The impact of global warming on sea surface temperature based El Niño–Southern Oscillation monitoring indices

Abstract

Sea surface temperature (SST) anomalies in the tropical Pacific are commonly used indicators for diagnosing the El Niño–Southern Oscillation (ENSO) state. Global warming has the potential to affect these indicators so that the indicators provide a less representative picture of El Niño/La Niña developments. The SST trend has not been uniform across the Tropics; hence, accounting for local trends may not account for widespread warming. A method is proposed to remove tropical SST trend from the Niño3.4 index, one of the most common indices for monitoring ENSO. The trend and climatology analysis periods are selected based on the Interdecadal Pacific Oscillation. The climatology period contains an equal number of years with positive and negative phases (1976–2014), while the trend is estimated over a longer period with no significant trend in the Interdecadal Pacific Oscillation (1962–2011). Furthermore, the trend is estimated using three SST datasets and sampling of the time period to account for uncertainty in measurements. Once the tropical trend is removed, new Niño3.4 values are calculated and a new ENSO classification proposed to re-classify ENSO events since 1976. The recent 3 years with the largest deviation due to global warming (2014–2016), once corrected contain the full ENSO cycle with neutral, strong El Niño, and La Niña years. These events based on the new classification align well with other ENSO predictors, such as outgoing longwave radiation and zonal wind at 850 hPa, particularly for marginal cases such as the 2016 La Niña event. These results have implications for how ENSO is monitored and predicted in relation to climate change.