Weather and climate modelers understand pretty well how seasonal winds and ocean currents affect El Niño patterns in the eastern equatorial Pacific Ocean, impacting weather across the United States and sometimes worldwide.
But new computer simulations show that one driver of annual weather cycles in that region—in particular, a cold tongue of surface waters stretching westward along the equator from the coast of South America—has gone unrecognized: the changing distance between Earth and the sun.
The cold tongue, in turn, influences the El Niño-Southern Oscillation (ENSO), which impacts weather in California, much of North America, and often globally.
The Earth-sun distance slowly varies over the course of the year because Earth’s orbit is slightly elliptical. Currently, at its closest approach—perihelion—Earth is about 3 million miles closer to the sun than at its farthest point, or aphelion. As a result, sunlight is about 7% more intense at perihelion than at aphelion.
Research led by the University of California, Berkeley, demonstrates that the slight yearly change in our distance from the sun can have a large effect on the annual cycle of the cold tongue. This is distinct from the effect of Earth’s axial tilt on the seasons, which is currently understood to cause the annual cycle of the cold tongue.
Because the period of the annual cycle arising from the tilt and distance effects are slightly different, their combined effects vary over time, said lead researcher John Chiang, UC Berkeley professor of geography.
“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.
Chiang noted that the distance effect is already incorporated into climate models—though its effect on the equatorial Pacific was not recognized until now—and his findings will not alter weather predictions or climate projections. But the 22,000-year phase cycle may have had long-term, historical effects. Earth’s orbital precession is known to have affected the timing of the ice ages, for example.
