Contrasting changes in the seasonal ITCZs between recent observations and future projection

Abstract:

The variability and changes of the Intertropical Convergence Zone (ITCZ) have been widely studied based on its annual-mean zonal-mean metrics, such as the hemispheric asymmetry and width. While these metrics provide important knowledge about the ITCZ, they overlook characteristics at finer scales beyond the annual-mean zonal-mean. By its nature, the ITCZ migrates north and south seasonally: its annual-mean state is not directly realized but an outcome of the seasonal migration. Analyzing the seasonal ITCZs will provide deeper insights into the ITCZ dynamics and impacts.

This study investigates the changes of the seasonal ITCZs both in recent decades and under future warming. We find that the ITCZ changes in recent decades are distinct from those projected under global warming. Specifically, the seasonal ITCZs have shifted poleward and narrowed in recent decades but are projected to shift equatorward and widen under future warming. We highlight that, while the annual-mean deep-tropical ascent is projected to get narrower (known as ‘deep-tropical squeeze’) as a result of the equatorward-shifted seasonal ITCZs, the seasonal ITCZs themselves actually get wider.

The contrasting ITCZ changes between recent observations and future projection are attributed to the distinct tropical warming patterns. Specifically, the equatorial Pacific has cooled in recent decades following a phase change of the PDO, in the long run however, anthropogenic warming will warm the equatorial Pacific more than the tropical mean. The relative equatorial warming shifts and changes the width of the seasonal band of the relative warm SST and high F_A/GMS (F_A: energy input into the atmospheric column from radiative and surface turbulent fluxes; GMS: gross moist stability), leading to consequent changes in the seasonal ITCZs. In the coming decades, a positive PDO can lead to concerted warming over the equatorial Pacific and amplify the anthropogenic-forced ITCZ changes.