Earth Ocean Currents
When choosing world map projections from the customary library of available layouts, the problem reduces to picking the least worst alternative. But, when our interest is ocean currents, the customary library works pretty well.
Here’s why: Earth’s rotation in space — spinning on the polar axis — drives the ocean currents. So a world map that centers up a pole shows the rotational axis as a point (a line seen on end appears as a point) and shows the currents with the same (or similar) clarity of understanding as an axial view of motor.
August 2012 EDIT: A real live professor of oceanography corrects me. Earth’s rotation in space does not drive the ocean currents; my expert source says the primary drivers are wind (for surface currents) and density differences (for subsurface currents). See comment(s) below by Petruncio for more detail. My info about rotation came from Nat’l Geo, and I may have misinterpreted their article. Need to check it . . .
[EDIT: Not to get overly grumpy, the fine people at Mail Online lifted the first map below and published it without credit, and without explaining to their readers that the image had nothing to whatsoever to do with the story they were reporting.]
The projection we want is an “equal azimuth,” and it looks like a circle which we may imagine to be spinning like a wheel. One pole is the point at map center, the other pole is the circle the makes the map’s edge. This is a little confusing, because now what used to be a point (the opposite pole from the one we put at map center) is now a circle within which is shown the whole surface of the earth. But that’s just the trick of equal azimuth — it gives a point (something of zero dimension) a size. The scale at map edge is infinite, but let’s not bog down. Besides, we may reverse our choice of pole for map-center and only think halfway (crop each map at the equator), employing two maps, each of which encompasses a hemisphere. Of course that defeats the purpose of using a world map, getting to see the interplay of all the currents all at once.
So give it a try. Work up to that outer circle slowly. Remember, it’s the patterns made by the ocean’s currents that we are contemplating, and earth’s rotation is the ocean’s motor.
Here we see SURFACE, MID-LEVEL, and BASEMENT CURRENTS in Red, Green and Blue respectively. Thanks National Geographic, for showing us how they go. (NG used an elliptical map centered on the Equator, and mislabeled 10 degrees south latitude as Equator, but hey, mistakes happen. Forgive.) I merely put them in equal azimuth projection, 10 degree graticles.
SEASONAL REVERSIBLE CURRENTS (culled from several sources) are in Yellow.
LAND is Grey, with land and ICE 10,000 feet and more above sea level in White.
My, how Antarctica’s two great gulfs play with the currents! There the mid-level flow drops to basement level, but let’s look at things from the opposite side of the solar system:
What’s that you say? A little hard to follow things — especially the play of Antarctica’s gulfs — at map edge? And what, around the periphery, are those trails of white dots?
Here’s what’s going on: On world maps, we traditionally want our continental shapes fair and regular, and let the smeary parts be the water. The top map’s edge is the North Pole, which is water, so it’s a little easier to spot the landmarks. The bottom map’s edge is the South Pole, so Antarctica gets smeared. But Antarctica has topographic structure; it, as all continents (islands too), is made up of watersheds, valleys separated by ridges. The White Dots are Antarctica’s RIDGES. and with constant-scale natural boundary mapping, we can use these ridges to form the edge of another map that, even though it cannot present Antarctica as a unified, conformal shape, it can present Antarctica’s constituent watersheds as unified, conformal shapes:
All right so far, you perhaps say. But what are the several shades of Grey? And, since Antarctica is covered with ice, what valleys do we see on this map, rock valleys or ice valleys?
Answers: Shades of grey are various categories of rock (in relation to sea level), and the valleys shown are rock. And, though you didn’t ask — yes, the map does fold up, to highly condensed globular object. Here’s a little poster with specifics.
Sorry for the low resolution on these images.