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For the past few years in succession India has witnessed extreme flooding events caused by monsoonal troughs colloding with what appears transitory mid latitude disturbances and these appear to trigger flash floods. Case in point - http://en.wikipedia.org/wiki/2013_North_India_floods and http://en.wikipedia.org/wiki/2014_India%E2%80%93Pakistan_floods. My question relates to the physics of such interactions.

Following are my questions:
1) The northward propagating waves emanating at the 850 hPa level from the MJO(BSISO) appear to be damped at the Himalayan foothills(30 N latitude). Following orographic interactions I presume the 850 hpA 's geopotential height appears to be raised. Is that an accurate statement i.e. in the mountains the 850 hPa monsoonal trough's geopotential height is higher(much higher) than at sea levels?

2) Then the planetary wave at the 500 hPa level then appears to be dipping down in the tropics and interacting with monsoonal troughs. How does a upper level trough "propagate" down ? Is this due to quasi geostropic theory?

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Here are two parts of an answer, but this is by no means complete.

  1. Most monsoonal research is focused on explaining the monsoonal passage through the plains of the Indian subcontinent. Very few papers can be found on how the same troughs cause rainfall at 3000 meters. I would like an authoritative reference on Monsoonal Mountain Meteorlogy. All that is known is that the MJO (BSISO) troughs and ridges at the 850 hPa seem to be damped at the Himalayan foothills.
  2. Regarding the 500 hPa propagating downwards I cannot get further more information than introducing the concept of a downward propagating Rossby wave. And as far as the physics that explains what happens when these two disparate systems meet it is at best an open research topic.
  3. I have read recent discussions(no reference available yet) that the two systems may need to "line up" in other words the geometric center of the upper level system needs to pass through the center of the lower level trough.Very recently Tropical Cyclone Hudhud moved up northwards towards the Himalayas and caused a blizzard in Nepal. There was a corresponding mid latitude disturabance at around the same time over the western Himalayas but extraordinary damage was prevented because the two systems did not line up.

What is of more serious concern is the fact that there have been four such incidents in recent times - 1) Pakistan Floods 2010 2) North Indian Floods 2013. 3) Indian Pakistan floods of 2014. 4) The near miss with Cyclone Hudhud. What climatic changes are causing these collisions between disparate systems is a new field of study. Both weather and climate models will be stretched to predict these sort of collisions.

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