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Other than WWV and WWBs, climate variability and trends outside the tropical Pacific are also important factors.
In 2014, the equatorial Pacific warm water volume (WWV) increased rapidly during the austral autumn following a strong westerly wind burst (WWB) event, reaching a level not seen since 1997 (McPhaden 2015).
A strong El Nino never materialized, which was later shown to be due to a combination of impeding factors, such as muted WWB activity (Menkes et al.
These characteristics include i) intense WWB activity in the western/central Pacific during event onset and development phases; ii) a dramatic eastward and equatorward shift of atmospheric convection as El Nino emerges and matures, thereby inducing unusually high rainfall in the climatologically dry and cold eastern equatorial Pacific; and iii) prominent eastward propagation of anomalous SSTs along the equatorial Pacific Ocean over event onset to decay phase.
These issues mean that multiple observational products and indices beyond the commonly used ENSO metrics in operational forecasting (e.g., Nino-3.4, WWV, WWB, Southern Oscillation index) are required to capture the diversity of ENSO extremes.
Wyrtki himself provided a conceptual framework to understand why some WWBs can impact the evolution of El Nino and not others, albeit 10 years after the 1975 expedition.
A series of WWBs in early 2014 excited large-amplitude downwelling Kelvin waves comparable in strength to those observed in early 1997 at the beginning of the strongest El Nino on record.
One immediate consequence of a WWB is the excitation of a downwelling equatorial Kelvin wave that propagates eastward along the equator.
Only the third of these Kelvin waves, the one in response to the January 1975 WWB, reached the eastern boundary (Fig.
While WWBs are commonly associated with MJO events, this particular separation of westerlies into two distinct bursts, as well as their vertical tilt during MJO2, bears the signature of convectively coupled Kelvin waves (Kiladis et al.
In the upper troposphere, two distinct positive potential temperature anomalies of about 3[degrees]C lead each of the WWBs by about one day.
4c) decreased during the WWBs. The SST and corresponding value of the sea surface specific humidity began to decrease as the daily-averaged net heat flux changed sign around 21-22 November.
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