Atmosphere Ocean Science Colloquium
Multiple Regimes and Thermohaline Variability in the Quasi-Adiabatic Pole-To-Pole Circulation
Speaker: Christopher Wolfe
Location: Warren Weaver Hall 1302
Date: Wednesday, November 12, 2014, 3:30 p.m.
In the limit of weak interior mixing, the ocean can support a pole-to-pole overturning circulation on isopycnals that outcrop in both the Northern Hemisphere and a high-latitude southern circumpolar channel. This overturning cell participates in a salt feedback, which strongly counteracts the precipitation-induced surface freshening of the northern high latitudes and weakly counteracts the surface freshening of the southern high latitudes. The net result is an increase in the range of isopycnals shared between the two hemispheres, which strengthens the overturning circulation. However, if precipitation in the Northern Hemisphere sufficiently exceeds that in the Southern Hemisphere, the overturning cell reverses and its southern endpoint moves equatorward of the channel. The reversed overturning circulation is shallower and weaker than its forward counterpart and is maintained diffusively. In a limited range of parameters, multiple equilibria are found for the same forcing configuration. For weak diapycnal diffusivity, the multiple equilibria are unstable to oscillations around each of the fixed points. The most prominent modes of variability are a decadal oscillation expressed in the Northern subpolar gyre and a multicentennial oscillation spanning both hemisphere. We discuss the dynamics of these oscillations and show how they mediate transitions between the multiple regimes.