29 July 2019

Cloud Radiative Feedbacks and El Niño–Southern Oscillation

Science

The role of cloud radiative feedbacks in the most important mode of interannual climate variability -- El Niño–Southern Oscillation (ENSO)  --  are directly investigated using the Community Earth System Model, version 1.2 (CESM1.2). The cloud radiative feedback is artificially disabled using a method called “cloud locking” in which an independent cloud dataset is used for radiative transfer, instead of the model’s predicted cloud fields. Removing the feedback between cloud radiative effects and the atmospheric state results in a shift of ENSO periodicity from around 4 years to decadal time scales. To understand this shift in the frequency in the absence of cloud feedbacks, several hypotheses are posited: 1) modulation of heat flux into the equatorial Pacific subsurface through negative shortwave cloud feedback on sea surface temperature anomalies (SSTA), 2) damping the persistence of subtropical southeast Pacific SSTA such that the South Pacific meridional mode impacts the duration of ENSO events, or 3) controls on the meridional width of off-equatorial westerly winds, which impacts the periodicity of ENSO by initiating longer Rossby waves.

Impact

The result of cloud-locking in CESM1.2 contrasts that of another study, which found that cloud-locking in a different global climate model led to decreased ENSO magnitude across all time scales due to a lack of positive longwave feedback on the anomalous Walker circulation. CESM1.2 contains this positive longwave feedback on the anomalous Walker circulation, but either its influence on the surface is decoupled from ocean dynamics or the feedback is only active on interannual time scales. The roles of cloud radiative feedbacks in ENSO in other global climate models are additionally considered. A conclusion of this study is that one cannot diagnose the role of cloud radiative feedbacks on ENSO with a multimodel analysis because each model may manifest a different set of feedbacks. Instead, the cloud radiative feedback should be directly altered in each model to assess the impact on ENSO variability.

Summary

Cloud locking is a methodology that directly disables the feedback between cloud radiative effects and the atmospheric state. This study uses cloud locking in a coupled setting to investigate the role of cloud radiative feedbacks on ENSO variability. In the absence of coupling between clouds and the atmospheric state, ENSO frequency shifts to the decadal timescale; events are larger and last longer, meaning that under normal circumstances in CESM1.2 cloud effects act to damp ENSO events. This result is explored using the surface energy budget, a simple model of ENSO, and the CMIP5 multimodel ensemble. It is difficult to pinpoint a single mechanism that explains the change, as several mechanisms appear viable. More importantly, it is pointed out that the result is model dependent, which casts doubt on multimodel analyses of the role of cloud feedbacks on ENSO variability. The cloud locking methodology offers a promising avenue to better understand how different models manifest different interannual variability, and potentially the response of such variability to changing climatic conditions.

Contact
Brian Medeiros
National Center for Atmospheric Research (NCAR)
Publications
Middlemas, E, A Clement, B Medeiros, and B Kirtman.  2019.  "Cloud Radiative Feedbacks and El Niño–Southern Oscillation."  Journal of Climate 32(15): 4661-4680.  https://doi.org/10.1175/jcli-d-18-0842.1.