On Spurious Interactions among a Mixed Layer Model, Convective Adjustment, and Isopycnal Mixing in Ocean Circulation Models

Andreas Oschlies Centre National de la Recherche Scientifique, Toulouse, France

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Abstract

Inconsistencies can arise in ocean circulation models when part of the physical processes responsible for vertical mixing is described in the usual differential form and part is formulated as adjustment processes. Examples for the latter class are explicit convective adjustment and Kraus–Turner type models of the surface mixed layer. Implicit convective adjustment as well as various representations of interior-ocean mixing are normally described in differential form. All these schemes mix density, with a mixing intensity that itself depends on stratification. This requires that information concerning static stability is passed through the individual mixing routines in a consistent sequence. It is shown that inconsistencies can arise when coupling a Kraus–Turner type model of wind-induced mixing with both a standard implicit convective adjustment as well as with an isopycnal mixing scheme. This leads to considerably overestimated mixed layer depths, for example, by hundreds of meters in the subpolar North Atlantic. The problem is eliminated first by ensuring that dissipation of potential energy during convection is included in the mixing scheme, even when considering wind-induced turbulence only, and second, by either calling the mixed layer routine before the differential vertical mixing scheme or tapering the vertical diffusivities to zero within the surface mixed layer.

* Current affiliation: Institut für Meereskunde an der Universität Kiel, Kiel, Germany.

Corresponding author address: Dr. Andreas Oschlies, Institut für Meereskunde an der Universität Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany.

Email: aoschlies@ifm.uni-kiel.de

Abstract

Inconsistencies can arise in ocean circulation models when part of the physical processes responsible for vertical mixing is described in the usual differential form and part is formulated as adjustment processes. Examples for the latter class are explicit convective adjustment and Kraus–Turner type models of the surface mixed layer. Implicit convective adjustment as well as various representations of interior-ocean mixing are normally described in differential form. All these schemes mix density, with a mixing intensity that itself depends on stratification. This requires that information concerning static stability is passed through the individual mixing routines in a consistent sequence. It is shown that inconsistencies can arise when coupling a Kraus–Turner type model of wind-induced mixing with both a standard implicit convective adjustment as well as with an isopycnal mixing scheme. This leads to considerably overestimated mixed layer depths, for example, by hundreds of meters in the subpolar North Atlantic. The problem is eliminated first by ensuring that dissipation of potential energy during convection is included in the mixing scheme, even when considering wind-induced turbulence only, and second, by either calling the mixed layer routine before the differential vertical mixing scheme or tapering the vertical diffusivities to zero within the surface mixed layer.

* Current affiliation: Institut für Meereskunde an der Universität Kiel, Kiel, Germany.

Corresponding author address: Dr. Andreas Oschlies, Institut für Meereskunde an der Universität Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany.

Email: aoschlies@ifm.uni-kiel.de

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