The representation of marine surface fluxes is linked to intertropical convergence zone biases

Abstract

Ocean-atmosphere coupled climate models struggle to produce a single northern hemisphere intertropical convergence zone (ITCZ), and instead simulate ITCZ bands in both hemispheres. This “double ITCZ’ bias can negatively impact representations of large-scale modes of variability, such as the Madden-Julian oscillation and El Ni\ no–Southern Oscillation. A new method to estimate model fluxes that would have been obtained with the COARE3.0 bulk flux algorithm indicates that twelve of fourteen CMIP6 models overestimate surface fluxes in the ITCZ region, suggesting that biases rooted in model flux algorithms may contribute to ITCZ biases. This finding is supported by atmosphere-only simulations of two models where the original flux algorithms are replaced with the COARE3.0 algorithm. In the experiments, precipitation root mean square errors in the double ITCZ region were reduced by 26\% and 15\%, respectively. We interpret these findings through the lenses of global energy constraints and convection-boundary layer interactions.