Understanding Tropical Atlantic Precipitation Bias Across CMIP5 Models

Tropical Atlantic and West African monsoon precipitation in 22 CMIP5 models is analyzed in order to compare variability across coupled (CMIP) and fixed sea surface temperatures (AMIP) models. The dominant mode of variability across CMIP models appears as a north/south (N/S) shift of the ITCZ, where the dominant mode across AMIP models appears as an east/west (E/W) shift in the precipitation maximum. This very different behavior is investigated by forcing the ITCZ locally (i) and globally (ii), using the Community Earth System Model (CESM) with a thermodynamic mixed layer “slab” ocean.

Locally (i), the Atlantic ITCZ is perturbed using surface fluxes that correspond to the E/W pattern of precipitation bias across the AMIP models. In a coupled setting these fluxes (forcing) would produce SST anomalies and a coupled feedback.  We apply these flux anomalies, as a forcing, to the slab ocean model and reproduce the N/S shift in the Atlantic ITCZ that represents the dominant mode of precipitation bias in the CMIP models.

In the global experiments (ii), the cross equatorial energy transport is altered by changing the ocean heat flux divergence in the extratropics, forcing the ITCZ to respond across the Atlantic. These runs are also performed with the SSTs fixed in the tropics, to see how the Atlantic ITCZ would respond to a global inter-hemispheric heat transport change when tropical SSTs are held constant. In all of these experiments the response of the tropical Atlantic and West African climate (focusing on precipitation and the Saharan Heat low) is investigated to better understand how local and global forcings associated with biases in energy transport and ITCZ location influence the West African monsoon.