Cloud Radiative Feedbacks and El Niño–Southern Oscillation (ENSO)

Cloud feedbacks play a potentially important role in determining ENSO variability, and these mechanisms can be tested in climate models. Using CESM1 the cloud feedbacks are evaluated using a technique referred to as ‘cloud-locking’. This technique essentially decouples the cloud radiative effects from the atmospheric state of the model as it evolves in time. By applying an external dataset of cloud characteristics for the calculation of radiation tendencies instead of the model’s prediction, the impact of feedbacks can be quantified and compared with other climate models.

A comparison of the cloud-locking experiment with a baseline CESM1 experiment, shows that cloud feedbacks within the tropics act to damp individual ENSO events. The exact mechanism involves a response in sea-surface temperature due to a change in the sensitivity of clouds to incoming solar radiation. This modifies wind-evaporation-SST feedbacks in the southeast Pacific, and alters the strength of tropical winds that impact ENSO periodicity through Rossby waves. When the cloud feedbacks are essentially removed artificially then ENSO period is increased dramatically, demonstrating their critical importance (Figure 1). Inter-comparing with the response in other climate models can give very different results to CESM1. It is clear that the role of cloud feedbacks on ENSO are marginally constrained and poorly understand.

Simulated winter temperature change associated with planting of tall (50cm, top panel) and short (10cm, bottom panel) cover crops
Figure 1: Power spectra from the control (gray and blue) and cloud-locked (red) simulation. Gray lines are the power spectra of 350-year segments of the Nino 3.4 SST anomaly index from the control simulations. The blue line is the average across all segments.

Reference

  • Middlemas, E.A., A.C. Clement, B. Medeiros, and B. Kirtman, 2019: Cloud Radiative Feedbacks and El Niño–Southern Oscillation. J. Climate, 32, 4661–4680, https://doi.org/10.1175/JCLI-D-18-0842.1