Southern Ocean deep convection in global climate models: A driver for variability of subpolar gyres and Drake Passage transport on decadal timescales

Abstract

We investigate the individual and joint decadal variability of Southern Ocean state quantities, such as the strength of the Ross and Weddell Gyres, Drake Passage transport, and sea ice area, using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA‐UKCA) model and CMIP5 models. Variability in these quantities is stimulated by strong deep reaching convective events in the Southern Ocean, which produce an Antarctic Bottom Water‐like water mass and affect the large‐scale meridional density structure in the Southern Ocean. An increase in the (near) surface stratification, due to freshwater forcing, can be a precondition for subsequent strong convection activity. The combination of enhanced‐gyre driven sea ice and freshwater export, as well as ongoing subsurface heat accumulation, lead to a time lag between changes in oceanic freshwater and heat content. This causes an ongoing weakening of the stratification until sudden strong mixing events emerge and the heat is released to the atmosphere. We find that strong convection reduces sea ice cover, weakens the subpolar gyres, increases the meridional density gradient and subsequently results in a positive Drake Passage transport anomaly. Results of available CMIP5 models confirm that variability in sea ice, Drake Passage transport, and the Weddell Gyre strength is enhanced if models show strong open ocean convective events. Consistent relationships between convection, sea ice, Drake Passage transport, and Ross Gyre strength variability are evident in most models, whether or not they host open ocean convection.

Energetics of the Southern Ocean Mode

“…Many coarse resolution GCMs exhibit Southern Ocean open ocean convection with varying localizations and strengths (Heuzé et al, ), much in disagreement with observations in which the Weddell Polynya only appeared twice since the beginning of satellite observations (Scambos et al, ). In particular, Behrens et al () investigated the connection between open ocean convection and many Southern Ocean state variables in the NIWA‐UKCA model and a suite of CMIP5 models. These models differ in two significant ways from our model setup, on one hand they are not eddy resolving, and on the other hand they are coupled to both an atmosphere and an interactive sea ice model.…”

“…Also, the Kiel Climate Model exhibits episodic or periodic deep convection on centennial timescales in the Southern Ocean depending on the details of the sea ice model setup (Martin et al, ). In general, the simulation of open ocean convection in the Southern Ocean is highly sensitive to freshwater fluxes (precipitation, evaporation, glacial melt runoff, and transport by sea ice and icebergs), the sea ice model setup (affecting heat and salinity fluxes), and the employed mixing scheme through their effects on the stratification, both seasonally and on the long term (Behrens et al, ; Kjellsson et al, ; Stössel et al, ). In addition, mesoscale eddies can help restratify the deep ocean, enabling periodic deep convection (Carsey, ; Dufour et al, ).…”

The Antarctic Slope Current in a Changing Climate

“…These climate modes influence polar regions largely through changes in atmospheric wind patterns that modify the surface stress of the polar gyres. However, other changes, for example, due to surface buoyancy forcing, may modify gyre circulations and, nonlocally, the ASC (Behrens et al, ).…”

Preindustrial Control Simulations With HadGEM3‐GC3.1 for CMIP6

“…Among models participating in the CMIP3, the ACC transport significantly correlated with the coefficient for the mesoscale eddy parameterization (Kuhlbrodt et al, ). In addition, the interannual variability on the mean has been attributed to zonal mean wind stress (Mazloff, ) and the strength of the Ross Sea and Weddell Sea Gyres (Behrens et al, ).…”