An unrecognized effect boosts or diminishes the Pacific cold tongue, likely impacting El Niño/La Niña events and North American weather. Weather and climate modelers understand pretty well how seasonal winds and ocean currents affect El Niño patterns in the eastern equatorial Pacific Ocean, impac
A temperature map of the Pacific Ocean for December 1993 showing a cold tongue of surface water stretching westward along the equator from the coast of South America. The temperature and extent of the cold tongue changes with the seasons, but new climate simulations show that the annual change in Earth’s distance from the sun also affects the cold tongue seasonal cycle. This influences El Niño conditions that impact weather in North America and globally.
The cold tongue, in turn, influences the El Niño-Southern Oscillation , which impacts weather in California, much of North America, and often globally. “The curious thing is that the annual cycle from the distance effect is slightly longer than that for tilt — around 25 minutes, currently — so over a span of about 11,000 years, the two annual cycles go from being in phase to out of phase, and the net seasonality undergoes a remarkable change, as a result,” Chiang said.
For example, ENSO events tend to peak during Northern Hemisphere winters, she said, and they don’t typically persist beyond northern or boreal spring months, which scientists refer to as the “spring predictability barrier.” Because of these linkages, it is reasonable to expect that the distance effect could also have a major impact on ENSO — something that should be examined in future studies.
The yearly hemispheric changes in seasonal temperature alters the strength of the trades, and thus cause a yearly cycle in the temperature of the cold tongue. This, in turn, has a major influence on ENSO, which typically peaks during Northern Hemisphere winter. “Our field is focused on El Niño, and we thought that the seasonal cycle was solved. But then we realized that the result by Erb and Broccoli challenged this assumption,” he said.
“When Earth is closest to the sun, these winds are strong. In the offseason, when the sun is at its furthest, these winds become weak,” Chiang said. “Those wind changes are then propagated to the Eastern Pacific through the thermocline, and basically it drives an annual cycle of the cold tongue, as a result.”
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