National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2014

Subseasonal atmospheric variability and El Niño waveguide warming: Observed effects of the Madden-Julian Oscillation and Westerly Wind Events

Chiodi, A.M., D.E. Harrison, and G.A. Vecchi

J. Climate, 27(10), 3619–3642, doi: 10.1175/JCLI-D-13-00547.1 (2014)

Westerly Wind Events (WWEs) have previously been shown to initiate equatorial Pacific waveguide warming. The relationship between WWE and Madden-Julian Oscillation (MJO) activity, and 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. 43 of these MJO events have one or more embedded WWEs and 21 have not. We examine 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. We find waveguide warming for the MJO+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 SSTA changes are well reproduced in an ocean general circulation model forced with the respective composite wind stress anomalies. Further, we find that the occurrence of an MJO event does not significantly affect the likelihood that a WWE will occur. These results extend and confirm the results of Vecchi (2000) with a near doubling of the period of study. We suggest that understanding the sources and predictability of tropical Pacific Westerly Wind Events remains essential to improving predictions of the onset of El Niño events.

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