Ocean bottom pressure data are examined from a year-long deployment of two bottom pressure recorders, separated by 417 km, in the Argentine Basin. Three forms of variability are found. At high frequencies (periods shorter than 2 hours) the signal appears to be due to gravity wave propagation with the waves preferentially coming from the west. At intermediate frequencies (periods between 2 hours and 12 days), the two records are highly coherent and in phase, reflecting large-scale coherent fluctuations which are well reproduced in a barotropic ocean model. However, the dominant mode is at a period of 20 days, with a phase lag of about 90--100¿ between the two records, consistent with the a mode of variability previously inferred from satellite altimetry and current meters, although the slightly shorter period is consistent with the suggestion that more than one mode contributes to the altimeter observations. Comparison with altimetry demonstrates that aliasing of higher-frequency signals is not a significant source of error and confirms that a previously used altimetry mapping technique reduces the apparent amplitude of the mode. The pressure records show a transport change of about 280 Sv over one 12-day period, and altimetry suggests that regions of higher variability exist. The mode is only weakly excited in barotropic models but is strongly excited in a baroclinic model with a realistic eddy field, suggesting that it is intrinsically linked to interactions between eddies, mean flow, and topography.