Thermosteric Sealevel Change Revisited - 2

Fig. 1 "It depends"

Revisiting the thermal dynamics of fresh water generally begins with remembering that the maximum density temperature is critical.

The graphic in the first post helps us grasp the reality involved with whether fresh water will expand or contract when we add or remove heat from a mass of it, because that depends on its temperature at the instance we add or remove that 'heat'.

For example, if the temperature of the mass is below 4 deg C (e.g. 3 deg C) when we add heat the mass will lose volume until the temperature reaches 4 deg C but will increase in volume if we continue to add heat beyond that maximum density temperature point.

Sea water is the same in principle in the sense that whether it will expand or shrink in volume if we add or remove heat depends in substantial part on its temperature at the instant we add or remove that heat.

But since seawater also has salinity as an additional factor to be considered, the graphic at Fig. 1 indicates that requirement by using 'X' as the maximum density variable (instead of a fixed temperature).

The TEOS-10 library can be used to our advantage when we want to determine the 'X' factor of seawater (see "IV. Important TEOS-10 Functions" here, noticing the all-important gsw_alpha function).

Fig. 2

TEOS-10 can be used to determine whether a mass of seawater will expand or contract in volume if heat is added to or removed from that mass of seawater.

To develop visual aids for delving deeper into the dynamics of this subject, I designed an experiment as follows: 1) determine which WOD zones have PSMSL tide gauge stations in them; 2) load the WOD temperature, salinity, depth, latitude, longitude, length, width, depth, and volume of those WOD zones; 3) load the PSMSL tide gauge records of sea levels dating from 1950 through 2023; 4) calculate the average tide gauge sea level for each year; 5) calculate the thermal expansion and contraction of the sea water at up to 33 depth levels based on the WOD data.

The exercise is to compare a zone's thermal dynamics with it's tide gauge recorded sea levels.

Fig.3

But more than that, the zones involved are restricted to Atlantic Ocean zones and tide gauge stations, Pacific Ocean zones and tide gauge stations, and Indian Ocean zones and tide gauge stations (see Fig. 2, Fig. 3, and Fig. 4 graphs).

What these graphs show is that there is no correlation between steric and thermosteric dynamics.

If we were talking about humans we could say they each have a mind of their own.

But since we are talking about abiotic rather that biotic entities, we can say that different dynamics determine the shape of the graphs.

Fig. 4

The tide gauge station sea level records are not based on temperature, salinity, depth, latitude, or longitude, they are based only on the surface location over a span of years.

On the other hand, the thermal expansion and contraction is based on temperature, temperature change, salinity, depth, latitude, longitude, and the mass of the depth slice of the ocean upon which the TEOS-10 calculations are applied.

The formula and some of the TEOS-10 considerations were pointed out in the previous post of this series (Thermosteric Sealevel Change Revisited), however, let me point out and emphasize that today's post is not about the same subject matter or the same WOD zones.

The WOD zone data involved in today's posts are only zones with coastlines within them, because tide gauge stations are located only on shorelines, thus the zones are located in very different geographical areas.

Thus the in situ measurements are less in number than in the previous post which also included zones without coastlines in them (e.g. zones far offshore).

The graph lines that show Tidegauge SLC are very different from the graph lines below them which show Thermal SLC (thermal expansion and contraction) even though they are in the same zone or zones.

Not to worry, because that difference is to be expected if we consider how different the two dynamics (steric, thermosteric) are.

Today's appendix (Appendix HTML) contains the list of zones per ocean used in Fig. 2, Fig. 3, and Fig. 4, and also the HTML data further emphasizes the differences between the two SLC types in terms of the magnitude of steric SLC compared to thermal SLC.

Thermal expansion is a minor part of sea level rise and fall, but that is not well understood , so, "worse than previously thought" is how glacial melt is described all too often because every once in awhile it becomes clear that more Cryosphere ice is melting than previously thought (Widespread seawater intrusions beneath the grounded ice of Thwaites Glacier, West Antarctica, dated May 20, 2024; "Glaciers suffer largest mass loss in 50 years" - State of Global Water Resources 2023, dated 07 October 2024).

The previous post in this series is here.

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