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Meteorology (Weather)/Koppen climate zone variations

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Question
Hi. Why are India, Southeast Asia+South China, and the Southeast US all wet climates to some degree, but the Arabian Peninsula is so arid (BWh)?  Similarly, why is southern South America so different from southern Africa, at corresponding latitudes?  

I'm a weather geek and I cannot figure this out (google isn't much help; wikipedia has tons on Koppen in general but nothing on the 'why' of the variations).  Arabian Peninsula is right on the ocean just like India; no mountains on the coast to create a rain shadow. I know moisture comes up from the Gulf and in from the Atlantic in the US, so why doesn't it move up from the Arabian Sea onto the Peninsula?  I'm guessing it has something to do with wind patterns, so what's influencing those?

Thanks.

Answer
Hi Kat

Typically the bigger & wetter patterns of precipitation amounts are regulated by average storm paths which conform to jet stream locations:

http://www.srh.weather.gov/srh/jetstream/global/circ.htm

http://www.srh.weather.gov/srh/jetstream/global/jet.htm

http://www.srh.weather.gov/srh/jetstream/global/climate.htm

http://virga.sfsu.edu/crws/jetstream.html

http://apollo.lsc.vsc.edu/data/gopher/Pictures/wxgifs.hem/globjet.gif

http://www.climate-charts.com/World-Climate-Maps.html#temperature  

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Donald Rosenfeld

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