The impact of stochastic physics on El Niño Southern Oscillation (ENSO) is investigated in the EC-Earth coupled climate model. By comparing an ensemble of three members of control historical simulations with three ensemble members that include stochastic physics in the atmosphere, we find that the implementation of stochastic physics in the atmosphere improves the excessively weak representation of ENSO in EC-Earth. Specifically, the amplitude of both El Niño and, to a lesser extent, La Niña increases. Stochastic physics ameliorates the temporal variability of ENSO at interannual time scales, demonstrated by the emergence of peaks in the power spectrum with periods of 5-7 years and 3-4 years. Based on the analogy with the behaviour of an idealized delayed oscillator model (DO) with stochastic noise, we find that when the coupling parameter is small (large) the ENSO oscillation increases with an amplification (reduction) of the noise amplitude. This would imply that a too weak atmosphere and ocean coupling might be responsible for the underestimated ENSO variability in the EC-Earth control runs. The stochastic physics in the atmosphere increases Westerly Wind Burst (WWB) occurrences (i.e. amplification of noise amplitude) that would trigger more and stronger El Niño events (i.e. increase of ENSO oscillation) in the coupled EC-Earth model. Further analysis of the mean state bias of EC-Earth suggests that a cold SST and dry precipitation bias in the central tropical Pacific together with a warm SST and wet precipitation bias in the western tropical Pacific are responsible for the coupled feedback bias (weak coupling) in the tropical Pacific that results in the weak ENSO simulation in EC-Earth. The same analysis of the ENSO behaviour has been carried out in a future scenario experiment (RCP8.5 forcing), highlighting that in a coupled model with an extreme warm SST, characterized by a strong coupling, the effect of stochastic physics on the ENSO representation is opposite. This enhances our argument that the mean state bias of the tropical Pacific region is the main reason for the ENSO representation deficiency in the coupled climate model.
The impact of stochastic physics on ENSO in the EC-EARTH coupled model
Yang Chunxue;Corti Susanna;Davini Paolo;von Hardenberg Jost
2018
Abstract
The impact of stochastic physics on El Niño Southern Oscillation (ENSO) is investigated in the EC-Earth coupled climate model. By comparing an ensemble of three members of control historical simulations with three ensemble members that include stochastic physics in the atmosphere, we find that the implementation of stochastic physics in the atmosphere improves the excessively weak representation of ENSO in EC-Earth. Specifically, the amplitude of both El Niño and, to a lesser extent, La Niña increases. Stochastic physics ameliorates the temporal variability of ENSO at interannual time scales, demonstrated by the emergence of peaks in the power spectrum with periods of 5-7 years and 3-4 years. Based on the analogy with the behaviour of an idealized delayed oscillator model (DO) with stochastic noise, we find that when the coupling parameter is small (large) the ENSO oscillation increases with an amplification (reduction) of the noise amplitude. This would imply that a too weak atmosphere and ocean coupling might be responsible for the underestimated ENSO variability in the EC-Earth control runs. The stochastic physics in the atmosphere increases Westerly Wind Burst (WWB) occurrences (i.e. amplification of noise amplitude) that would trigger more and stronger El Niño events (i.e. increase of ENSO oscillation) in the coupled EC-Earth model. Further analysis of the mean state bias of EC-Earth suggests that a cold SST and dry precipitation bias in the central tropical Pacific together with a warm SST and wet precipitation bias in the western tropical Pacific are responsible for the coupled feedback bias (weak coupling) in the tropical Pacific that results in the weak ENSO simulation in EC-Earth. The same analysis of the ENSO behaviour has been carried out in a future scenario experiment (RCP8.5 forcing), highlighting that in a coupled model with an extreme warm SST, characterized by a strong coupling, the effect of stochastic physics on the ENSO representation is opposite. This enhances our argument that the mean state bias of the tropical Pacific region is the main reason for the ENSO representation deficiency in the coupled climate model.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
