La Niña Effects Alive and Well Across U.S. Corn, Soy Belt

“La Niña or not?” That has been one of the central debates in the agriculture markets in recent weeks.

Since the beginning of the year, meteorologists have been warning that La Niña, the cool phase of the tropical Pacific Ocean, could begin at the start of the U.S. summer and could potentially place a high weather risk on the world’s largest corn and soybean crops.

Since we are not yet in a full-blast La Niña, some have begun to doubt La Niña’s presence or whether it is coming at all. Many weather watchers in the commodities space correctly point to the fact that on the presumed journey to La Niña, we are lagging key analog years such as 1998 and 2010. (

But anyone assuming the United States is safe from La Niña’s impact this summer may want to hold that thought. The atmosphere is already in a La Niña-type pattern, and the weather forecasts are responding with scorching heat for the second half of July in key corn and soybean states.

Weekly sea surface temperature anomalies in the Niño 3.4 region of the Pacific have been at minus 0.4 degree Celsius for three straight weeks now, which is very close to the La Niña threshold of minus 0.5 degree C.

But we do not necessarily need an “official” La Niña to have problems with weather in the United States this summer.

At least if the atmosphere has anything to say about it.


The atmospheric angular momentum, or AAM, is the spin of the atmosphere relative to the earth’s surface. This might sound complex, but the application to climate is simple.

AAM is linked to a well-known phenomenon, the El Niño-Southern Oscillation (ENSO), and these two events tend to play off of one another. A highly positive AAM index, meaning that the earth’s atmosphere is spinning relatively fast, tends to occur in conjunction to El Niño, the warm phase of the equatorial Pacific Ocean.

When the atmosphere is spinning relatively slowly under a negative AAM state, La Niña tends to dominate. This is why AAM is informally dubbed “atmospheric ENSO.”

If El Niño is present and AAM is positive, or if both La Niña and negative AAM exist together, the Pacific Ocean and the atmosphere are considered to be in sync. And when each respective phase of ENSO is present or blossoming, a complementary AAM phase can help to strengthen it via the trade winds.

On the flip side, when the AAM phase opposes the ENSO phase, the progression or perpetuation of El Niño or La Niña can be slowed.

It is not entirely accurate to say that changes in AAM always precede changes in ENSO phases as the two events have a back-and-forth relationship, and there are other large-scale climate indices involved in this process. But in general, AAM is a very good leading indicator of how ENSO is likely to progress in the near term.


Agriculture market participants have had La Niña fears for this U.S. summer drilled in to their heads for many months now. This is because several summer heat waves and poor crop performances have been linked to La Niña in the past, especially when coming off of a major El Niño phase as has recently occurred.

The presence of La Niña or La Niña-like conditions in the Pacific Ocean does not guarantee U.S. heat. But if AAM is playing along, the chances increase significantly.

When AAM is negative, high-pressure ridge patterns are very common across the United States east of the Rockies. In the summer, this generally means hot and dry weather is likely, and depending on the strength, positioning, and duration of the ridge, conditions can be downright scorching.

This is notable because the AAM is currently negative and is forecast to go even more negative for at least the next two weeks. The most recent negative run for the index was in June, which helped produced the hottest June on record for the United States. (

The current weather forecast aligns with the AAM outlook. An extremely strong ridge is expected to set up over the central United States at the beginning of next week and will raise temperatures 10 to 15 degrees F above normal in parts of the Midwest and Plains. Rainfall is likely to be scarce during this time. (

One has to look no further than the AAM to see how the ridge pattern that dominated the United States in 2012 could have had such longevity. The index was negative from planting through harvest that season and not surprisingly, that summer was one of the hottest on record and many states observed disastrous crops, especially for corn. (

Needless to say, we are not facing a situation quite like 2012 because so far this year, weather conditions have been nowhere near as extreme. But the upcoming ridge pattern has not been seen since that summer and ridges of this strength are uncommon.

Going forward into August, the AAM tendency will be important to monitor because not only may it assist in the entry to a weak La Niña, but it could continue to bring extended hot and dry patterns across the country during the worst possible time for corn and particularly soybeans.

However, in the longer term, the AAM can only take La Niña so far. While it will be important for the strength of La Niña over the upcoming winter, other climate indices in the Pacific Ocean will need to play along for a “bona fide” La Niña episode to occur.

Source: Agweek

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