Weakening of the El Niño amplitude since the late 1990s and its link to decadal change in the North Pacific climate

Abstract

The amplitude of El Niño, measured by the Nino3.4 sea surface temperature anomaly (SSTA) index, has exhibited an interdecadal change with a weakening trend since the late 1990s, characterized by the distinct Bjerknes stability index between 1980-1998 and 1999-2014. Statistical results suggest that this was primarily induced by the attenuation of the zonal wind stress and low-level wind anomalies in response to the zonal equatorial SSTA gradient. The weakened atmospheric responses to the zonal equatorial SSTA gradient were associated with the obvious westward extension of the negative sea level pressure anomalies (SLPA) over the tropics. This was mainly attributed to the transition of the dominant atmospheric circulation over the North Pacific, where the Aleutian Low (AL) mode was replaced by the North Pacific Oscillation (NPO) mode after the late 1990s. Numerical experiments from the long-term historical simulations of the GFDL-CM3 model indicate that both of the AL and NPO were dominant over the North Pacific and alternated on the interdecadal time-scale. When the NPO mode was dominant, it became more effective at playing a role in triggering an El Niño in the central Pacific via the seasonal footprinting mechanism over the subtropical northeastern Pacific. Such a change might cause the westward shift of the tropical SLPA and the attenuated atmospheric responses to the zonal SSTA gradient, ultimately resulting in the weakening of the El Niño amplitude.

Weakened SST variability in the tropical Atlantic Ocean since 2000

“…Some studies have addressed multidecadal change of SST variability both in the equatorial Pacific (Hu et al 2013;Lübbecke and McPhaden 2014;Guan and McPhaden 2016;Hu et al 2017;Xu et al 2019) and in the equatorial Atlantic (Tokinaga and Xie 2011). While Tokinaga and Xie (2011) investigated trends in Atlantic cold tongue variability over the time period 1950-2009, several studies have discussed a shift in equatorial Pacific variability that occurred around the year 2000 and is clearly visible in many ENSO characteristics.…”

“…This shift has been explained by changes in the equatorial thermocline tilt along with a strengthening of the trade winds, which has hampered the eastward migration of warm water along the equatorial Pacific and hence reduced ENSO amplitude (Hu et al 2013). A more recent study from Xu et al (2019), also investigating the weakening of ENSO amplitude since the late 1990s, put it in the context of the transition from the Aleutian Low mode to the North Pacific Oscillation in the atmosphere that is responsible for a westward extension of negative sea level pressure anomalies. This shift is proposed to have weakened the atmospheric responses to the zonal equatorial SSTa, and hence ENSO amplitude.…”

Two types of warm blobs in the Northeast Pacific and their potential effect on the El Niño

“…Note that the El Niño signal was predominant near the dateline (Figure 3j–l), implying that the single‐peak warm blobs were more likely to induce a Central Pacific (CP) El Niño event, consistent with Xu et al . (2019, 2020) who revealed that the NPGO‐like SST anomalies can trigger a CP El Niño via the WES feedback.…”

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

“…4a), the SSTA associated with their cold tongue mode is confined within 108S-108N (Zhang et al 2010). Also, the change in ENSO amplitude might also be associated with changes in various extratropical forcings (Xie et al 2016;Yu and Fang 2018;Xu et al 2019).…”

“…McPhaden (2012), Hu et al (2013), Lübbecke and McPhaden (2014), and Xu et al (2019) noted that ENSO amplitude decreased after 2000. Later, it was suggested that the interannual variability of the entire tropical Pacific atmosphere-ocean coupled system decreased after 2000 (Hu et al , 2017a.…”