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 abs orption is consistent with several recent intercomparisons of models and ob servations. The primary reasons for introducing enhanced absorption are to improve the fidelity of the modeled shortwave fluxes in comparison with obs ervations, to improve the simulation of sea surface temperature in the trop ical Pacific, and to reduce or eliminate transient behavior in the model as sociated with compensating errors between latent heat fluxes and surface in solation. The simulations incorporate a new method to increase shortwave ab sorption in cloudy atmospheres. In previous studies, adjustments to cloud o ptical properties or modifications to the shortwave radiative transfer were used to increase shortwave cloud absorption. In the new "generic'' impleme ntation, the in-cloud shortwave flux divergences have been adjusted to yiel d the observed global surface insolation while leaving the top-of-atmospher e fluxes fixed. The simulations with enhanced absorption show that several aspects of the atmospheric state and ocean-atmosphere fluxes are much close r to observational estimates. All the terms in the surface and top-of-atmos phere heat budget of the western Pacific warm pool are within 4 W m(-2) of the measured values. The latent heat fluxes are within 10-15 W m(-2) of est imates from buoy data over most of the tropical Pacific. Without absorption , the model consistently overestimates the latent heat fluxes. The differen ces between the observed and simulated sea surface temperatures in the equa torial Pacific are reduced from 2 K to less than 1 K with the introduction of enhanced absorption.