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

Abstract

Oceanic intraseasonal variability (ISV) plays a vital role in the initiation and development of El Niño events. Mooring observations reveal contrasting characteristics of upper-ocean zonal current ISV during different El Niño 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 (~160°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~160°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 Niño 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 Niño events, namely, the 1997-1998 and 2015-2016 events. Although the two El Niño 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.

Observed Deep‐Reaching Signatures of the Madden‐Julian Oscillation in the Ocean Circulation of the Western Tropical Pacific

“…Ocean circulation in the western tropical Pacific plays an essential role in the redistribution of heat, freshwater, and biochemical properties over the Indo‐Pacific Oceans (e.g., Fine et al, ; Hu et al, ). Prominent intraseasonal variability (ISV; 20–90 days) in ocean circulation has been detected by in situ observations in this region and reported before (e.g., Azminuddin et al, ; Hu et al, ; Kashino et al, ; Lyu et al, ). One important origin of the upper‐ocean current ISV is the Madden‐Julian oscillation (MJO; Madden & Julian, , ), which is the dominant mode of intraseasonal oscillations of the tropical atmosphere.…”

Variability of the Mindanao Current Induced by El Niño Events

“…Recent studies have indicated that El Niño events show evident diversity and can be broadly classified into eastern Pacific, central Pacific, and mixed type events (e.g., Kao and Yu 2009;Kug et al 2009). Different types of El Niño events have been shown to exert different impacts on the western Pacific circulation (e.g., Hsin and Qiu 2012;Zhao et al 2013;Lyu et al 2018;Tan and Zhou 2018). They have different wind forcing characteristics and possibly affect the MC in different ways.…”

Interannual Variability of Lower Equatorial Intermediate Current Response to ENSO in the Western Pacific

“…From September 2014 to December 2018, a subsurface mooring was deployed at 0°, 142°E to monitor the variability of LEIC. This mooring is a part of the Scientific Observing Network of the Chinese Academy of Sciences in the western Pacific (Lyu et al, 2018; Ma et al, 2019; Song et al, 2018; Wang et al, 2016: Wang, Wang, et al, 2016). The temporal coverage of the mooring data allows the investigation of the interannual variability of LEIC in response to the strong 2015–2016 El Niño event.…”