Hysteresis of the intertropical convergence zone to CO2 forcing

Abstract

With the unprecedented rate of global warming in recent decades, whether or not anthropogenic climate change is irreversible is an important question. Based on idealized CO2 ramp-up until 1,468 ppm and symmetric ramp-down model experiments, here we show that the intertropical convergence zone (ITCZ) does not respond linearly to CO2 forcing, but exhibits strong hysteresis behaviour. While the location of the ITCZ changes minimally during the ramp-up period, it moves sharply south as soon as CO2 begins to decrease, and its centre eventually resides in the Southern Hemisphere during the ramp-down period. Such ITCZ hysteresis is associated with delays in global energy exchanges between the tropics and extratropics. The delayed energy exchanges are explained by two distinct hysteresis behaviours of the Atlantic Meridional Overturning Circulation and slower warming/cooling in the Southern Ocean. We also suggest that the ITCZ hysteresis can lead to hysteresis in regional hydrological cycles. In idealized model experiments where CO2 increases four-fold before returning to its original level, temperature and precipitation show almost linear responses to CO2 forcing. In contrast, the response of the Intertropical Convergence Zone lags behind CO2 changes, associated with delayed energy exchanges.