The interplay of thermodynamics and ocean dynamics during ENSO growth phase
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The interplay of thermodynamics and ocean dynamics during ENSO growth phase Tobias Bayr1 · Annika Drews1,2 · Mojib Latif1,3 · Joke Lübbecke1,3 Received: 30 January 2020 / Accepted: 18 November 2020 © The Author(s) 2020
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.
1 Introduction The El Niño/Southern Oscillation (ENSO) is the dominant mode of tropical climate variability on interannual timescales and driven by a complex interplay between amplifying (positive) and damping (negative) coupled ocean-atmosphere feedbacks (e.g. Jin et al. 2006). ENSO is associated with extreme weather such as heavy precipitation events and droughts in the tropical Pacific region and beyond (e.g. Philander 1990; Yeh et al. 2018). During its warm (cold) * Tobias Bayr [email protected] 1
GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
2
Department of Environment and New Resources, SINTEF Ocean AS, Trondheim, Norway
3
Faculty of Mathematics and Natural Sciences, Christian-Albrechts-University of Kiel, 24105 Kiel, Germany
phase, El Niño (La Niña), the sea surface temperature (SST) is warmer (colder) than n
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