A team of climate scientists in China and California wanted to disentangle the role of El Niño from all the other factors in climate change. So they posed the following question: “How much of the pattern of climate change would be the same if the sea-surface temperature pattern remained the same?”
The team, led by Zhen-Qiang Zhou of the Ocean University of China, used climate models representing the ocean and the atmosphere to explore the impact of global warming on El Niño-induced patterns of temperature and precipitation across the Pacific Ocean and North America. For the study, the researchers used a “composite” El Niño, the average winter sea-surface temperatures during four El Niño events in the Northern Hemisphere (1972-1973, 1982-1983, 1997-1998 and 2009-2010). They were interested in possible changes in atmospheric “teleconnections,” or statistical correlations, between North American climate and El Niño (the more technical term is El Niño-Southern Oscillation or ENSO).
The researchers found that, indeed, teleconnections may shift. ENSO-induced changes in eastern Pacific tropical thunderstorms — along with warmer northern Pacific sea-surface temperatures — affect precipitation on the U.S. West Coast, making it more variable and more intense. Land-surface temperature, too, was more variable and intense moving eastward across the United States, a result of migration of the Pacific North American Pattern.
The models used in their study have better spatial resolution and exhibit better distribution of El Niño sea-surface warming than other models using only mean global sea-surface warming. Also, isolating atmospheric factors shows larger anomalies for the “forced” versus “internal” Pacific North American Pattern mode. Finally, greater magnitude in changes in sea-surface temperature and El Niño teleconnections producing an eastward shift in U.S. temperatures will persist in the future, likely independent of climate-change effects on El Niño variability and behavior.
A caveat: By holding the El Niño sea-surface temperature pattern identical for present and future climate simulations, the study sidesteps major uncertainties in El Niño properties (such as amplitude and sea-surface temperature).