Important Milestone: The Current El Nino Has Hit Its Peak Early

The very strong El Nino appears to have hit its peak earlier than predicted a few months ago, suggesting a substantial weakening later this autumn and winter.

Let's begin by looking at the sea surface temperature differences from normal (called anomalies) for two critically important areas in the central tropical Pacific:  the Nino 3.4 and 4 areas (see map below).

The magnitude of El Nino events is usually judged by the sea surface temperature anomaly in the Nino 3.4 area... and I show the latest values below (from NOAA Climate Prediction Center).  The magnitude of this El Nino appears to have leveled off over the last few weeks, around 2.5 °C above normal.

The fact that the sea surface temperatures are pretty much unchanged during the past few weeks is illustrated by the sea surface temperature anomalies on October 1 and October 19 (see below).

Hardly changed.

 

To understand the future of an El Niño event, it is useful to look at the upstream region (Nino 4) to gain insight into the future evolution of temperature in the Nino 3.4 region during subsequent months.

Why look upstream?   Because the sea surface warming tends to move eastward with the Kelvin waves I described in my previous blog.

The Nino 4 temperature anomalies (from climatological conditions) are shown below.   The Nino 4 temperatures rose rapidly from April to late June and then declined rapidly in July and August.

This is consistent with the rapid rise in Nino 3.4 temperature a few months later, from July through September.

NOAA runs complex atmosphere/ocean models for seasonal prediction.  NOAA's primary model  (the CFSv2) is predicting a rapid decline in the temperature in the Nino 3.4 area starting right now!   

If that prediction is correct, it means we are now at the El Nino peak.

Other modeling systems are less rapid in the decline, such as the GFDL Spear system (see below), but all suggest we are near the maximum. 

In summary, it appears that we are now at the peak of the current El Nino, and the latest forecasts are suggesting a fairly rapid decline.   

However, there is a lot of uncertainty in such forecasts because of the highly convective (e.g., thunderstorm) nature of tropical meteorology, and because the interaction between the ocean and atmosphere is often poorly modeled.

Lots to keep graduate students busy over the next decade!

The implications of the upcoming decline in El Nino will be the topic of a future blog.

El Nino, Sea Level, and Kelvin Waves

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? 

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!

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.