A model study of the Southern Ocean dynamics: mean flow, intrinsic variability and teleconnections.

Abstract

The general circulation in the Southern Ocean plays a significant role in the global climate, as it connects all the major world basins, carrying anomalies around the globe through the Antarctic Circumpolar Current (ACC). A better understanding of the physical mechanisms that take place in this region can help improve the knowledge of the global ocean circulation. In this context, this thesis deals with the dynamics of the Southern Ocean and of the ACC by means of a numerical model approach. Results obtained from the primitive equation sigma-coordinate Princeton Ocean Model (POM) concerning both the mean circulation and the intrinsic variability in the Southern Ocean are presented and discussed. Preliminary simulations are performed in order to set up a consistent circulation for the Sothern Ocean, i.e. with a realistic structure of the ACC fronts and with acceptable values of the volume transport across the Drake Passage. Steady wind stress and heat fluxes are imposed based on climatological data in a periodic channel with realistic coastlines and bathymetry. In order to investigate the role of the bottom form drag on the dynamical balance of the ACC, a hierarchy of model implementations for a homogeneous ocean are first developed using both a flat-bottom and an idealized topography. Baroclinicity is also considered: in particular, the meridional density gradient is found to be a significant element, as it activates the JEBAR effect and prevents a total topographic steering. A consistent model for the ACC is finally proposed using realistic bottom topography and background stratification. An overview of the simulations in an eddy-permitting resolution shows evidence of intrinsic variability of the ACC. The latter is found to be strongly sensitive to the bottom topography, eddy viscosity and stratification. By analyzing the results of these numerical experiments we investigate the statistical interrelationships between different regions in the Southern Ocean, and give a first estimate of teleconnections in the Southern Ocean dynamics. Both the variability and the connectivity are found to be very significant in the Argentine Basin, a crucial region where strongly contrasted water masses encounter and mix. Recent observations in this region have evidenced a clear variability of the flow associated with the Zapiola Anticyclone, a strong counterclockwise current that encircles the Zapiola Rise (the main topographic feature located in the middle of the basin). Our simulations does show such a dynamical feature, along with the existence of two markedly different regimes of the circulation associated with the Zapiola Anticyclone on long-term time scales. In particular, the analysis of transport and sea surface height signals evidence a weakly variable regime related to a quasi-steady circulation around the Zapiola Rise, and a highly variable regime associated with fluctuations of the main flow and with a strong eddy activity. An abrupt transition from one state to the other characterizes such a behavior. Moreover, the counterclockwise topographic circulation around the Zapiola Rise occasionally collapses when, within the highly variable regime, the northern branch of the ACC frontal system (the Sub Antarctic Front) is able to shed this structure away. Interesting agreement is also found when comparing model results with altimeter data.