Meridional movement of wind anomalies during ENSO events and their role in event termination

Abstract

Observational analysis has shown that when El Niño‐Southern Oscillation (ENSO) events typically reach their peak amplitude in boreal winter, the associated zonal wind anomalies abruptly shift southward so that the maximum anomalous zonal wind is located around 5°–7°S. Here, an analysis utilizing multiple wind products identifies a clear ENSO phase nonlinearity in the extent of this meridional wind movement and its dynamically linked changes in equatorial heat content. It is shown that the meridional wind movement and its discharging effect increase with increasing El Niño amplitude, while both remain relatively small regardless of La Niña amplitude. This result implies that asymmetries in the extent of the meridional wind shift may contribute to the observed asymmetry in the duration of El Niño and La Niña events. We also evaluate the result sensitivities to wind product selection and discuss Eastern Pacific (EP) and Central Pacific (CP) El Niño event differences.

The role of the southward wind shift in both, the seasonal synchronization and duration of ENSO events

“…Therefore, in agreement with the previous studies of McGregor et al (2012aMcGregor et al ( , 2013) the combination of these two leading EOFs can be viewed to represent this southward shift of zonal wind stress anomalies during both El Niño and La Niña. It is worth emphasizing that McGregor et al (2013) utilised eight global wind products, ERAinterim among others, finding a very similar spatial patterns and temporal variability for the two leading EOF modes amongst all data sets (see their Fig. S1 and Table S1).…”

“…1 top). In contrast, during DJF of moderate El Niño events there is a much smaller southward wind shift, consistent with the findings of McGregor et al (2013) who utilised multiple reanalysis products, and virtually no zonal movement of the anomalous westerlies ( Fig. 1 middle).…”

“…This shift has been linked to the southward displacement of the warmest SSTs and convection during DJF (Lengaigne et al 2006;Vecchi 2006), and the associated minimal surface momentum damping of wind anomalies (McGregor et al 2012a), both of which are due to the seasonal evolution of solar insolation. McGregor et al (2013) also show that the discharging effect of the southward wind shift increases with increasing El Niño amplitude, while remaining relatively small regardless of La Niña amplitude. They suggest that this aspect may also help explain the ENSO phase duration asymmetry (i.e., why El Niño events have a shorter duration than La Niña events).…”

“…It has been shown above that the linear combination of both EOFs can reproduce quite well the southward shift of the maximum westerly wind anomalies and its related seasonal weakening of equatorial westerly wind anomalies, both of which have been proposed to contribute to the transition between El Niño and La Niña (e.g., Harrison and Vecchi 1999;Vecchi andHarrison 2003, 2006;Lengaigne et al 2006;McGregor et al 2012aMcGregor et al , 2013.…”

El Niño−Southern Oscillation frequency cascade

“…Internal unforced variability and different SSTA forcing pattern play an additional role for the observed anomalous circulation in the NWP region. For instance, the zonal location of the SSTA forcing plays a crucial role in determining the combination mode response (30,31). The fact that the estimated regression coefficients are largely independent of the forcing frequency indicates a robust atmospheric response to SSTA forcing.…”