On the Shortening of the Lead Time of Ocean Warm Water Volume to ENSO SST Since 2000

The possible factors associated with the shortening of lead time between ocean warm water volume (WWV) variability along the equatorial Pacific and El Niño–Southern Oscillation (ENSO) variability after 2000 are documented. It is shown that the shortening of lead time is due to frequency increases of both WWV and ENSO. During 1979–99 the dominant frequencies were 1.5–3.5 years for both the Niño3.4 and WWV indices. In contrast, during 2000–16, both indices had a relatively flatter spectrum and were closer to a white noise process with a relative maximum at 1.5–2.0 years for the Niño3.4 index and 0.8–1.3 years for the WWV index. The frequency change of ENSO and WWV were linked to a westward shift of the Bjerknes feedback region. The results here are consistent with previous argument that the westward shift of the air-sea coupling region will cause an increase of ENSO frequency, as the corresponding zonal advection feedback reduces the period and growth of coupled instability, thus favoring more frequent and weak El Niño events.