As monsoon season approaches, a billion people await the seasonal forecast

By Dr Andy Turner

As spring continues towards summer, a sense of foreboding and expectation builds – what will the monsoon bring to India this year? The Asian monsoon is one of the most dramatic expressions of the seasonal cycle on the planet], both in terms of its rainfall and winds, and is crucial for water supply for human health, agriculture and industry in some of the world’s major developing nations. India’s billion plus population relies on the monsoon between June and September for around 80% of its annual rainfall – averaging about 85cm across the country but reaching several metres in the wettest places. During spring the warming land surface, together with elevated heating over the Tibetan Plateau, generate a large-scale gradient in surface pressure that drags the monsoon winds towards India from over the Indian Ocean. This flow brings with it moist air, culminating in the monsoon as it converges over India, Southeast Asia and China.

So why am I writing about this now, in April? With the sort of forward planning you would expect in the world-leading centre for weather and climate research that is the Department of Meteorology, this blog coincides with the release of the official seasonal forecast of monsoon rain for the coming summer. In late April each year, the India Meteorological Department (IMD) issues their early-stage seasonal forecast for the summer rains. It comprises their traditional statistical forecast as well as more experimental dynamical forecasts. The statistical forecasts rely on inputs from five or six measured parameters known to perturb the monsoon, such as the evolving state of conditions in the Pacific Ocean and snow cover conditions over Eurasia – if there is excessive snow then the heating over the Tibetan Plateau can be reduced, weakening the monsoon. The problem is that they don’t represent the evolving physics of the situation, meaning they can quite easily go wrong. Dynamical forecasts are those from coupled-ocean atmosphere models, rather like weather forecasting models, run for the season ahead and started from our current best observations of the atmosphere and ocean. These represent the best future prospect for accurate seasonal predictions of the monsoon (e.g. the example model at the Met Office) since they attempt to represent the physical processes in the ocean and atmosphere essential to the monsoon. Current dynamical models suffer rapidly developing biases, however, which somewhat limit our confidence in them.

The seasonal forecasts are eagerly anticipated every year owing to the importance of the monsoon rains. It is not only the massive agricultural sector that relies on forecasts for estimations of total rainfall and the onset date in order to plant crops appropriately, but the stock market is also affected via the impact of the rains on the coal and steal industries among others. Large droughts such as 2002 have been shown to affect GDP.

This year, the dynamical and statistical forecasts are down. The statistical forecast comes in at about 95% of expected normal rainfall over the June-September season. Bearing in mind that typical year-to-year variation in the monsoon is around 10% (the standard deviation: in other words around two-thirds of monsoon seasons are within a plus/minus 10% range of normal conditions) this is relatively normal. Results from the experimental, dynamical model, are less cautious however, at 88% of normal rainfall. The large droughts in 2002 and 2009 summers saw in the region of 80% of normal rainfall.

The culprit this year seems to be the prospect of a developing El Niño – a warming of the central and eastern equatorial Pacific Ocean, known to be related to monsoon droughts since the days of Sir Gilbert Walker. Measurements of the warm water volume – a known precursor to El Niño – in both the central and east Pacific Ocean suggest conditions are ripening for a large event. Indeed the heat content measurements in both regions seem as large as any since the big El Niño of 1997.

As conditions continue to change during late spring the IMD will issue a renewed forecast for the remainder of the season in June, taking into account the real evolution of El Niño conditions amongst other factors.

 

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