Abstract
We show how, by focusing on bottom pressure measurements particularly on the global continental slope, it is possible to avoid the “fog” of mesoscale variability which dominates most observables in the deep ocean. This makes it possible to monitor those aspects of the ocean circulation which are most important for global scale ocean variability and climate. We therefore argue that such measurements should be considered an important future component of the Global Ocean Observing System, to complement the present open-ocean and coastal elements. Our conclusions are founded on both theoretical arguments, and diagnostics from a fine-resolution ocean model that has realistic amplitudes and spectra of mesoscale variability. These show that boundary pressure variations are coherent over along-slope distances of tens of thousands of kilometres, for several vertical modes. We illustrate the value of this in the model Atlantic, by determining the time for boundary and equatorial waves to complete a circuit of the northern basin (115 and 205 days for the first and second vertical modes), showing how the boundary features compare with basin-scale theoretical models, and demonstrating the ability to monitor the meridional overturning circulation using these boundary measurements. Finally, we discuss applicability to the real ocean and make recommendations on how to make such measurements without contamination from instrumental drift.
Highlights
In monitoring the global ocean circulation we are faced with a major challenge in the form of the wide disparity in length scales involved
To give an idea of the size of the signals we are interested in, a good rule of thumb is that, at mid-latitudes where the Coriolis parameter f is about 10−4 s−1, a sea level difference of 1 cm reflects a transport of 1 Sv (Sv stands for sverdrup, a unit of 106 m3 s−1), on the assumption that the associated geostrophic flow penetrates to 1000 m depth. This is the transport associated with about a 5% change in the Atlantic meridional overturning circulation (AMOC), for example, and is the size of change we might aspire to monitor if changes in global ocean circulation are considered
In addition to the NEMO data, we show some diagnostics from the Advanced Global Barotropic Ocean Model (AGBOM)
Summary
In monitoring the global ocean circulation we are faced with a major challenge in the form of the wide disparity in length scales involved. This is the transport associated with about a 5% change in the Atlantic meridional overturning circulation (AMOC), for example, and is the size of change we might aspire to monitor if changes in global ocean circulation are considered This is deliberately plotted using a saturated colour scale, in order to show how few regions approach variability of only a few centimetres.
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