Abstract
The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional
Corresponding author address: Wilbert Weijer, Los Alamos National Laboratory, CCS-2, MS B296, Los Alamos, NM 87545. Email: wilbert@lanl.gov
