Specifically ocean water.
And especially on most ocean water, which is located under the surface deep down all the way to the bottom.
There are exceptions as the World Ocean Database (WOD) data have shown.
That is, the surface can be colder than the deep water at times at various locations.
Yes, to add to the non-intuitive nature of some of the data, the surface can be the coldest part of the water column, depending on the WOD zone in question.
Not only that, like politicians, the ocean layers can flip-flop at times, changing from the warmer layer to the colder layer, and vice versa (NASA JPL).
|Fig. 2 Salinity matters
Clearly, it is not limited to thermal expansion in the form of the hypothesis "thermal expansion is the MAJOR cause of sea level rise" would have it, because there is thermal contraction too.
Anyway, in terms of temperature, the norm is for the water to become colder as the depth increases.
The factor of salinity also comes into play in ocean water, in terms of what effect a change in temperature has on that ocean water.
The graphic at Fig. 1 shows the effect of temperature on pure ("fresh") water, and the graphic at Fig. 2 shows the difference when that fresh or pure water becomes saline.
But more than that, Fig. 2 is a case where "freshening" has taken place to the point that the maximum density of that ocean water takes place at -2 degrees Celsius.
In the case depicted in Fig. 2, if heat is added to ocean water that is below the temperature of -2 C, it will contract or shrink until it reaches the temperature of -2 C, at which point if more heat is added it will then expand (it will also expand from that point if heat is removed).
In other words, when an area of either ocean water or fresh water is at its temperature of maximum density, changing that temperature to either a warmer temperature or a colder temperature will cause expansion.
In the context of this series, that factoid is significant and important to remember.
As more freshening takes place in an ocean area, the maximum density point rises toward 4 deg. C, and finally, if enough freshing takes place to make the ocean water virtually pure, the temperature of maximum density will also go up to or near 4 deg. C.
This is also relevant because freshening is a growing phenomenon:
Recent salinity changes in the Southern Ocean are among the most prominent signals of climate change in the global ocean, yet their underlying causes have not been firmly established. Here we propose that trends in the northward transport of Antarctic sea ice are a major contributor to these changes. Using satellite observations supplemented by sea-ice reconstructions, we estimate that wind-driven northward freshwater transport by sea ice increased by 20 ± 10 per cent between 1982 and 2008. The strongest and most robust increase occurred in the Pacific sector, coinciding with the largest observed salinity changes. We estimate that the additional freshwater for the entire northern sea-ice edge entails a freshening rate of −0.02 ± 0.01 grams per kilogram per decade in the surface and intermediate waters of the open ocean, similar to the observed freshening. The enhanced rejection of salt near the coast of Antarctica associated with stronger sea-ice export counteracts the freshening of both continental shelf and newly formed bottom waters due to increases in glacial meltwater. (Southern Ocean Salinity, emphasis added.(cf. Washington Post). Increasingly, as time goes on the calculation of whether adding warmth to ocean water will cause expansion or contraction of that water is something that takes a closer look in order to figure it out properly.
The cold meltwater that enters the sea water cools and freshens the ocean surface. Driven by the winds and other factors, this modified water mass then sinks below the warmer surface waters to form what is known as the Antarctic Intermediate Water, which has a comparatively low salinity. At depths of about 600 to 1,500 metres, this water spreads as a tongue to the north with its tip stretching as far as the Equator, and in the eastern Atlantic even as far as the coast of the Iberian peninsula. (Freshening of the Southern Ocean, emphasis added).
At any rate and at any temperature above the maximum density level temperature, when heat is removed there is a resulting contraction equal to the expansion which took place when that heat was added.
And, as regular readers know, I have recently provided a long list of graphs that are generated with WOD data.
They show that the surface layer, as well as deep layers gain and lose heat over time, thus, they expand and contract accordingly (it is not a one-way street).
Analysis of the degrees of that change, on balance, show that certainly thermal expansion is not a major cause of sea level rise.
Melting and decomposing ice sheets and non-ice sheet land glaciers have always been the major cause of sea level rise and fall.
The next post in this series is here, the previous post in this series is here.