIX), which is related to 10 % higher overflows that cause a stronger and deeper upper cell of the Atlantic meridional overturning circulation (AMOC) and thereby an enhanced northward heat transport and higher inflow of warm and saline water from the Indian Ocean into the South Atlantic. Saltier subpolar North Atlantic and Nordic Seas lead to increased deep convection and thus to the increased overflows. Due to the warmer SSTs, the extratropics of the Northern Hemisphere become warmer with TKE(+IDEMIX), weakening the meridional gradient and thus the jet stream. With KPP, the tropics and the Southern Hemisphere also become warmer without weakening the jet stream. Using an energetically more consistent scheme (TKE+IDEMIX) produces a more heterogeneous and realistic pattern of vertical eddy diffusivity, with lower diffusivities in deep and flat-bottom basins and elevated turbulence over rough topography. IDEMIX improves in particular the diffusivity in the Arctic Ocean and reduces the warm bias in the Atlantic Water layer. We conclude that although shortcomings due to model resolution determine the global-scale bias pattern, the choice of the vertical mixing scheme may play an important role for regional biases.
Highlights
Vertical mixing in the ocean is a complex phenomenon and its magnitude depends on processes acting over a large range of vertical and horizontal scales, from about 1 km to several metres down to centimetres (Fox-Kemper et al, 2019)
We find warmer sea surface temperatures (SSTs) in the North Atlantic and Nordic Seas using K-profile parameterization (KPP) or turbulent kinetic energy (TKE)(+IDEMIX), which is related to 10 % higher overflows that cause a stronger and deeper upper cell of the Atlantic meridional overturning circulation (AMOC) and thereby an enhanced northward heat transport and higher inflow of warm and saline water from the Indian Ocean into the South Atlantic
These results suggest about 10 % higher overflow is transported across the Greenland–Iceland–Scotland Ridge with KPP and TKE, which contributes to a stronger upper cell of the AMOC
Summary
Vertical mixing in the ocean is a complex phenomenon and its magnitude depends on processes acting over a large range of vertical and horizontal scales, from about 1 km to several metres down to centimetres (Fox-Kemper et al, 2019). Secondorder schemes (Mellor and Yamada, 1982), such as the turbulent kinetic energy (TKE) scheme (Gaspar et al, 1990), contain in addition to the mean quantities prediction equations for higher-order moments, i.e. for variance and covariance terms of heat and momentum These two most common approaches represent processes that result in vertical shear of the velocity and in changes of the buoyancy, e.g. due to convection. These schemes can become more complex by adding further subgrid-scale processes (Fox-Kemper et al, 2019), such as mixing by Langmuir turbulence The traditional schemes (PP, KPP, TKE) all artificially create energy for mixing, which is introduced by the arbitrary background diffusivity This spurious energy source is an unwanted effect in an ocean model.
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