Abstract
Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.
Highlights
Full depth open ocean deep convection has been observed only once in the Southern Ocean, during the Weddell Polynya of 1974–1976 (Gordon, 1978)
Langmuir turbulence is represented in NEMO by the parameterization of Axell (2002), which appears as an additional production term in the turbulent kinetic energy (TKE) budget equation: de W 3
Unpublished experiments with HiGEM (High-resolution Global Environmental Model; Shaffrey et al, 2009), whose ocean model is not NEMO, suggest that the open ocean deep convection area is reduced in both southern subpolar gyres when the background diffusivity increases linearly instead of being constant with depth
Summary
Full depth open ocean deep convection has been observed only once in the Southern Ocean, during the Weddell Polynya of 1974–1976 (Gordon, 1978). In state-of-the-art CMIP5 (Coupled Model Intercomparison Project Phase 5) models, Antarctic Bottom Water (AABW) is formed via open ocean deep convection (deeper than 2000 m and up to full-depth) which occurs most winters in the seasonally ice-covered southern subpolar gyres (Heuzé et al, 2013) and continues throughout climate change simulations (Heuzé et al, 2015). Is this process relatively unrealistic, it leads to spurious ocean properties, variabilities and drift
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