The Interdecadal Shift of ENSO Properties in 1999/2000: A Review

Abstract

Following the interdecadal shift of El Niño–Southern Oscillation (ENSO) properties that occurred in 1976/77, another regime shift happened in 1999/2000 that featured a decrease of variability and an increase in ENSO frequency. Specifically, the frequency spectrum of Niño-3.4 sea surface temperature shifted from dominant variations at quasi-quadrennial (~4 yr) periods during 1979–99 to weaker fluctuations at quasi-biennial (~2 yr) periods during 2000–18. Also, the spectrum of warm water volume (WWV) index had almost no peak in 2000–18, implying a nearly white noise process. The regime shift was associated with an enhanced zonal gradient of the mean state, a westward shift in the atmosphere–ocean coupling in the tropical Pacific, and an increase in the static stability of the troposphere. This shift had several important implications. The whitening of the subsurface ocean temperature led to a breakdown of the relationship between WWV and ENSO, reducing the efficacy of WWV as a key predictor for ENSO and thus leading to a decrease in ENSO prediction skill. Another consequence of the higher ENSO frequency after 1999/2000 was that the forecasted peak of sea surface temperature anomaly often lagged that observed by several months, and the lag increased with the lead time. The ENSO regime shift may have altered ENSO influences on extratropical climate. Thus, the regime shift of ENSO in 1999/2000 as well as the model default may account for the higher false alarm and lower skill in predicting ENSO since 1999/2000.

Origin of Weakened Interannual Sea Surface Temperature Variability in the Southeastern Tropical Atlantic Ocean

“…One of them might be the interdecadal shift of El Niño/Southern Oscillation (ENSO) that occurred around 2000 (Hu et al, 2013, 2017, 2020; Lübbecke & McPhaden, 2014; McPhaden, 2012). This shift featured a decrease in ENSO variability, an increase in ENSO frequency (Hu et al, 2013, 2020) as well as a profound westward shift of the Walker Circulation in the equatorial Pacific (Li et al, 2019). The latter might have altered ENSO teleconnections, and thus, could have contributed to the weakened SST variability in the tropical Atlantic.…”

“…In order to investigate the changes in the interannual SST variability in the ABA since 2000, we compare the periods January 1982 to December 1999 and January 2000 to December 2017. The year 2000 was chosen as the year of separation because major shifts occurred around this year both in the tropical Atlantic (Prigent et al, 2020) and tropical Pacific (Hu et al, 2013, 2017, 2020; Li et al, 2019; Lübbecke & McPhaden, 2014; McPhaden, 2012). All analyses use monthly‐mean anomalies computed by subtracting the climatological monthly‐mean seasonal cycle derived separately for each data set and period.…”

On the Interdecadal Variation of the Warm Water Volume in the Tropical Pacific Around 1999/2000

“…According to An and Wang (2000), the zonal migration of the atmosphere and ocean coupling region affects the frequency of ENSO: westward shift (eastward extension) favors higher (lower) frequency of ENSO. Thus, the westward shift of the atmosphere‐ocean coupling region since 2000 is a crucial factor causing the frequency increase of ENSO and lead‐time shortening of WWV to ENSO (Hu, Kumar, Huang, et al, 2020; Hu, Kumar, Zhu, et al, 2017; McPhaden, 2012).…”

“…(a) Monthly mean Niño3.4 (line) and WWV (shading) indices from January 1979 to December 2019; variance dependence on time scales of the (b) WWV and (c) Niño3.4 indices for the average in January 1979 to December 1999 (bar) and January 2000 to December 2019 (curve), based on the time scale decomposition of wavelet (Hu & Nitta, 1996; Meyers et al, 1993). Panels (b) and (c) are replotted with updated data after Hu, Kumar, Zhu, et al (2017) and Hu, Kumar, Huang, et al (2020). …”

“…This scenario is consistent with the fact that the equatorial band WWV discharge during CP El Niño seems weaker than during EP El Niño (Singh & Delcroix, 2013). Therefore, understanding the interdecadal variation associated with WWV may help us to better understand the interdecadal shift of ENSO properties around 1999/2000 and the decline of ENSO prediction skill since 2000 (Barnston et al, 2012; Hu, Kumar, Huang, et al, 2020; Neske & McGregor, 2018; Wang et al, 2010).…”