NOAA forecasts strong El Nino: What it is and how it impacts Indian monsoon

New Delhi, Jul 13 (PTI) The United States’ National Oceanic and Atmospheric Administration (NOAA) last week announced that El Nino conditions over the tropical Pacific Ocean could intensify in the coming months and become one of the most intense events in history. El Nino typically leads to record-breaking temperatures around the world and less monsoon rainfall in India. Its opposite phase, called La Nina, has a cooling effect globally. When neither El Nino nor La Nina exists, there is a neutral phase. Put together, these three phases make up the El Nino Southern Oscillation (ENSO) cycle, a crucial climate phenomenon, which affects the global climate patterns. The emergence of these phases depends on two main factors in the tropical Pacific Ocean -- the sea surface temperature (SST) and the sea-level air pressure between the western and eastern parts of the ocean. For instance, El Nino occurs when air pressure conditions change in such a way that trade winds -- a permanent system of winds that flow in Earth’s equatorial region -- become weak. In this situation, trade winds, which normally blow from east to west, are not able to push the ocean’s relatively warmer water westwards (towards Indonesia). As a result, the central and eastern Pacific Ocean, off the coasts of Ecuador and Peru, becomes warmer than normal (cold water from deeper ocean does not rise up to replace warmer water). In an interview with PTI, M Ravichandran, former secretary of the Ministry of Earth Sciences and currently visiting professor at IIT-Madras, said that the warmer water then affects the air circulation above through convection — warm, moist air rising from the sea surface. The impact on air circulation causes less rainfall in Indonesia, its neighbouring regions, and India. The exact opposite happens during La Nina. The trade winds become stronger than usual, pushing more warmer water towards Indonesia, resulting in a colder eastern Pacific. This leads to an increase in rainfall over Indonesia, its neighbouring regions, and India. Note that for El Nino and La Nina to occur both SST and atmospheric conditions have to be in sync. There could be instances when the ocean can seem like it is in an El Nino or La Nina phase, but the atmosphere is not playing along (or vice versa). “Both El Nino and La Nina occur on average about every two to seven years… However, scientists do not know why in some years the ocean warms and cools in other years,” said Ravichandran. PTI ALC NB