Two Stable Equilibria of the Atlantic Subpolar Gyre

Abstract

The cyclonic circulation of the Atlantic subpolar gyre is a key mechanism for North Atlantic climate variability on a wide range of time scales. It is generally accepted that it is driven by both cyclonic winds and buoyancy forcing, yet the individual importance and dynamical interactions of the two contributions remain unclear. The authors 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 cannot develop. This situation corresponds to a second stable circulation mode. A hysteresis is found for variations in surface freshwater flux and the salinity of the near-surface boundary current. An analytical solution is presented and analyzed.

Mechanisms of decadal variability in the Labrador Sea and the wider North Atlantic in a high-resolution climate model

“…This suggests that the influence of the EGC on Labrador Sea density is related to the occurrence of extreme salinity events. A four-box conceptual model by Born and Stocker (2014) supports the importance of salt fluxes from the boundary currents to drive convection in the Interior Labrador, and the SPG variability. The cross-correlations in Fig.…”

Response of the North Atlantic dynamic sea level and circulation to Greenland meltwater and climate change in an eddy-permitting ocean model

“…4, 5a). A similar mechanism, connecting the baroclinic flow, deep mixing in the Labrador Sea interior and surface buoyancy forcing, is discussed by Born and Stocker (2014) using an idealized model. The weak sensitivity of V bt results from the changes in V bt and V bt (Fig.…”

An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes

“…Broadly speaking, the strength of the SPG is largely controlled by the horizontal gradient in upper-ocean densities between the gyre's center and its boundaries, and by the wind stress over the subpolar area (e.g., Häkkinen et al 2011;Born and Stocker 2014). The former is herein represented as the difference between upper-ocean (500 m) densities in the western subpolar basin, averaged between 55°N-60°N and 50°W-60°W, and the eastern basin, averaged between 55°N-60°N and 10°W-20°W, further smoothed with an 11-year running mean (Fig.…”

“…In its climatological mean, late-winter cooling of the upper Labrador Sea induces strong vertical mixing down to a depth of several hundred meters (Fig. 2c, contours) that forms a core of very dense waters within the SPG's center, which in turn strengthens the zonal gradient of upper-ocean densities and, thereby, the gyre's circulation (Born and Stocker 2014). After the SPG shift, however, ocean deep mixing is locally hampered as fresher, hence lighter, surface conditions develop in the Labrador Sea (Figs.…”