Monsoons are complicated, and nobody understands them.
There are five distinct things going on in a monsoon climate:
- ordinary seasonal variation is at a peak. Ideally, the ICTZ is over a region in one season and a long way away for another season, so you get strongly seasonal rainfall.
- rainshadow effects. Mountains can cause the heavy rainy season to be even heavier, exagerating the difference. Even more exceptionally, if seasonal variations cause the direction of winds to change considerably, seasonally, this can become very important when there's a mountain range involved - the rain can fall on one side in the wet season and the other in the dry season (when there isn't much to fall anyway), which further exagerates the difference.
- big blobs of land. These result in stronger pressure fluctuations between seasons, which results in air being pulled onto or off the land. The differences are higher on the eastern sides of continents.
- mountains! As well as creating rainshadows, mountains also exagerate pressure differentials - high mountains have low pressure air which helps suck up wet water from the ocean.
-longitudinal effects. We easily understand latitudinal circulations, but longitudinal effects are a lot harder to work out. Basically, there's a big pressure differential between eastern asia and the western pacific, which causes wind to move between them - but this interacts in a complex way with normal latitudinal pressure differences, through mechanisms such as counter-currents and ocean heights. In essence: when you've got an ocean with an equatorial current, warm water is moved from east to west. This makes the sea higher in the west. This causes water to move away - partly in the powerful western boundary currents, but also partly in a counter-current that slips between the two main equatorial currents, in the opposite direction. The relative balance of these currents is vital. Strong boundary currents reduce the warm peaks, but disperse warm water poleward, bringing rain further from the equator; strong counter-currents move warm water back to the east. If the counter-currents are very weak compared to the equatorial currents, there is a huge mass of very warm, wet, high-pressure water in the west. In summer, this combines with the low pressure over land to create lots and lots of rain. Eg China. But if the counter-current is strong, there is less warm water, so the monsoon effect is weaker. But we don't know exactly what governs the strength of the currents. AND the currents change over time - hence 'El Nino'. AND in the Indian ocean we have complications because the basin isn't big enough for a nice symmetrical current system.
Also, I don't think 'monsoon' is that helpful a term. It's nothing unique, just an exageration of normal processes - a dividing line will be artificial.
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!