What Goes on in the Pacific Doesn't Stay in the Pacific

Whether you want to know if you'll need an umbrella or if you need to water your fields, weather forecasts are quite useful. But beyond about ten days, the chaotic nature of our climate system makes it difficult to predict what the weather will be. Muhammad Adnan Abid, a postdoctoral fellow in ICTP's Earth System Physics section, was investigating a longer time scale – whole seasons – when he started thinking about how to predict longer than two weeks and shorter than a season.

"If there's any predictability beyond two weeks, there must be some sort of driving forces that we can try to understand," says Abid. For Abid and his collaborators, which included Fred Kucharski, a staff scientist in the Earth System Physics section, that meant looking at ocean temperatures, sea ice temperatures, and larger climate patterns. "Anything that has a longer memory," says Abid. "Oceans take a long time to heat up and cool down, so it has a long memory of earlier perturbations or changes." Kucharski adds, "In order to get any predictability beyond two weeks, we needed to look at perturbations- disturbances of patterns."

Abid is an ICTP Diploma Programme alumnus from Pakistan, a graduate of the first class of Diploma students in Earth System Physics. He returned to ICTP to work with Kucharski on climate teleconnections, starting with a famous climate pattern: the El Niño–Southern Oscillation, or ENSO. It describes small changes in sea surface temperatures and wind patterns in the tropical Pacific Ocean, that can have large effects on weather patterns in other parts of the world. El Niño and La Niña are the extreme heating and cooling phases of this pattern, words that often come up in weather forecasts as the causes of high or low pressure systems bringing clear skies or rain.

"Changes in ENSO are like dropping a rock in water, the effects ripple out," says Abid. Those perturbations also affect other basins, like the Indian Ocean and the Atlantic. In the Indian basin, ENSO ripple effects create heating anomalies, an irregular seesaw pattern resembling the sea surface temperature-based Indian Ocean Dipole or IOD. An El Niño ENSO phase makes the IOD cause more rain in the western Indian Ocean and less in the eastern Indian Ocean, and vice versa with La Niña. In turn, the IOD has been linked to heavy rainfall in East Africa and bushfires in Australia.

The Indian Dipole also affects early winter temperatures in Europe, as Abid, Kucharski, and colleagues outline in research published recently in the Journal of Climate. But the effect is not consistent across the whole season. Early in the winter, what's happening in the Indian Ocean has a big effect on the Europe-Atlantic system. Later in the winter, this effect weakens, and the direct ENSO forcing starts to play a bigger role in influencing winter in Europe.

Climatologists exploring the effects of ENSO around the globe have recently been looking at the connections between ENSO and winter conditions Europe. But instead of looking at the average temperature across the typical winter period, from December to February, researchers started looking at the differences between early and late winter, and the shoulder seasons of November and March.

The results were surprising: the dominant forcing on patterns in Europe changed between early and late winter. Without looking at the early and late winter data separately, this possibility for greater predicting power would never have been found. "Over the whole winter, the Indian Dipole and ENSO's distinct effects on European weather get mixed up and become hard to distinguish," says Abid.

"We wanted to know the mechanism of the indirect connections between ENSO and changes in the European-Atlantic region," says Kucharski. The team's paper describes the mechanism of this teleconnection between the Indian Ocean and Europe. "Indian Ocean predictability can be a clue for improving predictability in Europe, now that we know how these teleconnections work," says Abid.

"It was suggested before that ENSO affects Europe, but with very modest signal," says Abid. "It turns out, one of the mechanisms is the Indian Ocean." There is very 'noisy' weather over Europe, notes Kucharski. "If we can investigate the forcings and separate them out, we can hopefully get some predictability beyond two weeks."

Read the paper: Separating the Indian and Pacific Ocean Impacts on the Euro-Atlantic Response to ENSO and Its Transition from Early to Late Winter

DOI: https://doi.org/10.1175/JCLI-D-20-0075.1

---- Kelsey Calhoun