Variability of deep-ocean mass transport: Spectral shapes and spatial scales

This paper studies the variability of deep-ocean mass transport using four 1000-yr integrations performed with coupled general circulation models. Sta tistics describing the spectral and spatial features are considered. It is shown that these features depend crucially on the time-mean state. For the transport of tropical and subtropical water masses in three of the integrat ions, the spectral levels continually increase with decreasing frequency an d do not show isolated peaks at low frequencies. The slope of the low-frequ ency spectrum (in a log-log plot) changes with increasing depth. It has val ues of about 0 near the surface, about -1 at intermediate depth, and about -2 at or near the bottom. The result indicates that the maximal memory time scale for deep-ocean mass transport is longer than a few centuries. The sit uation is different in the fourth integration, which has a different mean c irculation pattern. In this case, the low-frequency spectrum is more or les s hat in the tropical and subtropical oceans below 2000-3000 m, indicating weak low-frequency variations. The dominant spatial covariance structures d escribe an anomalous recirculation of intermediate water masses, which is c onfined to a large extent to each ocean basin. The spatial scale of the dom inant modes is therefore smaller than that of the time-mean circulation.