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.

Previous work has established that warming is associated with an increase in dry static stability, a weakening of the tropical circulation, and a decrease in the convective mass flux. Using a set of idealized simulations with specified surface warming and super-parameterized convection, we find support for these previous conclusions. We use an energy and mass balance framework to develop a simple diagnostic that links the fractional area covered by the region of upward motion to the strength of the mean circulation. We demonstrate that the diagnostic works well for our idealized simulations, and use it to understand how changes in tropical ascent area and the strength of the mean circulation relate to changes in heating in the ascending and descending regions. We show that the decrease in the strength of the mean circulation can be explained by the relatively slow rate at which atmospheric radiative cooling intensifies with warming. In our simulations, decreases in tropical ascent area are balanced by increases in non-radiative heating in convective regions. Consistent with previous work, we find a warming-induced decrease in the mean convective mass flux. However, when we condition by the sign of the mean vertical motion, the warming-induced changes in the convective mass flux are non-monontonic and opposite between the ascending and descending regions.

Surface latent heat fluxes help maintain tropical intraseasonal precipitation. We develop a latent heat flux diagnostic that depicts how latent heat fluxes vary with the near-surface specific humidity vertical gradient (dq) and surface wind speed (|V|). Compared to fluxes estimated from |V| and dq measured at tropical moorings and the COARE3.0 algorithm, tropical latent heat fluxes in the NCAR CEMS2 and DOE E3SMv1 models are significantly overestimated at |V| and dq extrema. Madden–Julian oscillation (MJO) sensitivity to surface flux algorithm is tested with offline and inline flux corrections. The offline correction adjusts model output fluxes toward mooring-estimated fluxes; the inline correction replaces the original bulk flux algorithm with the COARE3.0 algorithm in atmosphere-only simulations of each model. Both corrections reduce the latent heat flux feedback to intraseasonal precipitation, in better agreement with observations, suggesting that model-simulated fluxes are overly supportive for maintaining MJO convection.