The porous boundary of the Caribbean Sea results in this oscillation influencing a mass exchange with the wider ocean, leading to an almost uniform bottom pressure variability over the Grenada, Venezuela, and Colombia basins.
These observations are based on satellite observation of the sea-level, monthly means of basin-averaged ocean bottom pressure using GRACE data, tide gauge measurements, and data from a bottom pressure recorder.
The oscillation was first found in a numerical modelling simulation,[1] from which is shown one cycle of the least squares fit of (left) sea level and (right) bottom pressure on basin averaged bottom pressure in the Caribbean Sea.
The net inflow and outflow required to explain the basin-averaged pressure signal is only a tiny fraction (about 1/1000) of the observed changes in flows through individual straits.
This means that the bottom pressure signal is a side effect of the oscillating mode operating in a basin which can exchange mass with the wider ocean.