The EconomistJune 29th 2019 Essay |The South Asian monsoon 43
I
n 1976 RaghuRai,thephotographerwhosepictureofgathering
cloudsgracesthebeginningofthisessay,tooka pictureofwater,
hopeanddaring.ModerntowersinthebackgroundspeakofIn-
dia’sfuture;ancientmasonryintheforegroundofitspast.A lithe
boyisonthecuspofflight,hangingabovethedeepwatersbelow.
Thesettingofthepicture(reproducedoverleaf )isoneDelhi’s
oldeststructures.AgrasenkiBaoli—Agrasen’sstepwell—wasbuilt
(orperhapsrebuilt)inthe14thcenturyadbyfollowersofa mythi-
calkingwhoselifeischronicledintheearliestSanskritepics.The
countryiscoveredwitha vastnumberofwatertanksandstep
wells,someofthemstunninglyelaborate,usedvariouslyforirri-
gation,drinkingwaterandreligiouspurification.Thuscontained,
watersignifiespoliticalandreligiouspower—aswellaslifeitself.
YettodaythebottomofAgrasenkiBaoli,farbelowthesoaring
boy,isjustdust.ThatmightbereadastestamenttotheheatwaveThedecisive moment
T
here is onething the locals do not talk about in the wettest
place on Earth: the rain. What, ask the townsfolk of Mawsyn-
ram, is the point?
Their settlement sits on a hilltop plateau in the north-eastern
state of Meghalaya—“Abode of the clouds”. Immediately to the
south, the hills fall away to Bangladesh’s steaming plains nearly
1,500 metres (5,000 feet) below in a sharply picturesque way—
“Danger: Selfie Zone” a road sign warns. To the north are the Khasi
Hills, standing athwart the path of moisture-laden southerly
winds eager to continue north. The hills scoop the wet air up-
wards, wringing out its rain.
The consequence, in Mawsynram, is daily rain for a good nine
months of the year. The settlement sees 11.9 metres (467 inches) of
rainfall in an average year, over a dozen times that seen in Man-
chester, say, or Seattle: in an exceptional year it can see 16 metres.
Before the monsoon it comes in heavy night-time thunderstorms.
From late May, it is nearly continuous, sometimes a steady drizzle,
sometimes rods that rattle tin roofs too loudly for conversation.
Why a tin roof? Dead wood rots quickly, so the locals often build
with metal—or with wood that is still alive, training the aerial
roots of Ficus elastica, the rubber fig, to form bridges over the rivers
and streams by which the water flows down to the plains below.
The certainty of daily rain, though, marks Mawsynram out as
unusual. In most places, there is a great deal of variation in rainfall
over the course of the monsoon. The season is marked by promi-
nent “active” spells, typically when depressions travel west along
the monsoon front, followed by dry “breaks”. Timing the sowing of
a successful crop is a gamble on what breaks may come, and for
how long. Yet today’s meteorological models remain poor at pre-
dicting the monsoon’s weather more than a few days out.
Then comes the need for more localised predictions. Knowing
where storms will do their worst would save not just crops but
lives: lightning kills more Indians than cyclones do, though cy-
clones do much more damage to property. Monsoon meteorolo-
gy’s big challenge, then, is to improve predictions of these intra-
seasonal shifts. That means combining its models of global
climate with an understanding of local peculiarity. The source of
much of that can be found in the source of the rains that roll down
the Khasi Hills: the Bay of Bengal.
Thanks to satellite imagery and cloud-busting radar, R. Venka-
tesan of the National Institute of Ocean Technology in Chennai
points out, people now know pretty well what is happening in the
atmosphere. But “the ocean remains very murky.” His high-roofed
warehouse in Chennai is a treasure trove of toys dedicated to de-
murkifying it: instrument-laden, unmanned sea kayaks for taking
surface measurements; buoys that anchor to the sea bed with sen-
sors to measure temperature, salinity and current; autonomous
floats that drift about collecting data at the surface and at depth; an
engineless underwater “glider” that soars through the water col-
umn, surfacing every few days to tell satellites what it has gleaned.
Many of them are about to be deployed in the Bay of Bengal.
The bay, the world’s biggest, has a notable oddity. It boasts a dis-
tinct surface layer of light, comparatively fresh water floating atop
its deep-sea salinity. This layer is maintained by two things. One is
the Brahmaputra, the Ganges and the rest of the lesser rivers, such
as the Godavari and the Krishna, that drain most of the subconti-
nent’s rainfall into the sea to its east. The other is rain that falls
onto the bay itself. This fresher water means the top few metres of
the bay are much less well mixed than is the norm in oceans—and
thus that the sea-surface temperature can vary more quickly. This
thermodynamic skittishness is passed on to the air above.
In 2015 a joint American-Indian mission used Mr Venkatesan’s
hardware, among other tools, to study interactions between the
bay and the weather in unprecedented detail. The study reinforced
the scientists’ belief that the bay’s quick changes are the key to the
breaks between monsoon rains. Rain falling onto the bay itself
cools the surface layer enough to limit the convection that would
produce more rain for a while before the surface heats back up
again. This year’s follow-on mission will try to refine the analysis
by gathering real-time temperature, salinity, current and wind
data to put together a better picture of how the Bay of Bengal makes
the weather, propagating bursts and hiatuses.
Meanwhile, in the Khasi Hills, pepper gardens and betel-leaf
plots on the hillsides will go untended as the rain pours down. In-
stead, people will lend their hands to the rain’s long-term project
of dismantling the hills themselves. In recent years local kingpins
have been putting the industrious locals to new work: felling the
hills’ forests to get at the coal, limestone, China clay and even gold
underneath. An orgy of illegal quarrying and “rat-hole” mining is
disfiguring the landscape and eroding the hillsides and stream-beds.Khasimenandwomensitbyroadsideswithball-peenham-
mersreducingboulderstopebblesthesizeofpeasthatwillde-
scendtotheplainsnotbystreambutbylorry,theretofeedanin-
frastructureboomonthealluvialplainsofBangladesh.
Damagingandunsightlythoughitmaybelocally,themining
andquarryingissmallbeercomparedtotheworkofnature.Allthe
sedimentsintheplainswheretheBangladeshisarebuilding come
fromhillsandmountainserodedawaybywinterice,springtime
meltsandmonsoonrainsfortensofmillionsofyears.Butifhu-
mansarenotallthatimpressiveonthemountain-levellingplains-
buildingsideofthings,theymorethanmakeupforitwhenit
comestoinundatingtheplainswithevergreaterburstsofflooding
andwashingthemawaywithsea-levelrise—processeswhichwill
haveconsequencesforthatinfrastructureboom infarlessthana
millionyears. 7Breaks and vagaries
The aerial roots of Ficus elastica, the rubber
fig, are trained into bridges over the streams
by which the water flows down to the plains1