Indian monsoon rains around five days late

The India monsoon, seasonal rains that spread across the country each year, are progressing northwards around five days later than normally expected.

The annual monsoon, that takes place between May and September, is a vital source of rainfall across the country, relied upon heavily for agriculture.

It is caused primarily by the differential heating between the Indian landmass and the oceans to the south of the country, which effectively sets up a huge sea breeze effect.

As summer takes hold in the northern hemisphere, the land over India heats up quickly. This leads to hot air rising upwards from the surface, high into the sky, causing low pressure to form.

At the same time, very moist air feeds in off the oceans from the south or south west, in order to replace the air over the land that has risen up from the surface.

When this moisture-laden air encounters the intense heat and mountains over the land, it rises on a large scale, causing intense rain and thunderstorms that drift northwards across the country over the space of around six weeks.

Year to year variation

As with any atmospheric phenomenon, no two seasons are the same, influenced by a variety of complex ocean-atmosphere interactions around the globe.

Last year, in 2013, the monsoon rains arrive in Kerala on 1 June, two days ahead of the official forecast and in line with the long-term average.

The season then went on to deliver above-average amounts of rainfall across India, resulting in a record grain harvest.

However, this year, the rains are running around five days late. The map from the India Meteorological Department below shows the current position of the monsoon rains (green solid line), compared to the average position (red dotted line).

Normally on the 13th June, the rains would stretch from southern Gujarat in the west to Bihar in the east. But at the moment they stretch from Maharashtra to Butan.

2014 monsoon prediction

Each year, the Indian Meteorological Department produces a seasonal forecast on how it expects the monsoon to behave – both in terms of its arrival time and how much rain is likely in comparison to average.

This year, its quantitative forecast expects the monsoon seasonal rainfall to be below normal, at 95 per cent of the long period average, with a model error of plus or minus 5 per cent.

An additional factor to take into account this year is the highly likely occurrence of El Nino – a naturally occurring ocean-atmosphere phenomenon that is characterised by unusually warm surface water temperatures in the central and eastern Pacific.

When El Nino occurs, rainfall between May and September tends to be below normal – sometimes to the extent that drought takes hold in north western parts of the country, as well as in Pakistan.

Even though the progress of the rain is around five days late, there are signs that it will catch up to where it should be this weekend and into next week.