1. Two stable equilibria of the Atlantic subpolar gyre
A. Born1,2 and T.F. Stocker1,2
We propose a simplified four box model representing the convective basin of the Labrador Sea and its shallow and deep boundary current system, the western subpolar gyre. Convective heat loss drives a baroclinic flow of relatively light water around the dense center. Eddy salt flux from the boundary current to the center increases with a stronger circulation, favors the formation of dense waters and thereby sustains a strong baroclinic flow, approximately 10-25% of the total. In contrast, when the baroclinic flow is not active, surface waters may be too fresh to convect and a buoyancy-driven circulation can not develop. This situation corresponds to a second stable circulation mode. A hysteresis behavior is found for variations in surface freshwater flux and the salinity of the near-surface boundary current. An analytical solution is presented and analyzed
Constant seasonal forcing
Three years to equilibrium
Increase of 10 Sv (1/3) over barotropic (wind) transport
9.2 Sv increase in lower boundary current, 0.8 Sv in upper
Increase in lower level density due to cooling, not salinification
Compares well with the formation of Labrador Sea water
(1) Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland
(2) Oeschger Centre for Climate Change Research, University of Bern, Switzerland
Hysteresis
Positive feedback of stronger circulation => more salt flux into the center => more convective cooling => stronger circulation
Strong SPG circulation sustains itself
Location of hysteresis depends on salinity of lower boundary current (dashed lines)
Analytical solution
Eddy salt flux balanced by surface freshwater flux, baroclinic flow scales with radial density gradient
Quadratic equation, two solutions
Formally identical to Stommel's model
Identification of flow regimes (thermal, haline, etc.)
Found in general circulation models
CCSM4 preindustrial control run
Spontaneous onset of deep convection in the Labrador Sea, 120 years duration, and spontaneous shut-down
Anomalies in deep convection, temperature, salinity (not shown) and depth-integrated stream function as predicted by the box model
Summary
1) Buoyancy-driven circulation contributes up to 30% to total, but it is probably more variable than wind forcing due to nonlinearity.
2) The subpolar gyre has two stable modes of circulation, consequence of mutual intensification of flow and salt transport
3) Bistability also found in comprehensive, last-generation climate models.
References
A. Born and T.F. Stocker (2014): Two stable equilibria of the Atlantic Subpolar Gyre, Journal of Physical Oceanography 44,
A. Born, T.F. Stocker, C.C. Raible and A. Levermann (2013): Is the Atlantic subpolar gyre bistable in comprehensive coupled climate models?, Climate Dynamics 40,