Subseasonal Atmospheric Variability and El Niño Waveguide Warming: Observed Effects of the Madden–Julian Oscillation and Westerly Wind Events*

Abstract

Westerly wind events (WWEs) have previously been shown to initiate equatorial Pacific waveguide warming. The relationship between WWEs and Madden-Julian oscillation (MJO) activity, as well as the role of MJO events in initiating waveguide warming, is reconsidered here over the 1986-2010 period. WWEs are identified in observations of near-surface zonal winds using an objective scheme. MJO events are defined using a widely used index, and 64 are identified that occur when the El Niño-Southern Oscillation (ENSO) is in its neutral state. Of these MJO events, 43 have one or more embedded WWEs and 21 do not. The evolution of sea surface temperature anomaly over the equatorial Pacific waveguide following the westerly surface wind phase of the MJO over the western equatorial Pacific is examined. Waveguide warming is found for the MJO with WWE events in similar magnitudes as following the WWEs not embedded in an MJO. There is very little statistically significant waveguide warming following MJO events that do not contain an embedded WWE. The observed SST anomaly changes are well reproduced in an ocean general circulation model forced with the respective composite wind stress anomalies. Further, it is found that the occurrence of an MJO event does not significantly affect the likelihood that a WWE will occur. These results extend and confirm the earlier results of Vecchi with a near doubling of the period of study. It is suggested that understanding the sources and predictability of tropical Pacific westerly wind events remains essential to improving predictions of the onset of El Niño events. * National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory Contribution Number 3825.

The very strong coastal El Niño in 1925 in the far-eastern Pacific

“…Chiodi et al 2014) that depress the thermocline and raise the sea level. However, in contrast to other EN events, the sea level data at Balboa, Panama, does not show positive anomalies in early 1925, and in fact they are negative in February (Fig.…”

Asymmetry of Westerly and Easterly Wind Events: Observational Evidence

“…On the other hand, the cyclonic and convective signal prior to the WWE, propagating eastward slowly from −50° to 0° in RL and having a baroclinic structure (Figs. S10a and S11a), is not robust as mentioned in previous studies (Seiki and Takayabu 2007a;Chiodi et al 2014), implying the importance of local development processes for WWEs. The westward propagation of cyclonic vorticity lagging the WWE (Fig.…”

“…Several criteria have been used to detect WWEs in surface wind datasets (Hartten 1996;Harrison and Vecchi 1997;Seiki and Takayabu 2007a;Chiodi et al 2014). Following the previous investigations into WWEs, we extract WWEs and EWEs based on the method in Seiki and Takayabu (2007a) using the HF surface wind anomaly averaged between 2.5°S and 2.5°N.…”

“…Short-lived anomalous westerly wind epochs over the equatorial Pacific, westerly wind events (WWEs) (e.g., Luther et al 1983;Hartten 1996;Harrison and Vecchi 1997;Seiki and Takayabu 2007a, b), act to warm the eastern equatorial Pacific (Vecchi and Harrison 2000;Lengaigne et al 2002;Chiodi et al 2014) and sometimes trigger El Niños (McPhaden 1999; Lengaigne et al 2004). Eisenman et al (2005) showed that WWE occurrence is dependent on sea surface temperature (SST) variability in the western Pacific warm pool associated with the El Niño/Southern Oscillation (ENSO) phenomenon (see also, Lengaigne et al 2003;Vecchi et al 2006).…”

Importance of background seasonality over the eastern equatorial Pacific in a coupled atmosphere-ocean response to westerly wind events

“…On the whole, warming was observed in the central and eastern Pacific relative to a composite SSTA without any WWEs. In the WWE region the west of the dateline, the sea surface was cooled, whereas it was warmed in the east, implying a dependency of the oceanic response on the longitudinal location of WWEs (see also, Chiodi et al 2014). The oceanic response to WWEs may also depend on the seasons, according to several ocean model studies.…”