How El Niño fuels storms along the Pacific coast of the Americas
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Version 0 of 1. With the UK Met Office predicting that an El Niño event is on the cards this year, we ask expert Dr Kevin E Trenberth from the US National Centre for Atmospheric Research to explain the phenomenon. What is El Niño? An anomalous warming of the central and eastern Pacific ocean. Under normal conditions, trade winds across the Pacific blow the surface waters towards the west. These winds pile up the water [in the far western Pacific] but every now and again it gets to such a point that the ocean says: "There is too much warm water over here!" and so the warm water spreads across the Pacific. This [El Niño event] can evolve over a period of a year to 18 months. And La Niña? In some respects it's the opposite of El Niño. It's a strong version of "normal conditions" where the warm pool region is in the far western Pacific. You tend to swing from one extreme, from El Niño, to the other extreme of La Niña. It's sort of like going from summer to winter in mid-latitudes – you don't have the average for very long, you get it very briefly in the spring. How often do El Niños occur? On a quasi-regular basis about every three to seven years. What actually triggers the event is one or more fairly random atmospheric events. What are the effects? [Warmer water means more evaporation] – this cools the ocean but it moistens the atmosphere, and [hence] fuels storms. As the warm water spreads across the Pacific, it goes along the coasts of the Americas, and causes a lot of extra rainfall. As the air goes up in the Pacific, where the high sea temperatures [and hence high rainfall] are, it has to come down somewhere – and that subsiding air tends to suppress activity in the Atlantic and the Indian oceans. The monsoons in India and south-east Asia tend to be weaker, Indonesia is drier, Australia, parts of Africa and the north-east part of Brazil all tend to be drought-prone. Is climate change affecting El Niño events? There is a lot of variability from one El Niño to another. However the rainfalls tend to be a bit heavier nowadays because the waters are a bit warmer, there is a bit more evaporation going on. Certainly the consequences of El Niño are a little greater – in places where there are droughts, the droughts are a little more severe; where there is flooding, then that flooding is a bit worse. Is there anyway to counter the effects? It is difficult to mitigate against El Niño, because of the time frames involved, but we can respond to minimise the effects or even take advantage of them. Already in Peru and Ecuador, they are very well aware that there is an El Niño happening. One of the ways to respond is to say: "Instead of being rather dry, this year is going to have wet conditions, therefore we should plant different crops." In places like Australia, where the risk is for drought, wildfires and heatwaves, cutting down on fire risk or trying to preserve water can help. How likely is a strong El Niño this year? It's not altogether clear. It started off in a way which looked encouraging for a strong event but it has petered out a little bit. What happened in the first part of this year was most like the 1997/98 El Niño event which is probably the biggest one on record. Just how large [these effects] end up being remains to be seen. Will the UK feel the effects? That again depends a little bit on how large this El Niño is. If the sea temperature anomalies get to be very large then there is unusual weather around the world and very active weather systems. As we go further into the year we look for the connections to the jet stream in the mid latitudes. It is mainly in the winter time when there are prospects for some influences across the UK. |