BackDoes the Panchangam provide a clue to predicting the monsoon?
Does the Panchangam provide a clue to predicting the monsoon?
PremiumHeavenly alignments: A farmer in a parched field in Maharashtra. Photo by Hemant Mishra/Mint
New Delhi: When it comes to predicting the monsoon, meteorological scientists may do well to look for clues in an ancient Hindu astrological almanac besides studying the El Nino weather phenomenon.
While 2012 may be horologically mundane—other than it being a leap year—it’s of special interest to those who follow the Panchangam, one of the words by which the almanac, which is based on planetary positions, is known; orthodox Hindus consult it to decide dates for everything, from moving house and starting a business to getting married.
The extra month results from having to synchronize Indian calendars, which try to align the cycles of the moon, with the conventional solar calendar.
The 12 lunar months of the traditional calendar add up to less than a full year whereas 13 lunar months are more than a year. To solve this problem, the Indian calendar defines a normal year to have 12 lunar months and every few years an extra lunar month is “intercalated" to keep in step with the solar year.
By the befuddling omniscience of statistics, it turns out that of the 19 such intercalated years since 1966, 10 have been years with below normal rainfall. Below normal rainfall, according to the India Meteorological Department (IMD), refers to a year when India got less than 96% of the 50-year-average of 88 cm.
That means any given intercalated year, the chances of below normal rains are 52%, an eye opener because the department data itself shows that chances of below-normal rains in any random year, are 33%.
Again, only three of these 19 years saw above average, or rainfall greater than 104% of the normal, when according to IMD any chosen year has a one-third chance of such bountiful rains.
While such a statistical alignment of the sun, moon and monsoon ought to be bizarre—as there’s no physical reason why they should be related—the Met department itself uses climatic variables that make sense only statistically but not in a physical, intuitive way.
“Relying on statistical models means that you use parameters which have some mathematical relationship," said Ravi Nanjundiah, a professor of atmospheric sciences at the Indian Institute of Science, Bangalore. “But you don’t necessarily know how that particular variable actually—in terms of the physics—influences the monsoon’s performance. That’s the big problem with statistical models."
Moreover, six of the 19 intercalated years correspond to so-called meteorological droughts, where the June-September rainfall was less than 90% of the long term average. That’s a 31% chance of any such year being a rainfall-deficient year and a significant twice the meteorological estimate of 16%.
To be sure, meteorological droughts are less serious from an economic and agricultural perspective than agricultural droughts that span at least 20% of the geographical area of the country.
Moreover, none of the recent agricultural drought years of 1987, 2002 and 2009 were intercalated years.
For preparing the long-range forecast, where it estimates the quantity of rainfall between June and September, the Met department uses a method called ensemble forecasting.
It picks six weather variables—North Atlantic sea surface temperature (December + January), equatorial SE Indian Ocean sea surface temperature (February + March), East Asia mean sea level pressure (February + March), Central Pacific (Nino 3.4) sea surface temperature tendency (March to May-December to February), North Atlantic mean sea level pressure (May) and north Central Pacific zonal wind at 850 Hectopascals May). Hectopascal is a metric measurement unit of pressure.
Combinations of these variables are selected and compared with a historical dataset to check how well they’ve managed to tally with deficient, normal or excess rains.
“The best models are selected and these are further averaged with some models given more weight (mathematical importance) than others and what is publicised is the average," said Madhavan Rajeevan, a former IMD forecaster who developed the models.
Forecasters understand the overall physical relationship between the parameters and their influence on the monsoon but don’t know how changes in the values affect the monsoon in any particular year, said Ajit Tyagi, former IMD director general.
“We know the range of values that a parameter can take and its influence... from experience and history," he said.
The way out is dynamical modelling, say experts. That relies on the actual state of the atmosphere on any given day and extrapolates into the future via number crunching and utilizing the physics related to the atmosphere and the ocean.
“Right now, even the dynamical models aren’t reliable for Indian monsoon," said Nanjundiah. “We need to understand so much more about the physics as well as access better data."
The Met department uses dynamical modelling for its short-term forecasts and recently launched a ₹ 400 crore plan, called the Monsoon mission, to improve its dynamical forecasting abilities.
jacob.k@livemint.com
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