The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. This body of water and air mass is just west of the United States. Due to the Coriolis Effect, the jet stream flows primarily in a west to east motion (which is why our weather mostly comes from the west) and is fundamentally why the ENSO pattern is so significant to the weather in the United States.
There are two phases to the ENSO pattern: El Niño and La Niña. The pattern shifts back and forth between these two phases, irregularly about every 2-7 years. In between these phases, we call it ENSO-neutral.
ENSO Neutral & Deviations
During the neutral phase of ENSO, the trade winds (which flow from east to west) push warm surface water in the Pacific Ocean from the Americas towards the west Pacific. Cold water below the ocean rises and replaces the warm surface water. This environmental setup brings what we would call normal or ‘neutral’ weather to the United States. See Image 1 below.
In some years, these trade winds weaken and that warm water is allowed back towards the east. The warmer surface temperatures push the jet stream down to the south. We call this warm phase El Niño.
On the opposite end of the spectrum, when the trade winds are even stronger than usual, they push even more warm water west towards Asia, bringing even more cold water up to the surface coasts of the Americas. This larger area of cold water pushes the jet stream north. We call this cooling phase: La Niña.
This climate tug of war is being constantly measured. In fact, there is a specific area of the Pacific Ocean that is observed to determine which side is “winning”. This location is called the Niño 3.4 which can be seen in Image 2 below. While Niño 3.4 has been standardized for overall ENSO monitoring, it it might not be the best area for actually determining a potential La Niña event. Since the Niño 4 (western Pacific) area would typically be warmer by default anyway, monitoring it for anomaly cooling temperatures would be a better indicator of potential La Niña.
Measuring ENSO Phase Shifts
By now you probably have guessed, one of the primary parameters used to measure the ENSO pattern changes is the temperature of the ocean. These temps are referred to as the Equatorial Pacific Sea Surface Temperatures (SSTs) and they can be measured as deviation from “neutral” (0°). Believe it or not, the threshold for significant impact is only +/- 0.5°C (0.9°F)! You can see the warm and cool phases in the image below.
The National Oceanic Atmospheric Administration (NOAA) has an amazing page for monitoring SSTs.
https://www.ncei.noaa.gov/access/monitoring/enso/sst
Below is a screen shot from NOAA’s page. Their chart allows you to switch between the different Niño regions as well looking at the actual SSTs and Anomalies.
Let’s dive in a bit deeper on each of the ENSO phases.
El Niño – Warm Phase
El Niño is the warm phase of the pattern. Since the jet stream is pushed south, the US sees overall warmer conditions, especially in the winter. The jet stream also brings wetter air to the south and leaves the northern parts of the US drier than usual.
Tornados are definitely affected by the El Niño pattern, but the biggest difference is where they occur. Because the jet stream is pushed south during this phase, we may see more severe weather trend south of normal, in places like Texas, Louisiana, and into the SE coast.
La Niña – Cool Phase
La Niña is the cooler phase. It pushes the jet stream north brining cooler temps to the US overall (especially winter). The south US will likely see more droughts, while the north will see wetter than usual conditions. Flooding in Pacific Northwest and eastern Midwest more likely too.
La Niña also brings with it increase in severe hurricanes. Again, tornados are definitely affected by the La Niña pattern, but the biggest difference is where they occur. Because the jet stream is pushed north during this phase, we may see more severe weather trend northern than usual, into Nebraska, Iowa, etc.
Additional Resources
NOAA Climate Prediction Center – Advisory
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml
NOAA Climate Prediction Center – Probabilities by 3-Month
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/strengths/index.php
NOAA Climate Prediction Center – Three Month Outlooks
https://www.cpc.ncep.noaa.gov/products/predictions/90day/
NOAA Climate Prediction Center – Consolidate SSTs Forecast
https://www.cpc.ncep.noaa.gov/products/predictions/90day/SSTs/
NOAA Climate Prediction Center – Prognostic Discussion
https://www.cpc.ncep.noaa.gov/products/predictions/90day/fxus05.html