Dynamical Downscaling of Climate Simulations in the Tropics

Abstract

The long‐existing double‐Intertropical Convergence Zone (ITCZ) problem in global climate models (GCMs) hampers accurate climate simulations in the tropics. Using a regional climate model (RCM) over the tropical and sub‐tropical Atlantic with a horizontal resolution of 12 km and explicit convection, we develop a bias‐corrected downscaling methodology to produce limited‐area simulations with a realistic ITCZ, despite the double ITCZ in the driving GCM. The methodology effectively removes GCM biases in the RCM boundary conditions, such as to produce more realistic large‐scale driving conditions. We show that the double‐ITCZ problem persists with conventional dynamical downscaling, but with bias‐corrected downscaling the RCM simulations yield credible ITCZ with a realistic seasonal cycle. Detailed analysis attributes the main cause of the double‐ITCZ problem of the selected GCM to the sea surface temperature bias. Compared to the GCM's AMIP simulations, RCMs with higher resolution allow explicit deep convection and enable a better simulation of tropical convection and clouds.