Contrasting Intraseasonal Variations of the Equatorial Pacific Ocean Between the 1997-1998 and 2015-2016 El Nino Events

Oceanic intraseasonal variability (ISV) plays a vital role in the initiation and development of El Nino events. Mooring observations reveal contrasting characteristics of upper-ocean zonal current ISV during different El Nino events. Compared to the 1997-1998 event, ISV of the 2015-2016 event was significantly weaker (by around 30-50%) over the equatorial Pacific basin, and the variability maximum was shifted to the east (similar to 160 degrees E). These differences can be largely explained by wind forcing features. ISV of surface winds in 2015 is overall weaker than in 1997. Intraseasonal westerly wind bursts in 1997 occurred near the western boundary and originated from the Indian Ocean, whereas in 2015, perturbations were mainly propagated from the northeast Pacific and achieved the peak power at similar to 160 degrees E of the equator. These changes are likely associated with the decadal warming of the northeast subtropical Pacific and weakening of the Indian Ocean intraseasonal oscillation. Plain Language Summary Sea surface warmings of strong El Nino events are usually triggered by intraseasonal oscillations prior to the events. Particularly, intraseasonal eastward ocean currents generated by westerly wind bursts carry the warm water of the western Pacific warm pool to the central Pacific and contribute to the surface warming. In this paper, we compare the intraseasonal zonal current variations during two strong El Nino events, namely, the 1997-1998 and 2015-2016 events. Although the two El Nino events were of comparable strengths, intraseasonal variations in 1997 were stronger than those in 2015 by 3050%. These differences can be largely explained by the contrasting features of wind forcing. Intraseasonal oscillations were originated from the Indian Ocean in 1997, whereas those in 2015 were mainly from the northeast subtropical Pacific. The varying origin is likely associated with the warming of the northeast subtropical Pacific and weakened Indian Ocean intraseasonal oscillation during recent years.