资源环境科技发展态势分析平台

The amplitude of El Nino, 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 Nino 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 Nino amplitude.