Cloud and Radiative Balance Changes in Response to ENSO in Observations and Models

The authors use observations and four GFDL AGCMs to analyze the relation between variations in spatial patterns and area-averaged quantities in the top-of-the-atmosphere radiative fluxes, cloud amount, and precipitation related to El Nino over the period 1979-2008. El Nino is associated with an increase in tropical average sea surface temperature of order +0.1 K (with a maxima of +0.5 K), large local anomalies of +2 K (maxima +6 K), and tropical tropospheric warming of +0.5 K (maxima +1 K). The authors find that model-to-observation biases in the base state translate into corresponding biases in anomalies in response to El Nino. The pattern and amplitude of model biases in reflected shortwave (SW) and outgoing longwave radiation (OLR) follows expectations based on their biases in cloud amount: models with a positive cloud amount bias, compared to observations, have too strong local responses to El Nino in cloud amount, SW, OLR, and precipitation.Tropical average OLR increases in response to El Nino in observations and models [correlation coefficients (r) with Nino-3.4 index in the range 0.4-0.6]. Weaker correlations are found for SW (r: -0.6 to 0), cloud amount (r: -0.2 to +0.1), and precipitation (r: -0.2 to 0). Compositing El Nino events over the period 2001-07 yields similar results. These results are consistent with El Nino periods being warmer due to a heat pulse from the ocean, and a weak response in clouds and their radiative effect. These weak responses occur despite a large rearrangement in the spatial structure of the tropical circulation, and despite substantial differences in the mean state of observations and models.