From the Cliff Mass Weather Blog
Cliff Mass
The media recently reported on oceanic Kelvin waves and their connection to El Nino. See samples below.
Some of the information is correct; some is problematic or exaggerated (samples below).
So, what are the actual facts, and why should you care?
The U.S. has satellites in orbit that can monitor the height of sea level around the world.
Here is the sea level elevation on April 1, before the current El Nino really got going.
I am showing the difference from normal (the anomaly in meters), with red being higher and blue being lower than normal. Things were pretty normal (yellows and light blue) at this time
Compare this to the situation two days ago (below). Big rise in sea level near the equator and along the Mexican coast, with some enhancement along the Southern California coast.
Not much sea level enhancement for Northwest coastal waters (sorry, Seattle Times climate lab!)
Let’s look at two West Coast sites.
At San Diego, the observed sea-level heights (red lines) are substantially higher than expected without El Nino (blue line), by roughly one foot. This contributes to localized flooding for Southern California.
In contrast, there is virtually no enhancement along the Washington coast at Westport. This will continue to be the case through the winter.
So what are these Kelvin Waves that the media are headlining? (and often explaining them wrong).
They are an essential part of the El Nino phenomenon.
During normal (non-El Niño or La Niña) situations, the easterly (from the east) trade winds in the tropics push warm tropical waters to the west, resulting in higher-than-normal sea level to the west (see below; red is higher than normal sea level). The situation last winter is shown below.
Then the easterly trade winds weakened, and warm water surged east as a Kelvin Wave during this summer (see the situation in early September). Higher than normal sea level along the equator and near the west coast of South America (dark red).
You can watch an animation of this happening below (click on the arrow). Pretty neat!
https://www.blogger.com/video.g?token=AD6v5dwK9GGIWRE5GWxyp9Z-pcYW_EvL-0FZ7b9rqbHjmGnnWHfW9B3t3oOrQ9XcCgR0gz9HLMW4QrOUD18pAP_wGzpj3LnuWIOHg3zhz2jqNyjxYRSzRDMNdofNseGAv-1KjPC9JsE&origin=cliffmass.blogspot.com
The eastward movement of the warm water occurred in a series of distinct pulses of deepening warm water (Kelvin Waves), with the figure below showing a cross section through one of the waves (the blue color is the warm water).
Such Kelvin waves are trapped near the equator by the Coriolis force resulting from the rotation of the Earth.
When the wave hits the west coasts of South and North America, the wave pushes poleward along the coasts, resulting in increased sea-level elevation in the coastal regions north and south of the equator.
You can see the enhanced sea level elevation along the West Coast of North America below (red is higher than normal sea level).
If you were in one of my classes, I would show a figure such as below.
An elevated water column is centered on the equator, producing a pressure force to the north and south. This pressure force is balanced by the Coriolis force (due to the rotation of the Earth); this balance keeps the surge in water intact. This balance only works for water surging to the east.
That is why Kelvin waves only move in one direction (east).
Don’t worry if this is a bit too much detail: providing it for the weather nerds 😀
There is no “sloshing” back to the west when the elevated warm water hits the west coasts of North and South.
So this is not like sloshing in a bathtub, as claimed by some media.
In any case, we are in the middle of a very strong El Nino and strong Kelvin wave pulses of warmer water towards the east are expected.
