1941- The Wettest Year in New Mexico Part 2 (ENSO/PDO) {Photo Credit: Earth Systems Research Laboratory}

Extended MEI (Multivariate ENSO Index) uses historical bi-monthly records of sea-level pressure and sea surface temperature over the Pacific to define El Niño/ La Niña years. The red peaks represent positive departures and indicate a warm phase. The years with a large negatives mark cold phases (shown in blue). The near-zero years are neutral years. The largest peak from 1915-1965 is reached around the year 1941.

La Niña has been a recent "cold button" issue with equatorial sea surface temperatures below average across the central and eastern Pacific. However, many sources have described 1941 as being a strong El Niño year and even as one of the top 10 strongest and longest El Niño events of the 20th Century. El Niño-Southern Oscillation (ENSO) is one of the most important coupled ocean-atmosphere phenomena associated with global climate variability on interannual time scales, typically defined by tropical Pacific sea surface temperature and air pressure. The changes in sea surface temperatures in the Pacific not only affect the distribution and strength of precipitation in the United States, but also result in modified atmospheric circulations that can change the positions of the jet stream and storm tracks in the Northern Hemisphere.

Pacific Decadal Oscillation (PDO) is a term used to describe multi-decadal variability of sea surface temperature anomalies in the northern Pacific. During a “warm” or positive phase, the western North Pacific becomes cool and part of the eastern ocean warms. The southern U.S. (including New Mexico) receives more precipitation than usual during El Niño, particularly during the cool seasons of winter and spring. Generally, ENSO conditions are likely to precede an extreme precipitation anomaly. The relationship between ENSO and PDO is debated, but the prevailing hypothesis is that PDO is caused by an increasingly positive El Niño-Southern Oscillation combined with stochastic atmospheric forcing.

The Multivariate ENSO Index (MEI) is an index of ENSO cycle that can be used to define El Niño/ La Niña event years (not including PDO), showing the duration and severity of the ENSO events. The only other year that comes close to the 1941 PDO value (+1.99) is the year 1987 with an annual average PDO index of +1.82. Multiple months of large precipitation amounts cannot be attributed to a strong monsoon or a strong wintertime ENSO anomaly alone, but the the combination of positive Pacific Decadal Oscillation values with a moderate El Niño can certainly enhance the precipitation anomaly. Another positive ENSO/PDO in the future may lead to above average precipitation, but it is unlikely that we will see values equaling those of the 1941 precipitation anomaly. 

To learn more about the role of ENSO/PDO and climate, please click here!

© 2018 Meteorologist Sharon Sullivan