A mystery in the Pacific is complicating climate projections

By | April 24, 2023

Yale Climate Connections reports:

Nothing has a bigger influence on year-to-year variations in the global climate than the El Niño-Southern Oscillation, commonly called ENSO. And the tropical waters at the heart of ENSO aren’t behaving exactly as climate scientists expected they would in a warming world, with potentially major implications for Atlantic hurricane seasons, droughts in the U.S. Southwest and the Horn of Africa, and other weather phenomena around the world.

ENSO is a recurring ocean-and-atmosphere pattern that warms and cools the eastern tropical Pacific through El Niño and La Niña events that last from one to three years. Once El Niño or La Niña emerges, the odds reliably shift toward hotter, colder, wetter, or drier conditions for various parts of the globe, from Oceania to North America to Africa. But though ENSO’s effects are well known, the phenomenon itself is notoriously tough to predict. And its slippery nature is complicating crucial multi-decade projections of climate.

Many aspects of human-caused climate change are playing out as long predicted, including overall warming of the global atmosphere and oceans as well as the intensification of rainfall extremes and the drying of many subtropical areas.

Not so for ENSO. Top global climate models have predicted for more than 20 years that the tropical Pacific would gradually shift toward an “El Niño-like” state, with the surface waters warming more rapidly toward the east than toward the west.

Instead, just the opposite is going on. The western tropical Pacific has warmed dramatically, as predicted, but unusually persistent upwelling of cool subsurface water has led to a slight drop in average sea surface temperature over much of the eastern tropical Pacific. [Continue reading…]