Modal Variability of the Argentine Basin

Altimeter observations analyzed by Fu et al. (2001) shows a very pronounced mode of variability in the Argentine Basin: a dipole in sea-surface height appears to rotate around a seamount -called Zapiola Rise- every 25 days. Bottom pressure measurements by Hughes et al. (2007) confirm the presence of strong barotropic oscillations, but at a period of only 19.6 days.

In Weijer et al. (2007a), we show that this discrepancy can be explained by the presence of multiple oscillatory modes. Two statistical techniques (POP and CEOF analysis), applied to 10 years of altimeter data, show several distinct statistical modes; one matches the Fu et al. (2001) observations, another those of Hughes et al. (2006).

As a matter of fact, we identified a whole spectrum of modes in a shallow-water model of the Argentine Basin. The dominant mode with a period of 19.6 days clearly corresponds to the Hughes et al. (2006) oscillation. However, the mode displaying the clearest similarities (in spatial structure) with the Fu et al. (2001) observations shows up with a period of 31.5 days.

In Weijer et al. (2007b), we used a continuation method to link the spectrum of modes of the Argentine Basin to the Rossby basin modes in the flat, rectangular basin of classical theory. To this end, we defined a gradual transformation that morphed the full-bathymetry domain into a rectangular basin; and although in reality the modes are strongly controlled by bathymetry, (almost) each of them could be linked to a distinct classical Rossby basin mode.

Currently, we are analyzing the spectrum of the Argentine Basin in a two-layer shallow-water model. We want to find out how stratification affects the oscillation period and spatial pattern of the otherwise barotropic modes.

References:

Fu, L.-L., B. Cheng, and B. Qiu (2001). 25-Day Period Large-Scale Oscillations in the Argentine Basin Revealed by the TOPEX/Poseidon Altimeter. J. Phys. Oceangr., 31, 506-517.

Hughes C. W., V. N. Stepanov, L.-L. Fu, B. Barnier, G. W. Hargreaves (2007). Three forms of variability in Argentine Basin ocean bottom pressure, J. Geophys. Res., 112, C01011, doi:10.1029/2006JC003679.


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