UCI Scientists Predictor for Southwest Winter Rainfall — Look to New Zealand in the Summer

Scientists and engineers at the University of California, Irvine (UCI) may have a new method for projecting wet or dry weather in the Southwest for the coming winter. Lead author Antonios Mamalakis, a UCI graduate student in civil & environmental engineering, said the UCI New Zealand Index study produced an unexpected result — the discovery of persistent sea surface temperature and atmospheric pressure patterns in the southwestern Pacific Ocean that exhibited a strong correlation with precipitation in Southern California, Nevada, Arizona and Utah.

Although the familiar El Nino conditions have long been considered a reliable tool for predicting future precipitation in the southwestern United States, its forecasting power has diminished in recent cycles, possibly due to global climate change. But UCI researchers have found that a sea surface temperature anomaly that begins in July and August, in the southwestern Pacific Ocean, triggers an interhemispheric teleconnection. As the sea surface temperature in the region cools down or heats up, it causes a change in the southern Hadley cell, an atmospheric convection zone from the equator to about the 30th parallel south, close to New Zealand.

“The interhemispheric teleconnection that we have discovered promises earlier and more accurate prediction of winter precipitation in California and the southwestern U.S.,” said study co-author Efi Foufoula-Georgiou, UCI distinguished professor of civil & environmental engineering. “Knowing how much rain to expect in the coming winter is crucial for the economy, water security and ecosystem management of the region.”

As the southern Hadley cell either heats or cools it prompts a commensurate anomaly east of the Philippine Islands, which, in turn, results in a strengthening or weakening of the jet stream in the Northern Hemisphere, having a direct influence on the amount of rain that falls on California between November and March. An interdisciplinary team of scientists analyzed sea surface temperature and atmospheric pressure in 1- and 2-degree cells around the globe from 1950 to 2015 to test the theory.

Mamalakis said the unexpected result was the discovery of persistent sea surface temperature and atmospheric pressure patterns in the southwestern Pacific Ocean that exhibited a strong correlation with precipitation in Southern California, Nevada, Arizona and Utah. Conversely, strongly positive El Niño conditions in recent years did not bring a lot of rain to California, as they had in the past. Yet, while the usually arid California received heavy precipitation in the winter of 2017, it was considered a neutral El Niño season.

“With the New Zealand Index, we can predict from late summer the likelihood of above- or below-normal winter precipitation in the southwestern U.S., with a correlation in the order of 0.7 – compared to the El Niño-Southern Oscillation technique, which has a correlation around 0.3 to 0.4,” said Mamalakis. “Our research also shows an amplification of this newly discovered teleconnection over the past four decades.”

“Predicting drought in the southwestern U.S. is a critical issue for food production and local economies,” said Tom Torgersen, director of the National Science Foundation’s Water Sustainability & Climate program, which funded the research. “The discovery of an interhemispheric bridge that affects the winter U.S. jet stream holds the promise of improved precipitation predictability and drought forecasts.”