Ag Weather Forum

Ever since El Nino developed early 2023 and was forecast to become very strong by the end of the year, the forecast has been for a developing La Nina in 2024. That is not because La Nina always follows an El Nino, but because since the 2023 El Nino was so strong, a so-called "super El Nino" by meteorologists and climatologists, the likelihood of developing La Nina the following year was at a highly increased probability.

El Nino is the positive side of the El Nino Southern Oscillation (ENSO), when trade winds near the equator are weak and sea-surface temperatures in the tropical Pacific Ocean are higher than normal. A deviation of just 0.5 degrees Celsius above the historical average over a 3-month period is all it takes to be called El Nino. The peak intensity of the 2023 event was 2.0 degrees Celsius, a mark that has only occurred six times since 1950.

An interesting note about those other five events is that each were followed by a La Nina (at least 0.5 degrees C below the historical average) by the following winter. And, in all cases, have been followed by two straight winters of La Nina. Now, each of these has been at a different intensity, and the conditions following the 1965-66 super El Nino technically only got to the cold side of neutral the following winter and not a true La Nina, but were right on the edge. Therefore, the forecast for La Nina was early and forecasters were talking about its development sometime in 2024 going back to summer of 2023. Models, and in particular the American CFS climate model, suggested the building La Nina in forecasts dated last fall to occur sometime in summer of 2024. The CFS was bold in quickly turning around the sea-surface temperatures to La Nina by early June.

Other models have been much more subdued in its development over the last few months, but the quicker change seemed reasonable given the expanse of a pocket of cold water just below the surface that was making its way eastward through the Pacific Ocean during the winter. I remember talking about it at several farm shows and events in February. The pocket of colder water has indeed moved into the eastern Pacific and the sea-surface temperatures have cooled as a result. You can find more information about both here on pages 11 and 12: https://www.cpc.ncep.noaa.gov/…. But while the surface waters have cooled and have come down to neutral in the region we monitor for ENSO, and even slightly below normal for the far eastern side of the region, the coldest waters remain below the surface. That is important for two reasons. One is that the interface between the ocean and the atmosphere is more closely looking like a normal sea-surface temperature pattern, but it also shows that the much-colder water is just below the surface and could arise in quick fashion.

While the CFS model may have been technically too quick to turn sea-surface temperatures colder in the Pacific, it has the cold pool of water in the right place at the right time. Disturbances moving through the atmosphere and ocean could quickly change these conditions toward La Nina over the span of a few weeks if they occur.

But forecasts do not show the potential for this for at least the next month. Upwelling waves in the ocean itself can bring up the colder water to the surface. They usually start near the dateline and move eastward, but there is not one being analyzed right now.

Another mechanism is an increase in the trade winds around the equator. These increased trade winds push warmer surface water westward toward Australia and Indonesia, forcing water from below to rise up to the surface near the coast of South America. But model forecasts are not showing any enhanced trade winds for at least the next month. Therefore, neutral conditions are likely to hang around for at least the next month, and delay the onset of La Nina until deeper into summer or potentially even the fall.

The longer that ENSO-neutral conditions persist in the Pacific Ocean, the less of an effect it can have on the North American summer weather pattern. Typically, La Nina does not produce its strongest influence over the summer months anyway, but in years with a developing La Nina, historical data suggests an upper-level ridge of high pressure parking over a section of the U.S. An upper-level ridge promotes hot and dry weather underneath its axis, building and sustaining heatwaves and potentially producing drought.

When looking at that history, the ridge tends to pick a spot and sit there for much of the summer. In 2020, it was in the Plains, in 2010, it was the East, in 1983, it was the Corn Belt. So, this ridge is not consistent during any of these years, but shifts.

The question about the 2024 season has been where does this ridge set up and will it be stagnant, like in other La Nina years, or will it shift around? The quicker that La Nina sets in, the more likely it will be to become stagnant. But even 1983 saw a stagnant ridge when La Nina didn't fully develop until early fall.

When ENSO is neutral and not in control of the weather patterns across the U.S., other climate influences, which operate on a much shorter timescale and are therefore harder to predict more than a few weeks, will take over the weather patterns for North America. If they move around frequently enough, so too will the upper-level pattern.

A shifting upper-level ridge, while not ideal, would be the more preferred scenario for U.S. agriculture. While areas get hot and dry for a period, like what happens during every summer, the shifting ridge would mean these hot and dry stretches can be replaced by milder and wetter ones and come to different parts of the country at different times. A shifting pattern brings at least some beneficial precipitation to most places, and an overall decent situation for most of America's croplands east of the Rockies.

If the ridge becomes stagnant, large areas could undergo harsh hot and dry conditions, leading to widespread areas of drought and reduced yields. We are entering our first widespread heatwave of the summer as a ridge shifts to the eastern U.S. this weekend. You can read more about that here: https://www.dtnpf.com/….

What can always throw a wrench into the precipitation pattern is a tropical storm. In a drier pattern, a tropical storm could bring much-needed rainfall, or cause catastrophic flooding. And both could be possible this season. La Nina tends to promote an increased Atlantic hurricane season and all forecasts that are publicly available, as well as the forecast from DTN, are calling for an above-normal season, which just started on June 1. While no tropical storms have developed yet this year, there is some potential during the next week in the Gulf of Mexico and off the Atlantic coast. No U.S. landfalls are currently being monitored, but the reduction of wind shear across the Atlantic during the next few weeks and the near-record warmth in sea-surface temperatures across the tropical Atlantic and Gulf of Mexico provide plenty of opportunity.

The shift toward a later-developing La Nina could hold off the summertime tropical storm systems toward or beyond the peak, which starts in mid-August and lasts through early October. Tropical storms that form later will have less of a positive impact on crops, and instead produce more of a detriment to harvest and quality when coming during the fall if they indeed make landfall in the U.S.

To find more weather conditions and your local forecast from DTN, head over to https://www.dtnpf.com/…

John Baranick can be reached at john.baranick@dtn.com

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