WOD Layers Revisited

Fig. 1 WOD layers

I. History?

The World Ocean Database (WOD) project lays out its data in a "zones" format.

Each WOD zone is given a unique number based upon its latitude and longitude location on the globe.

Various instruments have been used over the past couple of hundred years to collect in situ data which is stored in a manner that allows users to select data based on their preferred method of collection (Dredd Blog has tended to use all of them: World Ocean Database Project, 2, 3, 4, 5, 6, 7, 8).

At least the WOD used to be available, but the traditional link has not worked for several days now (Wayback Machine, traditional WOD link).

Let's hope this isn't a result of the supremes' decision (Here Come De Conservative Judges - 14), or Helene (its HQ is in Ashville, NC).

Fig. 2 Layers 14-16

II. Onward

At least for now let's pretend that everything is fine with the history of ocean heat temperatures stored in the WOD.

Anyway, today's graphs are not records of temperatures they are records of sea level at tide gauge stations which are stored in the PSML database.

The traditional WOD layers are shown in Fig. 1 so as to give the geographical perspective of the latitude layer of the tide gauge data from around the globe (with layers listed in red numbers).

The graphs (Fig. 2 - Fig. 6) have one line for each layer listed in that graph (each line is a different color).

Fig. 3 Layers 10-13

Those lines are the RLR average sea level change (in RLR millimeter format) for each tide gauge station in each WOD zone in that WOD layer. 

The graph at Fig. 7 is different in that it has a line for each one of all 17 layers.

This variety tells us that, among other things, sea level rise and fall is different at different latitudes of our planet.

Notice that Fig. 6 is a latitude band with noticeable sea level fall (green line) and noticeable sea level rise (orchid line).

This is the usual situation when large ice sheets are located at a given latitude (e.g. Greenland and Glacier Bay).

Fig. 4 Layers 7-9

III. The Nitty Gritty

As the ice sheets melt the sea level of the ocean around them decreases as the water is relocated toward the equator.

This happens because the loss of ice mass means the loss of the intensity of ice sheet gravity which had historically pulled the ocean water up against an ice sheet's shoreline.

The power of the ice sheet's gravity extends out for up to two thousand kilometers, so the quantity of the loss of water is very significant:

"To our knowledge, Woodward (1888) was the first to demonstrate that the rapid melting of an ice sheet would lead to a geographically variable sea level change. Woodward (1888) assumed a rigid, non-rotating Earth, and therefore self-gravitation of the surface load was the only contributor to the predicted departure from a geographically uniform (i.e. eustatic) sea level rise.

Fig. 5 Layers 4-6

This departure was large and counter-intuitive. Specifically, sea level was predicted to fall within ~2000 km of a melting ice sheet, and to rise with progressively higher amplitude at greater distances. The physics governing this redistribution is straightforward. An ice sheet exerts a gravitational attraction on the surrounding ocean. If the ice sheet melts, the net volume of water in the oceans increases, but the gravitational force exerted by the (now smaller) ice sheet on the ocean decreases. The latter leads to a migration of water from the near field of the ice sheet to the far field. Within 2000 km of the ice sheet, this migration dominates the sea level redistribution and the net result is a sea level fall. In the far field the migration adds to the general increase in ocean volume, leading to a sea level rise in excess of the eustatic."

(On the robustness of predictions of sea level fingerprints). These ice sheet melt caused sea level changes are difficult to understand, especially for people who have been taught that the ocean is like a bath tub where the surface water level is evenly distributed:

Fig. 6 Layers 0-3

"It is well known that complete melting of the Greenland ice sheet would, by itself, raise global-mean sea level by 7.42 ± 0.05 m,2 while melting of the entire Antarctic ice sheet would cause a rise of 57.9 ± 0.9 m.3 It is perhaps less well known that this rise would not be uniform around the globe. Counterintuitively, melting ice from Greenland lowers sea level at the Greenland shore, while far from Greenland it raises local sea level by more than average."

(Gravitational effects of ice sheets on sea level). The lines on Fig. 6 and Fig. 7 are an example of that sometimes perplexing reality.

IV. Closing Comments

Fig. 7 All Layers

One proof of that concept is more easily comprehended when one looks at the sea level history along the coast of south east Alaska (Proof of Concept, 2, 3).

In that case there is a long span of sea level fall along the coastline proceeding from the large ice sheet called the Glacier Bay ice sheet.

That large Glacier Bay ice sheet has been melting for some time causing the sea level to the south of it to decrease all the way down south of the Ketchikan area where the sea level begins to rise at the furthest point away from the influence of that ice sheet's gravitational pull on the ocean.

The sea level rise continues all the way down the west coast of the USA to the border with Mexico and beyond.