Welcome to my special post on what is know as “El Nino”. The goal of this post is to give everyone a general idea of what this weather pattern is and how it affects the climate. I will try not to get too detailed and keep things as simple as possible.
In reality “El Nino” is a phase of a weather pattern called ENSO which stands for El Nino Southern Oscillation. In a general sense this is an index that measures the water temperature profile off the west coast of South America. When water temperatures are warmer than normal (0.5 and above) it is an El Nino and when they are cooler than normal (-0.5 and below) it is called a La Nina. Today’s post will just focus on the warm phase or El Nino since it is relevant to the current state of our atmosphere. Below is a diagram showing this ENSO region..
Do not worry about the different sub areas as that is a topic for another day. So let’s focus now on what causes this area to develop warmer than normal ocean temps. It all has to do with the atmospheric wind pattern in this region. Normally winds blow from east to west and are called the “easterlies” This is due to higher pressure patterns to the east and lower pressure patterns to the west. The result is water getting driven westward, piling up, and warming due to solar heating. However, during an El Nino episode this wind pattern changes and the easterlies weaken. This is due to changes in the surface pressure pattern. The two main areas that are used to evaluate changes in the pressure pattern are Thaiti and Darwin. I have highlighted those two regions below..
El Nino episodes start to develop when higher pressure develops near Darwin and lower pressure near Tahiti. You can see that scenario on the third panel above.This is something called the Southern Oscillation Index. As the pressures rise near Darwin, the the easterlies weaken and winds start to blow from west to east. This causes the warmer water from the western Pacific to migrate towards the central and eastern pacific. Just this alone does not warrant classification of an El Nino. In order to be classified as an El Nino, the water must be at least 0.5 degrees above normal and last several months. The image below shows the current water temperature profile around the world. Notice the band of warmer than normal water developing in ENSO region just off South America. .
This is what is causing all the current headlines out there of an El Nino underway.
So now that we have a general idea of what El Nino is, lets talk about the effect it has on our climate. It is important to understand that in the tropics, warmer water causes convection or rising air. This rising air forms storm clouds and storm systems. As the warmer than normal water shifts to the east during an El Nino episode, so does the rising air and storm clouds. There is a definition for this and it is called The Walker Circulation but again I do not want to get too complicated here. Just know that El Nino episodes can be characterized by enhanced storm clouds or convection in the El Nino area. Now comes the key to this weather pattern- this enhancement of those storm clouds influences the jet stream. In an El Nino episode, the pacific jet stream strengthens effecting the weather in the USA and other parts of the Northern Hemisphere. The image below shows the typical jet stream set up during an El Nino..
Notice on this image the extended Pacific Jet Stream and how it effects the western, central and eastern parts of the country. It is not as simple as this however. The strength of the El Nino will determine the overall effect on the countries weather, especially in the winter season. A weak to moderate El Nino episode for example, could result in a cold and stormy winter in the east while a moderate to strong El Nino could cause a warm blowtorch winter in the east. The two images below from accuweather.com illustrate this..
If you think about it, the warmer the water is in the ENSO region, the stronger convection will be, thus the stronger the Pacific Jet Stream. If that Pacific Jet overpowers the weather pattern it just floods warm air into the country. However, as seen in accuweather.com's first image, if it is present but not overpowering then we get the chance of big winter storms. This is due to the moisture rich cold storm systems and dual phasing of the northern and southern jet streams.
The next image below I thinks sums up all of my points above in one picture..
As an aside, predicting how weak or strong El Nino will be is not easy. For example, take a look at model projections for the next 12 months..
All the different dots represent each models forecast of the magnitude of the El Nino. Notice by the time we get to DJF (Dec,Jan,Feb) of 2016 we have a range of 0.5 to 1.75. Where this ends up being will have a big impact on our Winter weather (in addition to the other factors I discuss In my winter outlooks). It will be interesting to see how this evolves over the next few months.
In terms of summer weather for the east coast during an El Nino, expect normal to slightly below normal temperatures with normal to slightly below normal precipitation. This means I am not expecting a brutally hot summer this year. To take it a step further, we do not normally see a high frequency of hurricanes during El Nino years either. This is due to the increased wind sheer in the tropics or in simple terms stronger mid-level atmosphere winds cutting off the ability for tropical storm systems to strengthen rapidly.
So there you have it a very general but hopefully helpful post on what El Nino is, how it forms, and its effect on our weather. Feel free to leave any questions in the comments section.
Thanks for checking in!