Mississippi State University Weather

A Crash Course in El Niño

1. Introduction

Perhaps you have recently heard about the climate teleconnection referred to as “El Niño”. It has been making headlines throughout the course of 2015. According to the Australian Bureau of Meteorology (ENSO Tracker), El Niño conditions have been present since May 12th, 2015.   Since being declared, it seems at times as if El Niño could be responsible for or linked in some way to any number of observed weather events. Why did it rain so much in South Carolina? El Niño. Why was there flooding earlier this year in Texas & Oklahoma? El Niño. Can you explain how the drought developed in Mississippi and Alabama late this summer? Sure, El Niño. Why are the tropics so quiet? El Niño. Joaquín? El Niño. Perhaps you would like to know more about what El Niño is and gain better understanding of how it can impact our everyday lives. You are in good luck, because I hope to answer those questions in this post. Before delving straight into the impacts of El Niño, we need to make sure that we are aware of and understand a few things first.

2.The Southern Oscillation Index (SOI)

In order to more fully understand El Niño, you have to first grasp the fact that it is just one of many climate patterns embedded within other more encompassing climate patterns. In the case of El Niño, the larger more encompassing pattern is known as the Southern Oscillation Index (SOI).  The SOI in simplest terms is the measure of the pressure differences between the Island nation of Tahiti and Darwin, Australia. Now why would anyone care about these pressures differences? They serve as a general measure for the intensity of the Walker Circulation which I will discuss more momentarily. The SOI consists of three distinguishable phases: La Niña, Neutral and El Niño. For the purposes of this post, I will only cover the latter two.

3.The “Neutral Phase”

Before you run, you walk, before you walk you crawl. It also true that before you can fully understand something that is abnormal (El Niño), you must understand the “normal” relationship between the atmosphere and ocean. In the neutral state (neither El Niño nor La Niña) trade winds blow east to west across the surface of the tropical Pacific Ocean, bringing warm moist air and warmer surface waters towards the western Pacific and keeping the central Pacific Ocean relatively cool. The thermocline is deeper in the west than the east (see Figure 1). Warm sea surface temperatures (SSTs) in the western Pacific pump heat and moisture into the atmosphere above. In a atmospheric process known as convection, this warm air rises high into the atmosphere and, if the air is moist enough, causes the formation of towering cumulonimbus clouds and rain. This now-drier air then travels eastward before descending over the cooler eastern tropical Pacific. The pattern of air rising in the west and falling in the east with westward moving air at the surface is referred to as the Walker Circulation. Measuring the intensity of the Walker Circulation is at the heart and soul of determining the current phase of the SOI. The Walker Circulation can be most fully comprehended as a planetary-scale vertical circulation occurring in the tropics. It is best visualized by taking a vertical cross-section along the equator extending across the Pacific Ocean basin (see Figure 2). The convective cells are primarily driven by pressure gradients produced by air rising over the warmer areas of the oceans and continents, and subsiding over the cooler regions.  The surface-based segment of the circulation describes the motion of the trade winds which typically blow from the northeast (southeast) in the northern hemisphere (southern hemisphere).

6-7_ENSO_12-07-12_neutral-outlines

Figure 1

enso_walker_mean

Figure 2

 4.What exactly is El Niño?

Now that we have some basic understanding, let’s address the main question: “What is El Niño?” The term “El Niño” is derived from the Spanish language meaning “The Christ Child”. We can thank Peruvian fishermen for this name, because they are largely believed to have grown intuitively familiar with the effects of El Niño over the course of many centuries. It is considered to bring respite from the Cold Peru Current (aka the Humboldt Current, see Figure 3), and provide welcome rains to an otherwise parched coastal region of South America.  Scientifically speaking, El Niño is an Oceanic-Atmospheric coupling that occurs when according to Australian BOM (ENSO Tracker), any three of the following four criteria are simultaneously met:

SciEMSI-PeruCurrent

Figure 3

       i.      Sea surface temperature: Temperatures in the NINO3 or NINO3.4 regions of the Pacific Ocean are 0.8 °C warmer than average. (See figure 4)

ninoregions

Figure 4

       ii.      Winds: Trade winds have been weaker than average in the western or central equatorial Pacific Ocean during any three of the last four months.

iii.      SOI: The three-month average SOI is –7 or lower.

iv.      Models: A majority of surveyed climate models show warming to at least 0.8 °C above average in the NINO3 or NINO3.4 regions of the Pacific until the end of the year.

El Niño can also be recognized as the warm phase of the SOI. Whereas La Niña is an exaggeration of the neutral phase, El Niño is a reversal of normal conditions. During an El Niño event, the trade winds weaken or may even reverse altogether, allowing the area of warmer than normal water to move into the central and eastern tropical Pacific Ocean. These warmer than normal ocean temperatures are associated with a deepening of the thermocline (due to less upwelling) in the central to eastern Pacific. A weaker upwelling of cooler ocean waters from below also contributes to warmer SSTs (See Figure 5). SSTs around northern Australia are cooler than normal and the focus of convection migrates away from Australia eastward towards the central tropical Pacific Ocean. El Niño is a fairly common phenomenon. It occurs roughly every two to seven years while and typically lasts anywhere between twelve and eighteen months.

6-7_ENSO_12-07-12_ElNino-outlines

 

Figure 5

There are several impacts that El Niño seems to bring upon the United States including:

  • Wetter conditions to the central and southern Pacific west coast are more common. (Especially in California)
  • A reduction or suppression of tropical activity in the Atlantic Basin and Gulf of Mexico.
  • The latter is due to an increased intensity of the subtropical jet stream which tends to create a high shear environment in the upper troposphere that works against tropical development.
  • The active subtropical jet also leads to stormier conditions across the southern tier of the country.

These are just a few of the impacts that become more likely or probable during El Niño events. With all that said, I must caution that no two El Niño events are ever identical.

There you have it that is El Niño in a nutshell. If you are still confused, do not feel bad. Even some professional meteorologists deal with confusion over what El Niño is and how much of an impact it can have on global climate. We are currently still in the El Niño phase of the SOI which many climate experts and seasoned weather forecasters are suggesting is now at or near its peak. If this is truly the case, then El Niño conditions should lessen in the months to come, and we may even find ourselves in a La Niña by next spring. That would be an equally interesting set up as La Niña comes with its own interesting set of impacts.

~Alan Matthew

Citations

“ENSO Tracker: An Alert System for the El Niño–Southern Oscillation.” ENSO Tracker, Bureau of Meteorology. Bureau of Meteorology. Web. 13 Nov. 2015. < http://www.bm.gov.au/climate/enso/tracker/#tabs=Summary>.

“ENSO Tracker: An Alert System for the El Niño–Southern Oscillation.” ENSO Tracker. Bureau of Meteorology. Web. 13 Nov. 2015. <http://www.bom.gov.au/climate/enso/tracker/#tabs=Criteria>.