Controlling high-latitude Southern Ocean convection in climate models. (February 2015)

• Record Type: Journal Article
• Authors: Stössel, Achim; Notz, Dirk; Haumann, F. Alexander; Haak, Helmuth; Jungclaus, Johann; Mikolajewicz, Uwe
• Is Part Of: Ocean modelling. Volume 86(2015:Feb.)
• Page Start: 58
• Page End: 75
• Publication Date: 2015-02
• Subjects: High-latitude Southern Ocean -- Climate models -- Surface buoyancy fluxes -- Convection -- Sea ice; Oceanography -- Periodicals; Océanographie -- Périodiques; Oceanography; Periodicals; 551.46
• DOI: 10.1016/j.ocemod.2014.11.008
• Languages: English
• ISSNs: 1463-5003

Abstract

Highlights: Investigation of the high-latitude Southern Ocean in different versions of an Earth System Model. Water-mass properties become realistic with extra freshwater flux. Southern Ocean of eddy-resolving ocean model improves considerably upon coupling, and higher-resolution atmosphere model. Switch to coupled mode leads to buildup of heat reservoir at depth. Spreading of glacial melt water improves near-boundary convection. Abstract: Earth System Models (ESMs) generally suffer from a poor simulation of the High-Latitude Southern Ocean (HLSO). Here we aim at a better understanding of the shortcomings by investigating the sensitivity of the HLSO to the external freshwater flux and the horizontal resolution in forced and coupled simulations with the Max-Planck-Institute Ocean Model (MPIOM). Forced experiments reveal an immediate reduction of open-ocean convection with additional freshwater input. The latter leads to a remarkably realistic simulation of the distinct water-mass structure in the central Weddell Sea featuring a temperature maximum of +0.5 °C at 250 m depth. Similar, but more modest improvements occur over a time span of 40 years after switching from a forced to a coupled simulation with an eddy-resolving version of MPIOM. The switch is accompanied with pronounced changes of the external freshwater flux and the wind field, as well as a more realistic heat flux due to coupling. Similar to the forced freshwater-flux experiments, a heat reservoir develops at depth, which in turn decreases the vertically integrated density of the HLSO and reduces the Antarctic Circumpolar Current to rather realistic values. Coupling with a higher resolution version of the atmosphere model (ECHAM6) yields distinct improvements of the HLSO water-mass structure and sea-ice cover. While the coupled simulations reveal a realistic amount of Antarctic runoff, its distribution appears too concentrated along the coast. Spreading the runoff over a wider region, as suggested in earlier studies to mimic the effect of freshwater transport through icebergs, also leads to noticeable improvements of the HLSO water-mass properties, predominantly along the coast. This suggests that the spread of the runoff improves the representation of Antarctic Bottom Water formation through enhanced near-boundary convection rather than weakened open-ocean convection.