Shifting dynamics of the ITCZ: from widening to narrowing in response to abrupt 4xCO2

The ascending branch of the Hadley circulation forms the band of heavy precipitation, the Intertropical Convergence Zone (ITCZ). Due to its sharp meridional gradient, even a small change in its structure can lead to drastic changes in local precipitation patterns, affecting large populations dependent on agriculture. Consequently, understanding its response to a warming climate has been a major focus of research over the past decades. Previous studies have identified a robust strengthening of the ascending branch of the Hadley cell, often referred to as a “deep tropics squeeze”, characterized by the narrowing of the ITCZ in response to increased CO₂. However, much of the research has concentrated on the quasi-equilibrated response, with little attention given to its response on shorter timescales, which are more relevant for near-term climate projections.

To examine the evolution of the ITCZ response, we construct a 30-member ensemble of abrupt 4xCO₂ simulations using the MPI-ESM. Our experiments reveal distinct phases in the Hadley cell response, starting with a weakening ascent and widening of the ITCZ in the initial years, followed by a reversal of these trends in subsequent decades. This reversal is primarily driven by a shift in the spatial pattern of sea surface temperature (SST) warming, transitioning from cooling to warming over the eastern equatorial Pacific due to dynamical ocean adjustments. Our findings underscore the value of large-ensemble 4xCO₂ experiments, which allowed us to identify the shifting dynamics of the ITCZ, with the fast response distinct from the well-established slow response.