El Niño – and why the Polar Vortex and MJO (not to be confused with mojo) matter

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El Niño conditions

(Editor’s note: We’ve been having “weather” in Arizona! And for water professionals, that always leads to a discussion of how the weather patterns might affect our water supply.

Previously, we talked about a weather pattern called La Nina, which had a three-peat – three years in a row. Now, since spring 2023, we’ve been in a weather pattern called El Niño. So, let’s take a look at what that is and what it could mean for us in Arizona and in the Colorado River Basin.)

What is El Niño?

By Orestes Morfin, Senior Policy Analyst

El Niño is often portrayed as a predictable, monolithic event that has the same effect around the globe whenever it appears. In reality, however, the effects of an El Niño event are not guaranteed, and this winter’s competing oceanic and atmospheric cycles are a perfect example of that. Context matters – none of these cycles is happening in a vacuum. And the way in which they interact can make or break expectations.

So, let’s break it down, by looking at the ocean, the atmosphere – and the complicating factors.


It’s important to understand El Niño as a seasonal pattern: a condition that persists over a period of several months. Defined by the trend in sea surface temperature anomaly (SSTA, or “how different current temperatures are from the long-term average”) in a designated region of the Pacific Ocean, it is primarily an oceanic phenomenon.

Figure 1 – The El Niño 3.4 Region
Figure 2 – Two-year trends in the El Niño 3.4 Region for every El Niño since 1950


In what we think of as a “classic” El Niño, the atmosphere couples with the ocean. The tradewinds weaken and may even stall, stranding a mass of warmer-than-average surface water along the Equator in the eastern Pacific. The cycle the tradewinds follow across the equatorial Pacific is known as the Walker Circulation (Figure 3) and, in a “classic” El Niño, the entire pattern shifts eastward.

El Niño conditions
Figure 3 – The Walker Circulation in “Neutral” and El Niño modes

None of this happens in a vacuum, and it’s relatively rare that we experience what we’d otherwise expect from a “classic” El Niño here in the Southwestern United States.

Complicating Factors

The predicted effects of El Niño face three complicating factors this winter.

1. This year’s El Niño is unprecedented in that it is occurring amongst record high average ocean temperatures worldwide (Figure 4). In past events, the El Niño signature was easier to differentiate from its surroundings, which drove what we think of as the “classic” pattern and resulted in warmer, wetter winters here in the Southwest. Without a significant difference between El Niño and its surroundings, the ability of a coupled ocean/atmosphere to move energy and mass where it needs to go on the globe is hampered.

Global Average Ocean Temperature Anomalies 2024
Figure 4 - Global Average Ocean Temperature Anomalies 2024

2. The picture is further complicated by a phenomenon called the Madden-Julian Oscillation (MJO). Active since November 2023, the MJO is an eastward traveling low/high pressure system that arises in the Indian Ocean off the coast of Africa (Figure 5). Unlike El Niño, the MJO does not stand still. It’s a short-lived system of intra-seasonal variability that circumnavigates the globe on roughly a 30- to 60-day cycle.

The Madden Julian Oscillation
Figure 5 – The Madden Julian Oscillation

The MJO matters because, depending on its position, it has the power to either suppress or enhance atmospheric convection in the Eastern Equatorial Pacific. As it enters the Pacific from the west, the downward flow on its eastern end can counteract or even temporarily cancel El Niño (in this case: its “destructive phase” with respect to El Niño). This is precisely what happened starting early January 2024.

On its journey east, however, the upward convection typical of the MJO may eventually overlap with the convection caused by El Niño (the “enhancing” phase) charging the atmosphere over the Eastern Pacific with heat and moisture. This is what has been happening in late January and is expected to continue into early February.

3. Finally, changes in the Polar Vortex have wreaked havoc on the jet stream. The Polar Vortex is a circular band of winds in the stratosphere (the extreme upper atmosphere, far above the zone where weather that affects Earth’s surface is typically generated). Normally, the Polar Vortex flows counterclockwise in a tight, well-organized (i.e. “strong”) manner (Figure 6).

The Polar Vortex in its “strong” and “weak” phases
Figure 6 – The Polar Vortex in its “strong” and “weak” phases

Occasionally, however, it weakens and breaks down, as it began to do in December/early January, when it shifted east and split over Greenland (Figure 7).

The Polar Vortex split in January 2024
Figure 7 – The Polar Vortex split in January 2024

The fact that the split is more pronounced in the lower stratosphere means it has the potential to disrupt that section of the atmosphere where surface weather is regulated. Extreme cold in the continental US in mid-January was testimony to this effect.

In sum, what would otherwise have been a “typical El Niño” has been complicated by some unprecedented factors this year. Historically warmer-than-average global oceans have taken the edge off the El Niño signal in the Pacific and disrupted the “typical” movement of energy and mass in the atmosphere, making it harder for El Niño to deliver on its promise of precipitation to the Southwest US. Add to that the shorter-term atmospheric variations caused by the MJO and a weak Polar Vortex, and we can expect to see some continued swings in the expected paradigm.

What does this mean for the Colorado River Basin?

During those periods of time when the MJO is in its “enhancing phase,” with respect to El Niño: we can expect more precipitation in the Southwest U.S. (in the Lower Colorado Basin, certainly, but … how far north into the Upper Basin this might extend is anyone’s guess).

Apart from that, the other complicating factors make it almost impossible to say whether or not El Niño will have a beneficial effect on this year’s snowpack. So far, these factors have kept the snowpack below-average.

Interested in monitoring Colorado River conditions, visit our Colorado River Conditions dashboard, which is updated twice monthly.