Abstract
Near the end of the calendar year, when El Nino events typically reach their peak amplitude, there is a southward shift of the zonal wind anomalies, which were centred around the equator prior to the event peak. Previous studies have shown that ENSO’s anomalous wind stresses, including this southward shift, can be reconstructed with the two leading EOFs of wind stresses over the tropical Pacific. Here a hybrid coupled model is developed, featuring a statistical atmosphere that utilises these first two EOFs along with a linear shallow water model ocean, and a stochastic westerly wind burst model. This hybrid coupled model is then used to assess the role of this meridional wind movement on both the seasonal synchronization as well as the duration of the events. It is found that the addition of the southward wind shift in the model leads to a Christmas peak in variance, similar to the observed timing, although with weaker amplitude. We also find that the added meridional wind movement enhances the termination of El Nino events, making the events shorter, while this movement does not appear to play an important role on the duration of La Nina events. Thus, our results strongly suggest that the meridional movement of ENSO zonal wind anomalies is at least partly responsible for seasonal synchronization of ENSO events and the duration asymmetry between the warm (El Nino) and cool (La Nina) phases.
Highlights
The El Niño-Southern Oscillation (ENSO) phenomenon is the main driver of Earth’s interannual climate variability (Neelin et al 1998; McPhaden et al 2006), leading to significant changes in the global atmospheric circulation (Ropelewski and Halpert 1989; Philander 1990; Trenberth et al 1998; Wang et al 2003)
We examine whether the ENSO phase asymmetry observed in this shift can account for the fact that La Niña events tend to persist for longer periods than El Niño (Okumura et al 2011)
In order to further examine the variability of the background stability in each calendar month, we have run a series of 12 perpetual month experiments with HCM1+2, in which the relationship between PC1 and PC2 was fixed to a given calendar month (i.e., PC2 is a function of PC1 only, while the coefficients which would vary with month are fixed to the prescribed month regardless the current calendar month of the simulations)
Summary
The El Niño-Southern Oscillation (ENSO) phenomenon is the main driver of Earth’s interannual climate variability (Neelin et al 1998; McPhaden et al 2006), leading to significant changes in the global atmospheric circulation (Ropelewski and Halpert 1989; Philander 1990; Trenberth et al 1998; Wang et al 2003). This shift in wind anomalies has been studied by Harrison (1987), Harrison and Larkin (1998), Harrison and Vecchi (1999), Vecchi and Harrison (2003); and more recently it has been proposed to explain the seasonal synchronization since the resulting reduction of equatorial westerly wind anomalies has been shown to drive: (1) strong thermocline shoaling in the eastern equatorial Pacific (e.g., Harrison and Vecchi 1999; Vecchi and Harrison 2003, 2006; Lengaigne et al 2006; Lengaigne and Vecchi 2009); (2) changes in equatorial warm water volume (WWV) (McGregor et al 2012a, 2013) and (3) interhemispheric exchanges of upper ocean mass (McGregor et al 2014).
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