27 July 2023, NOAA: Has climate change already affected ENSO? This is a guest post by Mike McPhaden, who is a senior scientist at NOAA Pacific Marine Environmental Laboratory in Seattle, WA. Mike has previously blogged with us and was lead editor of the book “El Niño- Southern Oscillation in a Changing Climate” published by the American Geophysical Union. He has had a prolific career, including spearheading development of the Tropical Atmospheric Ocean (TAO) buoy array across the equatorial Pacific Ocean, which is key to observing and understanding ENSO. For more than 30 years, climate researchers have been puzzling about how human-forced climate change affects the El Niño Southern Oscillation (ENSO), the warm phase of which we refer to as El Niño and the cold phase as La Niña. There are two aspects to this question:
- Has climate change affected ENSO already? and
- How will climate change affect ENSO in the future?
Arguably, we have made more progress on the second question than the first because greenhouse gas forcing in the future is expected to be stronger than it has been up to now; the stronger the forcing, the more obvious its impacts become. Whether climate change has already affected ENSO has been a harder nut to crack. A previous blog article by Tom DiLiberto summarized the conclusions from the latest IPCC report, which essentially found that there was no clear evidence yet for an impact of climate change on tropical Pacific sea surface temperature (SST) anomalies related to ENSO. However, in a recent study published in Nature Reviews Earth and Environment, Wenju Cai and colleagues revisit this question, reviewing past studies and performing new analyses to provide additional insights on this important question. One of the primary sources of information we have for past ENSO behavior is the instrumental record of sea surface temperatures (SST) from the tropical Pacific. Many have noted that the most recent period of the observed SST record in key ENSO index regions, like Niño-3.4, exhibits higher-amplitude variability than the early part of the record. The most recent 50-60 years, for example, appears to be more energetic, with larger swings up and down, than the previous 50-60 years. Cai and colleagues make this point using several different data sources and methods—evidence that the pattern is real, not just a data quality problem due to the relative sparsity of data before 1950. Read more here