March 24, 2023

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What determines the average position of the Jet Stream over North America? : meteorology

2 min read

I am this close to seeing the full picture of North America’s climate zones and how they form, all i need is this one bit of information, which i can’t seem to find anywhere, for it all to click.

And I’m not even American, so i don’t even know why I’m obsessed with this lol anyway, first some background.

A while ago, looking at a map of vegetation, i asked myself a question: why is the western part of N.America dry and the east part is lush? I looked into the three cell model, which explains why western N.America is dry (subtropical anticyclone caused by the descending arm of the Hadley cell), but not why the east is rainy. The southeast is lying at a subtropical latitude, it should be anticyclone-dominated too, right? but it’s the exact opposite!

So I looked into the Jet Stream and its wobbles, called Rossby waves, i watched -and took note of- a really interesting online lecture about the relationship between the Jet Stream and mid latitude cyclones, and found out that the way Rossby waves bend determines where we have our highs and lows, particularly the equatorward portion of a Rossby wave is favorable for anticyclones to form while the poleward portion is cyclone-friendly, as a result of the conservation of vorticity. Since the Jet stream bends equatorward on the western US and poleward on the eastern US, this explains why the former is dominated by dry conditions and the latter is rainy. Also obviously the warm Gulf waters supplying moisture. Great! I solved the mystery!

…But now I’m left to wonder why exactly the Jet stream bends equatorward when it crosses the Western US. Why does it do that?

I came across a few webpages and videos linking the presence of the Rocky Mountains to the jet stream wobble, it seems as though any large mountain range tends to bend the jet equatorward, triggering a Rossby wave, but they did not explain it very in depth, so I’m still wondering exactly why the presence of a mountain range bends the jet equatorward. Can you help me understand?


2022-09-24 09:12:08

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