Effect of the Plant Physiological Response to CO\(_2\) on Tropical Precipitation and the ITCZ

Abstract

The Intertropical Convergence Zone (ITCZ) is a band of intense precipitation located along the ascending branch of the Hadley circulation. Zonal mean ITCZ position is controlled by the atmospheric energy balance, and extratropical thermal forcings cause the ITCZ to shift towards the warmer hemisphere. Recent studies have found that the plant physiological response to increased atmospheric CO\(_2\) concentrations, known as the physiological effect, induces surface warming which differentially warms the Northern Hemisphere (NH). We hypothesize that the physiologically-driven surface warming in the NH generates an interhemispheric energy imbalance which should shift the ITCZ northwards. Using a suite of carbon cycle experiments from Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we find that the physiological effect moves the ITCZ north by 0.062±0.057° to 0.239±0.158° when CO\(_2\) concentrations are tripled. The small magnitude of change and large uncertainty is attributed to the low interhemispheric energy imbalances induced by the physiological effect. In a second set of idealized experiments with the AM2.5 climate model using dynamic vegetation, prescribed sea surface temperature fields, and excluding ocean dynamics, we find that the physiological effect at doubled CO\(_2\) concentrations has a negligible impact on ITCZ position. However, we do find that the physiological effect in Africa causes a poleward expansion of tropical rainforests which induces a 1-2 K cooling and 0.25-0.50 mm day\(^{-1}\) increase in precipitation. These results suggest that the physiological effect could be crucial to understanding the past and future dynamics of African climate.