Coastal desert feasibility

[spaceman]

Right, I've got a little question about the realism of coastal deserts. I've designed a map, as per the attached image, that purports to have a coastal desert in the purple-shaded area. Is this feasible? I haven't decided on the direction of rotation of this world, yet, although the day length and year length are to be pretty much the same as those of Earth. I'm aware that the answer will have something to do with ocean currents. The dark brown areas of the map indicate mountain ranges.

[Chick]

The issue is not that it's on the coast. There are plenty of coastal deserts on Earth. The issue is whether that region would be naturally arid, and that depends on two main things.

Deserts occur in the latitudes (on Earth, between 20 and 30 degrees N and S latitude, as for example the Sahara) where the Hadley Circulation causes dry descending airflow

https://en.wikipedia.org/wiki/Hadley_cell

or in the lee (called rainshadow) of mountain ranges (as for example, the Tibetan Plateau or the North American interior plains).

https://en.wikipedia.org/wiki/Rain_shadow

So if your prevailing wind is from West to East across those mountains, the purple area would be desert due to the rainshadow of the mountains. If the prevailing wind is from East to West, that would not be a desert unless it is in the desert latitudes.

[Ghostman]

The Atacama desert is the prime example of a coastal desert on Earth: https://en.wikipedia.org/wiki/Atacama_Desert

[Azélor]

Yes and no. If it rotate in the same direction as Earth, it's not very likely to happen. On Earth, the East coasts are never dry at these latitudes. (the east coasts are very rarely dry in general) Well, except in Argentina, probably for the reason specified by Chick in her post.

However, Argentina is the exception. The southern hemisphere is mostly made of water (especially at this latitude) and is colder than the northern hemisphere. Because of this, the two regions have different climates and it can be hard to compare or to extrapolate the results to an alternate world. Patagonia is dry but not New England because of the Jet stream. I can't explain the Jet Stream mechanics well but it's what makes the middle latitudes wet with a succession of sunny and rainy weather systems. https://en.wikipedia.org/wiki/Jet_stream

[spaceman]

Thank you for your comprehensive responses! In that case, let us assume that the direction of rotation is the opposite to that of the Earth. In this case, the purple area would be similar to the Atacama Desert (extremely dry, although not as hot as the Sahara), because:

The Coriolis effect (going anti-clockwise) would turn the surface water along the "purple coast" away from the shore, and you'd get an upwelling of cold air, which won't produce precipitation.
The mountains to the west of the purple area would still create a rainshadow for the purple region.

Does that sound right?

[Chick]

Thank you for your comprehensive responses! In that case, let us assume that the direction of rotation is the opposite to that of the Earth. In this case, the purple area would be similar to the Atacama Desert (extremely dry, although not as hot as the Sahara), because:

The Coriolis effect (going anti-clockwise) would turn the surface water along the "purple coast" away from the shore, and you'd get an upwelling of cold air, which won't produce precipitation.
The mountains to the west of the purple area would still create a rainshadow for the purple region.

Does that sound right?

No The surface water, ocean currents, and surface temperature have little to do with it. The Atacama Desert is smack in the middle of the 20-30degree latitude where the atmosphere circulates downward, causing extreme dryness.

On your map, if the prevailing winds are from the East, the only way that your purple area could be desert would be if it were in the desert latitudes (based on the atmospheric circulation of your planet). On a planet the size of Earth, the regions between 20 and 30 degrees N or S latitude are desert regions.

[Will Phillips]

I've nothing to contribute here other than how much I appreciate the climate insights. Keep it up!

[spaceman]

Thank you for all your help so far! (And I'm in full agreement with Will Phillips here.) I have changed things around quite a bit, and now I think that I've got it looking a bit more realistic. Here it is. (The purple area represents the purported desert region. As stated previously, the planet spins in the opposite direction to that of the Earth.)

[Azélor]

I think it make sense. On Earth, the position of these deserts is very predictable.