October 2, 2018 | Published by | ,

Forecasting El Niño Southern Oscillation (ENSO) events, and anticipating how they may change with global warming remains a significant challenge for climate researchers. This is vital research as the impacts of these events are global, often bringing extreme weather conditions to geographical regions separated by thousands of kilometres.

It prompted two workshops in 2017 that produced follow-up papers (see below).

The first workshop, the El Niño Complexity workshop, included 40 scientists from 11 countries took place at Pusan National University in October 2017 and produced a summary paper in Nature. The workshop aimed to re-examine the complexity of ENSO and determine if a framework could be developed that might improve our understanding and ability to forecast ENSO events.

The second workshop on ENSO dynamics with 25 Australian researchers was held at the University of New South Wales. It also produced a follow-up paper summarising what we know about ENSO and its predictability from an Australian perspective.

Together they summarised what we know about ENSO events and set directions for future research directions.

 

The combined findings
El Niño events are characterised by an unusual warming of the central to eastern equatorial Pacific, which can last up to one year. Many events subsequently transition to a La Niña (cold) state, with a typical duration of 1-2 years. El Niño events, which tend to peak in boreal winter, typically lead to a drying of Southeastern Asia and the Western tropical Pacific whilst enhancing rainfall near the eastern Pacific shores, in countries such as Ecuador and Peru. El Niño’s remote “ripple effects” can not only be found in the atmosphere, but also in ocean currents, ecosystems, the occurrence of natural disasters, global markets and national economies.

Despite some understanding of the processes that bring about the formation of ENSO events, we can generally only forecast these events with some certainty up to nine months ahead. This is primarily because of the complex series of factors that combine to produce ENSO events.

Our ability to predict El Niños and La Niñas has remained relatively unchanged for a number of decades and there was even a slight decrease in prediction skill at the turn of the 21st Century.

Not every El Niño is alike. Some are weak, others are strong. Some occur in the central Pacific, others in the east. These differences will determine which areas will be hit hardest by climatic extremes and which ones will be spared. Predicting El Niño events accurately requires a deeper understanding of its diversity or as some scientists call it — its “flavors”.

Analysing large amounts of climate observations and computer model simulations, the Pusan workshop unraveled the mechanism behind El Niño’s capricious behaviour. When the upper tropical Pacific Ocean stores more heat, El Niño events tend to peak in the Eastern Pacific and during boreal winter, whereas a cooler upper ocean system leads preferably to the development of Central Pacific El Niño events which exhibit a weaker seasonal coupling.

By running El Niño computer model simulations for different temperature, wind and ocean current configurations the research team found that Eastern Pacific El Niño events are characterised by a return time of 3-7 years, whereas Central Pacific events tend to recur on average every 2-3 years. The different character of these modes is determined by how strongly atmosphere and ocean interact with each other. The theoretical results presented in this study are consistent with observational datasets.

In reality however, the co-existing Eastern and Central Pacific warm/cold swings are far from periodic. The tropical Pacific climate system requires constant excitation, either through random weather events or through atmospheric circulation changes induced by temperature changes in the Indian and Atlantic Oceans. These interactions are an important source for El Niño irregularity, and limit how far ahead Tropical Pacific climate anomalies can be predicted.

It is hoped this proposed synthesis of two ENSO structures, their interaction with each other and how they respond to external forcing, will be the catalyst for future research and practical applications for forecasting and determining the impacts of present and future ENSO events.