When people search for “what is La Nada,” they are referring to the El Niño-Southern Oscillation (ENSO) neutral phase. “La Nada,” which translates to “the nothing,” is an unofficial nickname for this state of the tropical Pacific Ocean. This period is defined by the absence of the strong, predictable climate signals associated with the two opposite phases of the cycle. The neutral phase signifies that the massive Pacific Ocean system is operating close to its long-term average, rather than representing a halt of weather activity.
Clarifying the Terminology and the ENSO Cycle
The formal climate phenomenon is the El Niño-Southern Oscillation (ENSO), a major recurring climate pattern across the tropical Pacific Ocean. ENSO involves the coupled interaction between changes in the temperature of the ocean surface and the overlying atmosphere. This natural system shifts back and forth irregularly, typically over a period of two to seven years.
The cycle has three distinct phases, defined by the sea surface temperature in the central and eastern equatorial Pacific. The warm phase, El Niño, is characterized by warmer-than-average temperatures. Conversely, the cool phase, La Niña, features cooler-than-average temperatures. The neutral phase is the state between these two extremes, where ocean temperatures are near the long-term average.
The strong warming or cooling during El Niño or La Niña creates powerful atmospheric teleconnections that influence global weather. During the neutral phase, these strong global influences are significantly reduced. The term “La Nada” aptly describes this interim state, as it represents a period without the strong push of either the warming or cooling phase.
Metrics for Identifying Neutral Conditions
Scientists rely on specific, quantitative metrics to determine when the ENSO cycle is in a neutral state. The primary measurement is the Sea Surface Temperature (SST) anomaly in the Niño 3.4 region of the equatorial Pacific. This region spans \(5^{\circ}\) North to \(5^{\circ}\) South latitude, and \(170^{\circ}\) West to \(120^{\circ}\) West longitude.
The official threshold for neutrality is defined by the Oceanic Niño Index (ONI), based on the three-month running average of SST anomalies in the Niño 3.4 region. Neutral conditions are declared when these temperatures remain within \(\pm 0.5^{\circ}\) Celsius of the long-term average. If the anomaly exceeds \(+0.5^{\circ}\)C, it signals El Niño, and if it drops below \(-0.5^{\circ}\)C, it indicates La Niña.
Secondary atmospheric indicators must also be near average to confirm a neutral period. The Southern Oscillation Index (SOI) measures the standardized difference in sea level pressure between Tahiti and Darwin, Australia. During neutral periods, this pressure difference is close to zero, reflecting a near-average strength of the Walker Circulation, the east-west atmospheric overturning circulation along the equator.
Regional Weather Impacts of Neutral ENSO
The neutral phase is often associated with “typical” or “average” weather, but this does not mean a lack of weather variability or extreme events. Since the strong, coherent global patterns of El Niño and La Niña are absent, weather is instead primarily influenced by local and regional climate drivers. This makes long-term global weather prediction significantly less reliable during neutral periods.
In North America, for example, neutral winters tend to feature more moderate and less predictable temperature and precipitation patterns compared to the strong signals of the other phases. The southern United States may experience warmer probabilities, while the north-central and northeastern regions often favor colder conditions. This pattern is generally considered more “normal” but is far less globally predictable than the winter effects of a strong El Niño, which typically directs a powerful Pacific jet stream across the southern US.
The Atlantic hurricane season is another area where the neutral phase has a distinct impact, representing a baseline for activity. During neutral years, the mean number of landfalling hurricanes in the United States is generally higher than during El Niño years, but lower than during La Niña years. For instance, the mean number of US landfalling hurricanes is approximately \(1.61\) per year during neutral periods, compared to \(1.04\) during El Niño years. The absence of the strong ENSO signal allows other regional factors to dictate hurricane development. This can still lead to highly active seasons, such as the record-breaking 2005 season that occurred under neutral conditions.