Sea surface temperatures off the coast of South America are warmer than usual for this time of year, and NOAA’s 2018-2019 Winter Outlook hints that El Nino may be a factor in our weather this winter.
But what does that mean for the Tennessee Valley? And what is El Nino in the first place?
Two phases of an ocean-atmosphere system
El Nino is part of an overall ocean-atmospheric pattern called the EL Nino Southern Oscillation, or ENSO. ENSO conditions are observed in the Pacific Ocean, just south of the Equator and spanning from the South American coast westward toward Indonesia. Specifically, this area is known as the “Nino-3.4” region and is defined as the area between 5oN-5oS and 120oW-170oW.
La Nina is considered to be the “cool phase” of ENSO, when winds push the warm ocean waters further west, allowing much cooler waters to upwell near the coast of South America.
El Nino is considered to be the “warm phase” of ENSO, when warm ocean waters collect along the equator near the coast of South America.
Both phases irregularly shift back and forth over the course of a season or two, at times even spanning through the majority of a year.
Because ocean water temperature cools/warms the air above it, the weather across the Pacific Ocean — as well as across North America — can be impacted by the ENSO Phase. This is why ENSO is known as a “teleconnection”: The air/water interaction “far away” (the “tele-” portion of the word) is “connected” to the weather where we live.
How does El Nino impact the Valley’s weather?
In general, the warmer ocean waters in the Pacific create an area of upward-moving air from Alaska south along the West Coast of North America. This rising air produces an area of low pressure, which encourages rain to develop in this region.
As a result, a sub-tropical jet stream sets up over the South, driving storm systems into the region and creating wetter conditions over the winter and early spring months.
In a nutshell, El Nino conditions bring cool and wet weather to the Southeast, including the Tennessee Valley. This is a generalization, and is not the “end all, be all” for the season.
The ENSO teleconnection impacts many weather patterns across the globe, including severe weather and hurricane formation.