Effects of enhanced shortwave absorption on coupled simulations of the tropical climate system

The effects of enhanced shortwave absorption on coupled simulations of the tropical climate have been tested using the National Center for Atmospheric Research Climate System Model. The enhancement in cloudy-sky shortwave absorption is consistent with several recent intercomparisons of models and observations. The primary reasons for introducing enhanced absorption are to improve the fidelity of the modeled shortwave fluxes in comparison with observations, to improve the simulation of sea surface temperature in the tropical Pacific, and to reduce or eliminate transient behavior in the model associated with compensating errors between latent heat fluxes and surface insolation. The simulations incorporate a new method to increase shortwave absorption in cloudy atmospheres. In previous studies, adjustments to cloud optical properties or modifications to the shortwave radiative transfer were used to increase shortwave cloud absorption. In the new "generic" implementation, the in-cloud shortwave flux divergences have been adjusted to yield the observed global surface insolation while leaving the top-of-atmosphere fluxes fixed. The simulations with enhanced absorption show that several aspects of the atmospheric state and ocean-atmosphere fluxes are much closer to observational estimates. All the terms in the surface and top-of-atmosphere heat budget of the western Pacific warm pool are within 4 W m^-2 of the measured values. The latent heal fluxes are within 10-15 W m^-2 of estimates from buoy data over most of the tropical Pacific. Without absorption, the model consistently overestimates the latent heat fluxes. The differences between the observed and simulated sea surface temperatures in the equatorial Pacific are reduced from 2 K to less than 1 K with the introduction of enhanced absorption.