Atmospheric Feedbacks in the ENSO cycle and the Role of the Hydrological Cycle

We investigate processes that determine the behaviour of ENSO in CMIP5 models, with a particular focus on the role of latent heat fluxes in damping Sea Surface Temperature (SST) anomalies. We find that the latent heat flux damping has a wide range of values in the CMIP5 models and is generally underestimated when compared to the observed damping rate. A significant fraction of the spread of latent heat flux damping is driven by differences in the sensitivity of boundary-layer atmospheric humidity to SST anomalies. Variations in latent heat damping are related to spatial variations in the climatological mean rainfall in the CMIP5 models.

The results suggest a central role for the coupling between the hydrological cycle and ENSO in models and show how biases in the mean climate can lead to differences in variability. Future work is proposed to (i) extend the analysis to produce a more complete conceptual model for ENSO-hydrological cycle coupling, (ii) add further important feedbacks such as radiative damping from clouds, (iii) examine ocean feedbacks using a decomposition of the ocean heat budget and (iv) to develop a series of metrics that may be used in model development to improve ENSO simulation in models and to constrain future projections of ENSO characteristics.