The effect of Denmark Strait overflow on the Atlantic Meridional Overturning Circulation

Abstract

[1] Hydraulic constraints on the Denmark Strait overflow (DSO) are used as a parameterisation to improve the overflow representation in a global climate model. The parameterisation increases deep water formation in the Nordic Seas and strengthens the Norwegian Atlantic Current. Associated higher northward heat transport leads to a northward shift of the sea-ice edge and warming by 2.5°C in the eastern Nordic Seas despite a small effect on the Atlantic Meridional Overturning Circulation (AMO). This emphasises the impact of the DSO on climate even though the response in overturning due to the DSO representation in this model is less than expected from previous studies using ocean only models. In contrast to previous studies almost no stabilising effect of the overflow on the AMO is found to freshwater perturbations.

MOMBA 1.1 – a high-resolution Baltic Sea configuration of GFDL's Modular Ocean Model

“…A remedy for this model artefact, which is probably a consequence of the high spacial resolution, is still to be developed. We suggest to investigate approaches similar to the one developed by Kösters et al (2005) for the Denmark Strait overflow. …”

“…Additional and pragmatic remedies other than applying non-homogenous viscosities could be adopted from previous efforts to model the Denmark strait overflow: Kösters et al (2005), for example, parameterise the transport across the (Denmark Strait) sill as a function of the large-scale conditions and then -directly -impose this transport, thereby overriding the effects of resolved spurious circulation on the sill. It is worth noting that their improved and apparently more realistic model features significantly differing climate projections (Köller et al, 2010).…”

Modelling the Overflows Across the Greenland–Scotland Ridge

“…Another approach to parameterize the overflow in models with low resolution is to determine the transport directly from hydraulic relations (Kösters et al 2005). Their application of a hydraulic transport parameterization to a coarse resolution ocean model gave a considerable increase in both the AMOC and the meridional heat transport that resulted in 1 K warmer air temperatures over Europe compared to the standard model set-up.…”

Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team

“…Credible climate projections require that these overflow processes are represented in ways that reproduce both the modern overflow-derived plumes and any known variations in response to changes in the ambient ocean state. Models without accurate overflow representation may have deep-water formation sites in the wrong locations (i.e., dominated by open-ocean convection in the Irminger and Labrador Seas; Kosters et al 2005) and incorrect meridional overturning circulation (MOC) vertical structure and heat transport distribution. However, because these processes occur below the grid scale of our highest-resolution global ocean models (or even regional models), realistic representation of overflows poses a considerable challenge for numerical simulation.…”