Tuesday, May 29, 2018

Let's Not Be Too Dense

Fig. 1 Density Matters

I. Knowing Density

In a recent Dredd Blog post I quoted the World Ocean Database (WOD) Manual, and pointed out a very interesting quote in it:
The reason I have ventured into the sea water density issue is because of what is written in the TEOS-10 manual:
"Since the density of seawater is rarely measured, we recommend the approach illustrated in Figure 1 as a practical method to include the effects of composition anomalies on estimates of Absolute Salinity and density. When composition anomalies are not known, the algorithm of McDougall et al. (2012) may be used to estimate Absolute Salinity in terms of Practical Salinity and the spatial location of the measurement in the world oceans."
(TEOS-10 Manual, p. 14, p. 24 PDF).
(On Thermal Expansion & Thermal Contraction - 35, emphasis added). That could be quite significant.

As the graphs at Fig. 1 and Fig. 2 show, the maximum density temperature of the sea water is an important part of determining whether contraction or expansion will take place as the temperature of the sea water being analyzed changes.
Fig. 2 Opposite of Fig. 1

Unless one calculates the density and the maximum density temperature, one can not know exactly enough whether or not, or how much thermal contraction or thermal expansion is taking place at any given in situ context.

In other words, the degree of expansion or contraction taking place is impacted by how close CT is to the sea water maximum density in the in situ context the measurements were taken.

In other words one will not have a clue as to how much or even whether or not sea level is being impacted by the change in sea water temperature.

II. Easy

Furthermore, the density is quite easy to determine, and so is the maximum density temperature, when one is using the TEOS-10 software library to analyze in situ measurements.

Since the TEOS-10 library wast released about 8 years ago one does not need a pressure gauge onboard a CTD device or an ARGO float to determine sea water density.

That can be calculated using the TEOS-10 "gsw_ct_maxdensity" function.

III. Use Good Tools, Not Toys

While writing software modules and using actual in situ sea water temperature measurements around Antarctica, then generating graphs, I noticed that at times the Conservative Temperature and the Maximum Density Conservative Temperature could run parallel to one another.

"Does thermal contraction or expansion take place when they run parallel to one another?" I wondered.

Whether it does or not, thermal contraction and expansion can and does take place anyway.

The question is how much, because it is the net result (add expansion, subtract contractions) that matters.

IV. Conclusion

The graphs at Fig.1, Fig. 2, and Fig. 3 are hypothetical examples showing where the sea water temperature and the maximum density temperature could cause different and even opposite results.
Fig. 3

The initial data indicates that even though the sea water temperature (Conservative Temperature - CT) was increasing quite a bit the thermosteric volume was not.

I will finish those modules soon, and share the results.

Hopefully we can determine if and how much this impacts the thermal expansion hypothesis.

The hypothesis I am talking about is the one that alleges that thermal expansion is the main cause of sea level rise.

UPDATE conclusion: Fig. 2 is the most common of all by far.

The rare Fig. 1 and Fig. 3 phenomena are not major considerations, so from now on I will focus on the Fig. 2 phenomenon.

Here is a preliminary list I compiled after parsing all WOD zones (1968-2016) at all 33 depth levels:

year 2007, CT [0.756752], MDCT [2.16742] @ zone 7715 @ depth 0m
year 2007, CT [0.125456], MDCT [2.08418] @ zone 7715 @ depth 10m
year 2007, CT [-0.68055], MDCT [2.03381] @ zone 7715 @ depth 20m
year 2007, CT [-1.29632], MDCT [2.06461] @ zone 7715 @ depth 30m
year 2007, CT [-1.05918], MDCT [2.2507] @ zone 7715 @ depth 50m
year 2007, CT [-1.44098], MDCT [2.35989] @ zone 7715 @ depth 75m
year 1988, CT [0.744434], MDCT [2.81778] @ zone 1601 @ depth 0m

year 2015, CT [2.64246], MDCT [2.89663] @ zone 1602 @ depth 0m
year 2015, CT [2.63883], MDCT [2.87479] @ zone 1602 @ depth 10m

See updated graphs here.

The next post in this series is here.

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