<p>We investigate key mechanisms driving the Atlantic extratropical-tropical teleconnection and associated Atlantic Intertropical Convergence Zone (ITCZ) shift in boreal summer under a strong external freshwater forcing using a coupled climate model. Our analysis reveals that the wind-evaporation-sea surface temperature (SST) feedback is not the primary mechanism. Instead, the southward advection of the upper extratropical North Atlantic signal by the North Atlantic subtropical gyre along a horseshoe pathway is a key mechanism for forming the horseshoe pattern of cold SST anomalies. Additionally, the weakening of the Atlantic Meridional Overturning Circulation changes the upper tropical North Atlantic western boundary current. This change is amplified by enhanced surface wind stress curl over the tropical North Atlantic, contributing to warmer tropical Atlantic subsurface thermocline temperature and SST in the tropical South Atlantic. The dipole Atlantic SST anomalies lead to the trade wind response and associated southward ITCZ shift over the tropical Atlantic.</p>