Ag Weather Forum

Pacific Temperatures Show Slower La Nina Development vs. 2020

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Mid-September to mid-October Pacific Ocean temperatures show only pools of cooler water around the equator, a sign La Nina is slow in its development compared to the 2020-23 La Nina event. (NOAA graphic)

La Nina, the cool-water phase of the Pacific Ocean El Nino-Southern Oscillation (ENSO) recurring climate pattern, is certainly taking its time to develop. Pacific Ocean temperatures for the ENSO region, which extends from the International Date Line in the central Pacific to the Peruvian Coast in South America along the equator, show only a couple areas of cooling during the month from mid-September to mid-October. Even in those areas of cooler water, temperatures at most have declined to 2 degrees Celsius (3.6 degrees Fahrenheit) below average. Between those cooler sections, the equatorial Pacific had a large area with water temperatures anywhere from average to around 1 degree C (1.8 F) above average.

A look at how Pacific temperatures have trended during the past six months compared with 2020, using a multi-month temperature analysis called the Oceanic Nino Index (ONI), shows how La Nina's development this year is notably slower than four years ago. The NOAA Climate Prediction Center defines the ONI as the rolling three-month average temperature anomaly -- difference from average -- in the surface waters of the east-central tropical Pacific, near the International Dateline.

Back in 2020, the ONI was already showing La Nina, with a value of -0.6 (deg C) for the July-August-September rolling three-month average temperature anomaly. This year, 2024, that July-August-September temperature anomaly was still in the ENSO-neutral bracket at -0.1. The following three-month period, August-September-October, is still to be determined for 2024, but in 2020, that value was -0.9; a sign that what turned out to be a multi-year La Nina event was underway.

Forecasts for La Nina have had several revisions during the past three months. The NOAA Climate Prediction Center's latest forecast calls for a 60% chance for La Nina to emerge by November and persist through the first quarter of 2025 (January-February-March). DTN Long Range Weather Risk Analyst Nathan Hamblin also expects La Nina to develop soon, with its peak cooling approximately in December. Hamblin's forecast is for Pacific cooling to reach a maximum of -0.8 deg C. This is much warmer than the 2020-23 La Nina, which had a maximum ONI value of -1.3 deg C in October-November-December 2020. The 2020-23 La Nina also had 12 three-month periods with the ONI reading of -1.0 deg C or below.

A big follow-up question now is: How much of an impact will this expected La Nina have on central U.S. weather patterns? Across the contiguous U.S., La Nina tends to bring an amplified polar jet stream and a weaker subtropical jet stream than average. The strongest related weather trends are dry conditions in the Central and Southern Plains, above average precipitation in the eastern Midwest, and generally below average temperatures. This La Nina event could feature some differences. "It's important to note that the weak intensity of La Nina may result in less pronounced impacts going into the winter compared with other stronger ENSO events, so there could be a lot more variability in weather patterns throughout the winter," said Indiana State Climatologist Austin Pearson in a recent NOAA-USDA forecast webinar.

Recently dry primary U.S. crop areas might welcome variability from established La Nina winter precipitation patterns. As of mid-October, USDA's Ag in Drought assessment found that 62% of U.S. corn production, 54% of U.S. soybean production and 52% of U.S. wheat production were in areas affected by drought.

Bryce Anderson can be reached at bryce.anderson@dtn.com

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