A weak La Niña climate pattern has officially emerged in the equatorial Pacific Ocean, an arrival that complicates forecasts for the remainder of the 2025 Atlantic hurricane season. The event, characterized by cooler-than-average sea surface temperatures, was confirmed by the National Oceanic and Atmospheric Administration’s Climate Prediction Center in September. While La Niña typically supercharges the Atlantic hurricane season by creating more favorable atmospheric conditions, scientists say this particular event is weak and its late arrival makes its precise impact difficult to determine.
Forecasters are weighing the influence of this weak Pacific cooling against other competing and localized climate signals, including a separate and unusual cooling in the tropical Atlantic itself. This mix of factors introduces significant uncertainty into a season that has so far been relatively slow to ramp up. Historical data from years with similar late-season transitions to a weak La Niña suggest that a surge in tropical storm and hurricane formation is possible through October and November, but the overall weakness of the event could temper its influence, potentially leading to a less impactful season than typically expected under La Niña conditions.
Understanding the La Niña Phenomenon
La Niña is a naturally occurring climate pattern defined by a persistent cooling of surface waters in the central and eastern equatorial Pacific Ocean. This oceanic shift sets off a chain reaction in the atmosphere. The cooling in the Pacific alters global weather patterns, and one of its most significant effects is felt thousands of miles away over the Atlantic basin. Specifically, La Niña reduces vertical wind shear in the Atlantic.
Vertical wind shear is the change in wind speed and direction at different altitudes in the atmosphere. Strong wind shear acts as a disruptive force for developing tropical cyclones, essentially tearing them apart before they can organize and strengthen. During a La Niña event, the reduction of this shear creates a much more stable and favorable environment for tropical waves rolling off the coast of Africa to grow into organized, powerful hurricanes. This is why La Niña years are strongly correlated with more active and destructive Atlantic hurricane seasons.
A Complicating Factor: The Atlantic Niña
Adding a layer of complexity to the 2025 forecast is the recent emergence of a rare and lesser-known phenomenon dubbed the “Atlantic Niña.” This pattern is marked by unusually cool sea surface temperatures in the eastern and central tropical Atlantic, particularly in the Gulf of Guinea. This cooling has the opposite effect of its Pacific counterpart; it can suppress hurricane activity by strengthening trade winds and weakening the West African monsoon, which is the source of many of the tropical waves that seed Atlantic hurricanes.
Meteorologists note that the Atlantic Niña caused sea surface temperatures to drop by 2 to 4 degrees Celsius in some areas. However, most experts believe this cooling trend will be short-lived. Unlike the vast and persistent Pacific events, anomalous temperature patterns in the Atlantic tend to be more transient. Forecast models and subsurface temperature data suggest that the cooler waters are a shallow feature, with warmer-than-normal water lurking just 25 to 50 meters below the surface. Weaker trade winds are already allowing these waters to warm, and the influence of the Atlantic Niña is expected to fade as the season progresses.
Official Forecasts and Projections
Despite the mixed signals, major hurricane forecasting centers maintain predictions for an above-average season, largely based on the expected overarching influence of the Pacific La Niña. Colorado State University’s extended forecast, a benchmark in the field, calls for a total of 17 named storms, nine of which are projected to become hurricanes. Of those nine, four are expected to strengthen into major hurricanes of Category 3 intensity or higher. The university’s modeling gives a 56% probability of a major hurricane tracking through the Caribbean, a figure higher than the long-term average.
These forecasts were issued with the anticipation that either neutral conditions or a weak La Niña would dominate the peak months of the season. The official arrival of La Niña in September aligns with these projections. The primary driver for these forecasts remains the reduction in Atlantic wind shear that even a weak La Niña can produce, creating an environment conducive to storm development.
Timing and Historical Precedent
The timing of La Niña’s onset is a critical factor in determining its ultimate impact on the hurricane season. With the pattern not officially emerging until September, its influence is concentrated on the latter half of the season. This has led forecasters to examine analog years where a similar transition from an ENSO-neutral state to a weak La Niña occurred near the peak of hurricane season. The years 2000, 2016, and 2017 serve as key historical comparisons.
Each of these analog seasons was active and demonstrates how a late-developing weak La Niña can still fuel significant tropical activity. The 2000 and 2016 seasons both produced 15 named storms, with several systems forming late in the season. The 2017 season was one of the most destructive on record, producing 17 named storms, including the catastrophic hurricanes Harvey, Irma, and Maria. While other climate factors were also at play, these past seasons highlight that a transition to even a weak La Niña during the heart of the hurricane season can support a very active and dangerous environment.
Implications for Late-Season Activity
The convergence of a developing La Niña and waning influence from the Atlantic Niña points toward a potentially “back-loaded” hurricane season. Experts suggest that atmospheric conditions will become increasingly favorable for storm formation in October and November. Alex DaSilva, an AccuWeather hurricane expert, noted that the emergence of La Niña favors an active end to the season precisely because it reduces the wind shear that typically begins to increase in the autumn months. This could extend the period of significant threat for coastal regions.
While the season had a relatively slow start through early September, the most critical period may still lie ahead. The confluence of peak ocean temperatures, reduced wind shear from La Niña, and the steady progression of tropical waves from Africa creates a recipe for late-season development. The key uncertainty remains the potency of this particular La Niña. If it remains weak, as forecast, its ability to sustain an environment ripe for hurricanes could be less pronounced, but coastal residents are nonetheless urged to remain vigilant through the final months of the season.
