Physical oceanography

Abstract:

Abstract:

Density staircases are observed in an idealised model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centres of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.

Abstract:

We use an ocean general circulation model and its adjoint to analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea to variability in the deep branch of the Atlantic meridional overturning circulation (AMOC) on inter-annual timescales. Our study highlights the importance of the North Atlantic Current (NAC) for the north-to-south connectivity in the AMOC and for the meridional transport of Lower North Atlantic Deep Water (LNADW). We identify two mechanisms that allow the Labrador Sea to impact velocities in the LNADW layer. The first mechanism involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background NAC. The second mechanism plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea; the NAC adjustment modifies the northward transport of salt and heat and exerts a strong positive feedback, amplifying the upper ocean buoyancy anomalies. The two mechanisms spin up/down the subpolar gyre on a timescale of years, while boundary trapped waves rapidly communicate this signal to the subtropics and trigger an adjustment of LNADW transport on a timescale of months. The NAC and the eastern subpolar gyre play an essential role in both mechanisms linking the Labrador Sea with LNADW transport variability and the subtropical AMOC. We thus reconcile two apparently contradictory paradigms about AMOC connectivity: (1) Labrador Sea buoyancy anomalies drive AMOC variability; (2) water mass transformation is largest in the eastern subpolar gyre.