Modelling the Dynamics of Abyssal Equator-crossing Currents

Abyssal Abyssal flows, flows, as as part part of of the the global global thermohaline thermohaline circulation, circulation, make make a a sig­ sig­ nificant nificant contribution contribution to to the the flux flux of of heat heat over over the the earth, earth, and and therefore therefore affect affect the the planet's planet's climate. climate. In In the the Atlantic, Atlantic, the the deepest deepest flow flow consists consists of of Antarc­ Antarc­ tic tic Bottom Bottom Water, Water, which which originates originates in in the the Weddell Weddell Sea Sea near near Antarctica Antarctica and and flows flows northward northward along along the the western western boundary boundary of of the the Atlantic Atlantic ocean. ocean. While While part part of of this this flow flow recirculates recirculates within within the the Brazil Brazil Basin, Basin, remaining remaining in in the the southern southern hemisphere, hemisphere, part part of of the the flow flow is is observed observed to to cross cross the the equator equator into into the the northern northern hemisphere hemisphere Potential Potential vorticity vorticity is is conserved conserved following following the the flow flow if if friction friction effects effects are are neglected. neglected. However, However, the the fluid fluid is is relatively relatively quiescent quiescent before before and and after after cross­ cross­ ing ing the the equator! equator! that that is, is, planetary planetary vorticity vorticity dominates dominates relative relative vorticity. vorticity. Therefore, Therefore, since since the the planetary planetary vorticity vorticity changes changes sign sign over over the the path path of of the the 'flow, 'flow, the the potential potential vorticity vorticity of of the the fluid fluid has has also also changed changed sign, sign, and and so so is is certainly certainly not not conserved! conserved! This This violation violation of of potential potential vorticity vorticity conservation conservation in in cross-equatorial cross-equatorial flows flows and and the the breakdown breakdown of of the the geostrophic geostrophic approxima­ approxima­ tion tion at at the the equator equator constitute constitute two two significant significant challenges challenges in in modelling modelling these these flows. flows. We We present present a a simplified simplified model model of of large-scale large-scale flow flow across across the the equator. equator. (1998a, b) b) show show that that the the presence presence of of friction friction in in the the dynamics dynamics is is necessary …