The interplay of thermodynamics and ocean dynamics during ENSO growth phase

Abstract

The growth of El Niño/Southern Oscillation (ENSO) events is determined by the balance between ocean dynamics and thermodynamics. Here we quantify the contribution of the thermodynamic feedbacks to the sea surface temperature (SST) change during ENSO growth phase by integrating the atmospheric heat fluxes over the temporarily and spatially varying mixed layer to derive an offline “slab ocean” SST. The SST change due to ocean dynamics is estimated as the residual with respect to the total SST change. In observations, 1 K SST change in the Niño3.4 region is composed of an ocean dynamical SST forcing of + 2.6 K and a thermodynamic damping of − 1.6 K, the latter mainly by the shortwave-SST (− 0.9 K) and latent heat flux-SST feedback (− 0.7 K). Most climate models from the Coupled Model Intercomparison Project phase 5 (CMIP5) underestimate the SST change due to both ocean dynamics and net surface heat fluxes, revealing an error compensation between a too weak forcing by ocean dynamics and a too weak damping by atmospheric heat fluxes. In half of the CMIP5 models investigated in this study, the shortwave-SST feedback erroneously acts as an amplifying feedback over the eastern equatorial Pacific, resulting in a hybrid of ocean-driven and shortwave-driven ENSO dynamics. Further, the phase locking and asymmetry of ENSO is investigated in the CMIP5 model ensemble. The climate models with stronger atmospheric feedbacks tend to simulate a more realistic seasonality and asymmetry of the heat flux feedbacks, and they exhibit more realistic phase locking and asymmetry of ENSO. Moreover, the almost linear latent heat flux feedback contributes to ENSO asymmetry in the far eastern equatorial Pacific through an asymmetry in the mixed layer depth. This study suggests that the dynamic and thermodynamic ENSO feedbacks and their seasonality and asymmetries are important metrics to consider for improving ENSO representation in climate models.

ENSO Atmospheric Feedbacks under Global Warming and their Relation to Mean-state Changes

“…In (Bayr et al 2019(Bayr et al , 2021. In reanalysis data, 1 K of SST change during an ENSO event in the Niño3.4 region goes along with a forcing of + 2.3 K/K by the ocean dynamics and a Q net damping of -1.3 K/K, mainly by the SW feedback amounting to -0.8 K/K and the LH feedback amounting to -0.5 K/K (Fig.…”

“…The contribution of the thermodynamics and ocean dynamics to the SST change during ENSO growth in the Niño3.4 region is estimated by a method proposed by Bayr et al (2019Bayr et al ( , 2021: The net heat ux Q net is integrated over the 6 months preceding the maximum of each ENSO event:…”

“…where c p = 4000 J kg -1 K -1 is the speci c heat capacity at constant pressure of seawater, ρ = 1024 kg m -3 the average density of seawater, H = 50 m the mixed layer depth (MLD) in meters, and t the time in months. The MLD is set to a constant value of 50 m, as in the Niño3.4 region the differences in MLD between the models has a negligible in uence on the results of this method (Bayr et al 2021). We normalize the SST change (dSST) and the SST change caused by the heat uxes (dSST HF ) by dSST at the peak time (t = 0), yielding an SST change per Kelvin warming and enabling comparison of the individual simulations irrespective of their amplitude.…”

“…This further results in biased ENSO dynamics; roughly half of the CMIP models simulated a hybrid of winddriven and shortwave-driven ENSO dynamics and not purely wind-driven ENSO dynamics as observed (Dommenget 2010;Dommenget et al 2014;Bayr et al 2019Bayr et al , 2021. Finally, the biased ENSO dynamics can also explain why many climate models have problems in simulating important ENSO properties like the asymmetry and phase locking of ENSO Bayr et al 2019Bayr et al , 2021.…”

ENSO Influence on Regional Atmospheric Circulation and Thermodynamics Over Northern South America

“…They showed that water and energy flux anomalies during ENSO dominate the SPCZ shift. Other study by Bayr et al (2021) quantified the thermodynamic feedbacks between the ocean and the atmosphere through sea surface ENSO influence over Northern South America temperatures (SSTs) and heat fluxes simulated by climate models. They suggest that the better representation of thermodynamic feedbacks between the ocean and the atmosphere, the more realistic simulation of ENSO events.…”