The relationship between the Somali jet strength and the summertime South Asia high on interdecadal timescale is analyzed using the nearly 60-y NCEP/NCAR reanalysis data. The results are shown as follows. On interdecadal timescale, positive correlation between Somali jet strength and South Asia high is found. If the Somali jet strength is weak (strong), the summertime South Asia high is also weak (strong) and retreats westward (advances eastward). In different interdecadal backgrounds of Somali jet strength, meridional circulations of the eastern (75°-120°E) and western (20°-70°E) South Asia high are just opposite. During the period when the Somali jet is weak, the meridional circulation of the western South Asia high is strong, whereas the meridional circulation of the eastern South Asia high is dramatically weak, and vice versa. Meanwhile, zonal circulations to the south (20°-27.5°N) and north (27.5°-35°N) of the South Asia high also change differently. The zonal circulation of the northern South Asia high over the Tibetan Plateau is weaker (stronger) than normal, while the zonal circulation of the southern South Asia high over Iranian Plateau is significantly weaker (stronger) than normal during the stronger (weaker) Somali jet period.
Further studies verify that the interdecadal relationship between the Somali jet strength and South Asia high is related to the Pacific Decadal Oscillation (PDO). The phase transition of PDO first changes the subtropical westerly jet on interdecadal time scale, which causes the interdecadal variation in the strength of the South Asia high and the tropical easterly jet over the tropical northern Indian Ocean. The tropical easterly jet then sends signals of anomalies to lower level of the atmosphere by the zonal circulation over the tropical Indian Ocean. Finally, the strength of the Somali jet changes.