Distinctive role of ocean advection anomalies in the development of the extreme 2015–16 El Niño

The recent 2015–16 El Niño was of comparable magnitude to the two previous record-breaking events in 1997–98 and 1982–83. To better understand how this event became an extreme event, we examine the underlying processes leading up to the peak of the event in comparison to those occurring in the 1997–98 and 1982–83 events. Differences in zonal wind stress anomalies are found to be an important factor. In particular, the persistent location of the zonal wind stress anomalies north of the equator during the two years prior to the 2015–16 peak contrasts the more symmetric pattern and shorter duration observed during the other two events. By using linear equatorially trapped wave theory, we determine the effect of these off-equatorial westerly winds on the amplitude of the forced oceanic Rossby and Kelvin wave response. We find a stronger upwelling projection onto the asymmetric Rossby wave during the 2-year period prior to the peak of the most recent event compared to the two previous events, which might explain the long-lasting onset. Here we also examine the ocean advective heat fluxes in the surface mixed layer throughout the event development phase. We demonstrate that, although zonal advection becomes the main contributor to the heat budget across the three events, meridional and vertical advective fluxes are significantly larger in the most recent event compared to those in 1997–98 and 1982–83. We further highlight the key role of advective processes during 2014 in enhancing the sea surface temperature anomalies, which led to the big El Niño in the following year.