Thursday, March 15, 2018

Ocean Bottom Pressure Data

Oceans of the World
Today I am going to discuss some of the problems with Ocean Bottom Pressure (OBP) records.

What I did in preparation for this post was to generate data from the World Ocean Database (WOD) zones for each ocean area (see graphic to the left).

I used all PSMSL tide gauge stations in each ocean area, as well as all bottom pressure records from the same ocean areas.

Equatorial Indian
The tide gauge stations are found along the coasts of these ocean areas, while the OBP stations tend to be out to sea in deep waters.

I also included satellite based readings beginning in 1993.

Equatorial Pacific
The satellite dataset is the same for all graphs because it is the global mean average.

The tide gauge graphed lines are different in each graph because they are composed of tide gauge records from each individual ocean area.

The same goes for the OBP dataset, which is also from the different/individual ocean areas.

North Atlantic
You can see the OBP problem at once by comparing the sea level change shown by the tide gauges along the coast line, the global mean averages shown by the satellite data, and the two OBP panes.

North Indian
The OBP graph lines are all over the place and generally incoherent in terms of consistent accuracy (Estimating high frequency ocean bottom pressure variability).

This illustrates the major problems that arise in attempting to set up an OBP station that can watch the highest (a.k.a. deepest) sea level areas out in the deep ocean.

One paper indicates that we need a new and robust design for OBP stations:
"Long-term vertical seafloor displacements and geostrophic changes in the water column height could be easily monitored if pressure meters were less susceptible to drift. Currently, these signals, which have typical amplitudes
North Pacific
from decimeters to less than 1 mm/yr, cannot be differentiated from instrumental drift. In this paper, we introduce and outline a new constructional concept for an ocean bottom pressure meter that aims for unequivocal detection and monitoring of long-term trends. The concept is based on a differential pressure sensor that measures the pressure difference between the environment and a reference pressure within a sealed volume. This sealed volume conserves the instantaneous pressure at the moment of its closure at the monitoring location in a temperature-compensated manner. Furthermore, the approach enables easy in situ calibration of the differential pressure gauge by simply opening the reference pressure chamber to the environment and checking the zero point offset
(A new concept for an ocean bottom pressure meter capable of precision long-term monitoring in marine geodesy and oceanography). That is reason for hope.

South Pacific
The OBP stations do not establish the fact of sea level rise and fall, so we can determine the issues with what we have without them.

However, they are ultra critical when it comes to tsunami warnings and several related matters.

What they can offer, in terms of sea level change, is a monitoring system in the areas where sea level is highest.

That would be out in the areas that are generally far from civilization, far from the coastlines.

In that regard I am going to continue the search for the good datasets from good ocean area candidates.

That is, those in critical areas which are of long lasting quality and able to endure the great pressures down there in the deeper waters.

The next effort I will engage in is an attempt to round up all the active OBP stations in one subject area, and fully implement the story they tell.

South Atlantic
That may require additional focus and observation, because one malfunction can spoil things requiring a rejection of the data during the entire time span that the station is producing bad data.

Hopefully, the new instrument design mentioned in the paper I cited above will be active and in place shortly.

In the mean time I will continue to flag aberrant portions of the datasets I already have, and to clean them up.

There is 'out' and there are 'laws'. Lyrics to both here:


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