Projections of ENSO in sea surface temperature and atmospheric angular momentum/length of day from CMIP6

Abstract

The El Niño-Southern Oscillation (ENSO) is a natural climate pattern associated with a periodic fluctuation of 2 to 7 years in both sea surface temperature and air pressure across the tropical Pacific. Historically, the ENSO signal has been reflected in that of length of day (LOD) because, in addition to effects on regional weather, the warm and cold phases of ENSO (El Niño/La Niña) also entail large changes in atmospheric circulation. During the intense El Niño phase, the normal tropical trade winds from the east over the Pacific and elsewhere slacken or even turn westerly, and elsewhere some westerly winds strengthen, so that such episodes produce significant positive anomalies in zonal atmospheric angular momentum. Due to the conservation of angular momentum in the Earth system, such peaks are contemporary with the solid Earth’s deceleration and hence increase LOD. The opposite is the case in the La Niña phase. Future projections of Earth system variables provided from the global models of CMIP6 (Coupled Model Intercomparison Project Phase 6) allow for the investigation of ENSO reproducibility using two approaches. In this study, we use historical and 21st century simulations of the sea surface temperature from CMIP6 models to derive the Oceanic Niño Index (ONI), a measure used to classify the ENSO phases. These ONI values are then compared to the time series of atmospheric angular momentum computed from the same set of model simulations. The results evaluate the ability of the coupled models to realistically represent the temporal distribution of ENSO, and from these results, we can also better assess how such climate-related Earth rotation excitation signals can be derived from coupled climate simulations.