Impact of Tropical SSTs in the North Atlantic and Southeastern Pacific on the Eastern Pacific ITCZ

During boreal spring, observation shows double-ITCZ over the eastern Pacific, with the northern ITCZ stronger than the southern ITCZ. However, it is opposite in most climate models, which exhibit deficient (overestimated) rainfall over the northern (southern) ITCZ. These models also display cold bias in tropical North Atlantic (TNA) sea surface temperature (SST) and warm bias in southeastern Pacific (SEP) SST. In this study, the influences of TNA and SEP SSTs on the double-ITCZ bias during boreal spring are investigated by prescribing the observed SST in the NCAR CESM1. Although the SST change over the eastern Pacific is weak, the northern ITCZ is substantially enhanced and southern ITCZ is moderately reduced with the TNA SST prescribed. The southern ITCZ is largely reduced with SEP SST prescribed, while the northern ITCZ shows little change. The moisture budget analysis suggests that dynamical term, mainly the low-level convergence change, determines the above precipitation change. Further, a mixed layer model analysis suggests that the low-level convergence change is determined by the surface pressure from the planetary boundary layer. Over regions where large SST change exists, such as TNA and SEP, SST contributes much to the surface pressure change, which is further modified by boundary layer process. Over the TNA (SEP) region, boundary layer process shifts the SST-induced change westward (southward). Over the northern ITCZ, where the SST response is weak, boundary layer process plays a more important role. The low-level convergence changes the vertical distribution of moist static energy, which alters the atmospheric stability and eventually produces the precipitation change in the model.