How temperature-pressure tango crimps monsoon rain | Number Theory

The first part of this series showed that winds that blow from the southwest to cause monsoon rains are not just driven by faster heating of land than the ocean

The real gradient between the Indian Ocean and the Indian landmass exists higher up in the atmosphere. It is this gradient that drives the monsoon and which the El Nino suppresses. How exactly does the El Nino suppress the gradient? The short answer is that this happens through the interplay of temperature and pressure. The long answer is as follows.

Residents of Vivekanand Camp in Delhi's Chanakyapuri fill empty vessels from a water tank on a hot summer afternoon on May 26. (Vipin Kumar/ HT Photo)

How temperature-pressure tango crimps monsoon rain

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  Monsoon is caused by the general shift of a low-pressure region north of the equator

  The first part of this series explained that the monsoon is triggered by the temperature difference between the Indian landmass and the Indian Ocean becoming positive higher up in the atmosphere. Since air becomes less dense with heat, this also means that the pressure over Indian landmass is lower than over the Indian Ocean. A warm central-eastern Pacific – this is what El Nino is – is able to affect this because the gradient is not unique to India. How so? One theory is that it is set up simply as a result of the northward movement of the sun. As one moves from the spring equinox (sun falls directly over the equator on this day) month of March to the monsoon month of June, the latitude band of maximum solar heating shifts from the equator towards the latitudes of India, also bringing the band of low pressure along. This turns India into a place of lower pressure than the ocean, from where winds blow towards it.
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  El Nino affects this pressure gradient during monsoon…

  To be sure, the meridional (running north to south, along longitudes) pressure gradient described above is not the only game going on in tropical areas. In some ways, it also only explains the “south” part of the southwest monsoon. The maps above also show a zonal (parallel to latitudes) pressure gradient. This leads to what is known as the Walker Circulation — a giant east-west loop of rising and sinking air over the tropics. Air rises around Indonesia because of the relatively lower pressure there and falls over the eastern Pacific and weakly over the Arabian Sea. The higher pressure in the Arabian Sea is one reason we have westerly winds blowing for the monsoon in the lower atmosphere. The El Nino affects the Indian monsoon by shifting this entire zonal circulation spanning the globe further east, shown here as departure from normal pressure in June 2015, when an El Nino was underway and India saw a dry monsoon. The shift means that pressure is higher over India, decreasing the pressure gradient that leads to the monsoon. As map above also suggests, it is not just the strength of the El Nino that determines how the monsoon is affected, but also how far east it shifts the circulation.
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  …because of the warming in the central-eastern Pacific Ocean

  How does the El Nino come about? This is a bit of a chicken-and-egg problem. One way to understand this is that the central-eastern Pacific becomes warmer relative to normal during an El Nino, making central-eastern Pacific the region of low pressure rather than the western Pacific, which weakens the trade winds — the steady east-to-west surface winds that normally dominate the tropical Pacific. Of course, this can also be understood in reverse. The trade winds blowing from east-to-west become weaker, making it difficult to transport heat stored in the upper ocean layers to the west, making the western part cooler and of high pressure. Whatever starts this change in pressure though, it sets in motion the change that prevents air from rising for monsoon rains.