Eddy diffusivities from a doubly-periodic quasi-geostrophic model of the Antarctic Circumpolar Current

Note:The Antarctic Circumpolar Current (ACC), long recognized for its importance in world ocean and climate dynamics, is close to purely zonal in many regions. This means the majority of meridional transport of heat or other scalars is largely achieved by transient mesoscale eddies. Many long-running climate models are still unable to resolve the mesoscale; their effects are parameterized, most commonly using the GM90 parameterization which relates mass transport to the slopes of isopycnal layers via an eddy diffusivity, /'\, . In this project, the ACC was modelled using a two-layer quasi-geostrophic model with idealistic topography. A series of simulations were carried out at combination of topography heights and forcing strengths. Time-mean statistics were used to fit several forms of K,. A paradox was found in K,o, the cross-jet diffusivity. In a global sense, K,o increases as the strength of forcing and jet velocities increased, consistent with higher eddy kinetic energy. On the other hand, a local definition f K,o was found to be a local minimum in the jet core where the velocity peaked, consistent with research suggesting the ACC cores suppress mixing. In addition, a skew diffusivity, K,skew was fitted and found to be globally non-zero for the higher topography, corresponding to a net eastwards flux . For the ridge simulations, large-scale variation in K,o and K,skew was noted and attributed to the effect of local bottom topography on baroclinic instability. T his motivated a matrix definition of K, which included the bottom topography gradient as an additional parameter to explain along and cross jet mass ftuxes. In all forms the cross-jet diffusivity showed a large dependence on the density gradient.Le role du Courant Circumpolaire Antarctique (CCA) clans la dynamique mondiale des oceans est reconnu depuis longtemps. L'ecoulement du CCA est presque de purement zonal clans de nombreuses regions, ce qui signifie que la majorite du transport traversant les jets est effectuee par les remous meso-echelles. La plupart des modeles climatiques de longue duree sont encore incapables de resoudre la meso-echelle et les effets de ces remous doivent etre parametres. Actuellement, le modele GM90, qui estime le transport de masse a partir des pistes isopycnales en utilisant une diffusivite turbulente, K, est le plus frequemment employe. Dans ce projet, le CCA a ete represente par un modele quasigeostrophique de deux couches avec de la topographie idealiste. Une serie de simulations a ete realisee en changeant la hauteur de la topographie et le niveau du forc;age. Dans le but d'evaluer plusieurs formes de la diffusivite turbulente, une analyse des statistiques temporelles a ete realisee .La diffusivite turbulente normale clans la direction meridienne, Ko, a augmente avec le gradient de densite. Une definition locale de KQ, cependant, etait un minimum clans le noyau de jet, en accord avec des recherches qui suggerent la suppression du transport tranversant le CCA. Une diffusivite turbulente clans la direction du debit moyen, "'skew, a ete egalement estimee, montrant un flux net vers l'est pour les topographies et les forc;ages superieurs. Des repartitions spatiales ont ete notees clans les "'O et Kskew· Celles ont motive une matrice de diffusion turbulente afin d'essayer de prendre en compte l'effet de la topographie sur les flux de masse. Cette forme a ete appliquee globalement et localement, et a permis de reduire la variation spatiale de KO et Kskew