Salt, Rain, and the Logic Behind a Multiyear La Niña

Is there not a strange, terrifying elegance to the way nature organises its own disruption? We often mistake the ocean for a chaotic body—a thing of crashing waves and unpredictable swells. Yet, look closer. There is a rigid, almost mathematical logic at work. A system that remembers.

Recent observations and modelling experiments have turned our attention to the Pacific’s recent habit of getting stuck. We are seeing more frequent prolonged cold phases. To understand why, researchers have identified a mechanism that feels less like meteorology and more like a biological feedback loop: the relationship between rain and salt.

The mechanics of a Multiyear La Niña

The study highlights a simple input with a complex output. When La Niña cools the waters, rainfall drops. Less rain means the water becomes saltier. This is where the physics gets interesting. The research indicates that these positive mixed-layer salinity (MLS) anomalies in the western-central equatorial Pacific do not just sit there. They actively reinforce the cold.

It acts like a battery charging up. The data suggests that this rainfall reduction boosts the amplitude of the La Niña by 14% in the first year. Significant, but not overwhelming. However, by the second year, that influence jumps to 32%. The ocean holds a grudge. It remembers the dryness of the previous year and uses the accumulated salinity to drive the system harder.

Why would nature design a system that amplifies its own extremes? Consider the efficiency of a genome. Biological systems often rely on positive feedback loops to commit to a process—cell division, for instance—so that once a threshold is crossed, the action completes decisively. The climate appears to function similarly. It avoids ambiguity. Once the Pacific tips toward a Multiyear La Niña, the salinity feedback ensures it stays there, rather than wobbling back to neutral.

Kelvin waves and the slow response

The mechanism is a two-step dance. The researchers found that the salinity anomalies initially trigger equatorial Kelvin waves—rapid adjustments that cause surface cooling in the east. Later, a slower ocean circulation response kicks in, spreading that cooling across the entire basin.

This superposition of fast and slow responses creates a lock-in effect. The study implies that without this salinity feedback, these multiyear events might fizzle out much sooner. Instead, the lack of fresh water stiffens the ocean's resolve, sustaining the pattern. For forecasters, this is a vital clue. If we watch the salt, we might predict the duration of the drought.