Explaining the Year-to-Year Variability of the Eastern Pacific Intertropical Convergence Zone in the Boreal Spring

The year-to-year variability of the Intertropical Convergence Zone (ITCZ) in the eastern tropical Pacific is a significant phenomenon of the tropical climate system. Asymmetric interannual variation of precipitation on two sides of the Equator is often observed there in the boreal spring. We introduce a simple normalized asymmetric index and a double ITCZ index as well as a hierarchical clustering method to describe the ITCZ variation based on the tropical boreal spring precipitation from 1979 to 2017. We show that the indices and the clustering method give consistent categorization of asymmetric ITCZ variation. We find that the asymmetry of precipitation can be explained by the remote impact of the interannual variability of the central Pacific sea surface temperature (SST) on the horizontal gradient of surface pressure and surface wind convergence, especially the Modoki mode. The central Pacific SST also remotely modulates free-tropospheric temperature and vertical stability in the eastern equatorial Pacific, but this remote impact is secondary to the wind convergence. Interannual variability of local SST south of the Equator plays little role in the precipitation variability, which is supported by numerical experiments with a regional atmospheric model. Results imply that surface wind convergence should be considered in convection schemes to improve the simulation of precipitation in the eastern tropical Pacific in climate models.