In an early post here I wrote the following brief description of Tulavarsham (literally "the Showers of Libra"), the Keralan name for the North East or Retreating Monsoon:
"In late October - early November, Kerala experiences a very unique weather regime called 'Tulavarsham' - the mornings are sunny and clear; by midday it gets very sultry and the afternoon sees a rapid and massive buildup of clouds followed by a thunderstorm, which clears up in a couple of hours. The evenings are usually clear and humid."
And there are any number of other online pages which repeat: "During Tulavarsham, it commonly rains in afternoons with thunder and lightning".
That the Advancing (South West) Monsoon blows in from the Arabian sea due to the subcontinent getting hyper-heated in summer is no mystery. That the monsoon has to retreat once the temperature gradient levels off and then inverts in autumn is not hard to understand either. The Coriolis force explains why the air current advances and withdraws along a South-West to North-East axis rather than a North-South one - undergrad, if not high-school, stuff.
But there still remains plenty about the Thulavarsham phenomenon that I find very puzzling.
First up, why does it rain almost exclusively in the afternoons/evenings? Afternoon thunderstorms are quite the norm in equitorial rain forests (this again is school gyan). But there, what happens is local convection with no large-scale *horizontal* currents of air; here we do have a major and steady drift of air masses from the north-east.
One could guess: the moisture laden monsoon (the moisture having been collected from the Bay of Bengal) blowing in from the north-east collides above Kerala with the afternoon sea-breeze and this causes the disturbances and violent showers. The sea-breeze does set in in the afternoon. But is it strong enough to cause such precipitation? I really don't know.
There are other issues: Why does it not rain anywhere else on the western coast during Thulavarsham (rainfall is utterly scanty on the coast beyond north Kerala; Bombay, which gets much more rainfall from the main south-west monsoon than most places in Kerala, actually suffers a brief but sharp summer called 'October heat'!)? Why don't the Western Ghats cause a rain-shadow effect over Kerala for the North-East monsoon (they do cast a rainshadow over western Tamil Nadu during the South West monsoon)?
The first question is perhaps answered by the shape of the Indian peninsula - a triangle with apex at Kanyakumari. This coupled to the North-Easterly direction of the withdrawal of the monsoon suggests that over say Bombay or Goa, the withdrawing monsoon would have hardly any moisture since it would not have passed over any waterbody. In the south, the Bay of Bengal provides enough water to the withdrawing monsoon to drench a good half of our eastern coastline in pretty steady rain - and even after traversing Tamil-Nadu, the winds have enough moisture to give Kerala a sharp rainy spell.
I have no good idea why the Western Ghats have such an asymmetric rain-shadow effect. Unlike in Maharashtra/Karnataka where the mountains drop sharply to the west coast and slope much more gently eastwards, along Kerala-TamilNadu border, they are steep in both direction. A possible explanation could be the narrowness of Kerala which allows the sea-breeze to play a major role in causing precipitation - the Bay of Bengal is quite far from Western Tamil Nadu.
A legend from medieval Kerala relates: the Portuguese stealthily took away some pepper wines; the Zamorin remarked: "Nothing will happen! They can't take away our 'Edavappathi' (the South-West Monsoon)"
I beg to differ with Zam. the Edavappathi gives no *exclusive* benefits to Kerala - it gives good rains to Kerala all right but it gives much more copious precipitation to Coastal Karnataka, Konkan and so forth. But, the Thulavarsham is a different matter altogether; ignoring the west-coast above Kasaragod, it extends the Keralan rainy season almost to December. And this is precisely why Kerala has, by far, the most persistently lush landscapes in the country. One can see drought-resistant scrub and cactus hedges in the hills near Bombay but they are unheard of anywhere in Kerala except perhaps the farthest corners.
And seeing the rapid buildup and awesome approach of a Tulavarsham thunderstorm from the Ishanya (North East) corner of the October sky, one can't help recollect the inspired imagery of medieval Keralan poet Poonthanam's hymn addressed to Kali (Thirumandhankunnil Amma):
"Ghanasangham idayunna thanukanthi thozhunnen!"
("I behold and salute thy fearful form, a vision of dark, swirling rainclouds thunderously colliding with one another!")
