The haze is back in Singapore and rather invariably, everyone is comparing this year's situation with that of previous years. The most commonly heard haze years are 2013 and 1997. The 1997 haze was caused by the clearing of plantations in Malaysia and Indonesia using the slash-and-burn technique but it was exacerbated due to the El Nino weather condition.
El Nino is a weather phenomenon that occurs once every few years. It is due to changes in the pressure gradient over parts of the Pacific Ocean, also known as the El Nino - Southern Oscillation. Normally, in December, atmospheric pressure is higher over Tahiti, French Polynesia than Darwin, Australia. This means that there is a high pressure cell off the western coast of South America and a low pressure cell off Australia. However, when El Nino occurs, this pressure gradient is reversed, i.e. pressure over Australia is higher than in Chile, South America.
The change in pressure gradient causes a pool of warm surface water off the coast of Australia to travel across the Pacific Ocean towards South America. This warm surface water suppresses the normally cool water off the coast of South America. On the other hand, warm water off the coast of Australia is replaced by colder water. The cooler temperatures off the Australian coast prevents the formation of convection rain.
As seen in the diagram above, this results in prolonged dry weather in the western Pacific, i.e. Australia and Indonesia, while creating abnormally wet weather off the western coast of South America, i.e. Chile and Peru.
The dry conditions will thus exacerbate the haze situation in South-East Asia by prolonging and easing the burning of the trees since large swathes of land are likely to be extremely dry and parched. The lack of rain also makes it hard to put out the fires.
But that said, other atmospheric factors such as shorter-scale seasonal variations in precipitation levels and surface wind conditions are also important in influencing the severity of the haze. It is also important to note that the choking haze last June (2013) was NOT exacerbated by El Nino. In fact, 2013 was not an El Nino year. Rather, it was due to other conditions such as prevailing surface wind direction and a spike in the use of the slash-and-burn technique to clear land in Indonesia.
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