ENSO is predictable, but only in the big picture, meaning seasonal averages or longer. The signals that tell us that
El Niño or La Niña are on the way, such as a large amount of cooler or warmer water under the ocean surface, or a particularly strong, long-lasting shift in the trade winds, are reliable indicators. Also, our computer climate models, which look at current conditions and make predictions based on mathematical and physical equations, are pretty good, especially after the
spring barrier (a time of year when predictions are especially difficult).
However, small,
short-term fluctuations, such as the weaker equatorial trade winds that occurred during September, can’t be predicted more than a couple of weeks (at best) in advance. They tend to have a disproportionate impact during borderline, more marginal situations when we are hovering near our
ENSO thresholds. These small fluctuations can tip the scales one way or the other. In this case, they’ve added up to a slower and weaker La Niña development. That said, many of our models are holding steady for La Niña to develop shortly.
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