El Nino is the name given to the natural cycle that involves the eastern Pacific getting warmer in the area close to the equator. (La Nina is the name for the alternative cycle when the waters there are cooler. The two together are known as the El Nino Southern Oscillation - ENSO.) This is one of the most significant natural cycles and occurs every few years. It has the potential to change weather around the world. Take a look at this graphic showing the sea surface temperature anomaly. The darker the red, the warmer it is relative to the long-term average.
|Source: Climate Reanalyzer|
But, the important thing is to notice how the warm water stretches all the way across the Pacific Ocean. The significance of this lies in the fact that warm water creates atmospheric low-pressure areas which results in thunderstorms. There is now a Pacific Ocean-wide corridor of low pressure which will allow thunderstorms to develop and travel all the way from Asia to South America. One of the things this leads to is a change in the Hadley Cells.
Hadley Cells are circulation patterns in the atmosphere. Warm air near the equator rises and then travels towards the poles at high altitude. When the air reaches the mid-latitudes it sinks back to the surface and travels back towards the equator. This circulates heat and causes the trade winds. A stronger El Nino results in stronger Hadley Cells. Live Science has a nice graphic here showing how this all works.
You can probably see where this is going. More heat is being circulated through-out the world as a result of El Nino. Changes in the heat and water vapor input in a given region will result in changes to the weather in that region. How much of a change and what kind can be expected? That is a big variable. Some regions will experience greater rainfall. Others will experience droughts. Depending on the strength of the El Nino event, the effect could be anywhere from very mild to catastrophic.
Some of the most dramatic example of El Nino effects is a series of famines that have occurred in what is modern-day India, including the Great Famine of 1876-1878 (5.5 estimated dead) and the Bengal Famine of 1770 (10 million estimated dead). These famines occurred when the monsoons did not occur and the crops failed. The famines were greatly aggravated by British mismanagement.
What has been found is that severe droughts in India always occur during El Ninos, but not every El Nino leads to droughts in India. The apparent link seems to be where the Pacific is warmest. When it is warmer in the Central Pacific, India has droughts. When it is warmest in the Eastern Pacific, India is spared. Take a look at the plot of surface temperatures, similar to the plot above.
|Source: Climate Reanalyzer|
The figure above showed the difference from the average. This plot shows the actual average temperature. The way I interpret this data is that it is warmer in the Central Pacific region than in the Eastern Pacific region off of South America. This could be bad news for India. The good news is that Britain is not handling the management any more.
But, El Ninos are not bad news for everyone. Actually, for us in the U.S. it will be a good thing. A typical El Nino brings mild temperatures and more rainfall for the southern half of the country. This would be particularly welcome in the mid-Plains and the Southwest where drought has been raging for many years. In fact, several states out here are at risk of running out of water. More rain would be good.
So, let's talk about the White Elephant sitting in the middle of the room. Is global warming affecting the ENSO cycle? Quite simply, we don't know yet. There are some that believe a connection exists, but more data is needed. What is known for sure though, is the El Nino affects the short-term accuracy of our computer models. The models are highly accurate when predictable conditions exist. But, unpredictable events like ENSO and volcanic eruptions disrupt them. The good news is that when the events occur and are included in the models, the models once again become highly accurate - in excess of 95% accurate and getting better. I have not heard what the models are forecasting with the this current El Nino included, but I will keep a look out for any news.