Climatic Effects of the Indian Ocean Tripole on the Western United States in Boreal Summer

Abstract

Abstract The Indian Ocean tripole (IOT) is an independent mode of ocean–atmosphere circulation centered on the tropical Indian Ocean. This study explores the physical mechanisms of the IOT affecting the western United States climate variation during the boreal summer. We find that the IOT is significantly correlated with both western United States summer surface temperature and precipitation anomalies. During positive IOT events, the westerly wind anomalies over the northern Indian Ocean are intensified by two cross-equator airflows over the tropical eastern Indian Ocean and the east coast of Africa. The resulting convergence of air over the northern Bay of Bengal–Indochina Peninsula–northern South China Sea (NBB–IP–NSCS) region (15°–25°N, 80°–125°E) exacerbates the surplus precipitation there. Serving as a heat source, these NBB–IP–NSCS precipitation anomalies can excite a circumglobal teleconnection (CGT)-like pattern that propagates eastward from west-central Asia toward North America along the Asia subtropical westerly jet, further influencing local circulation anomalies. Development of strong anticyclonic circulation over the western United States enhances descending motion and divergence there, resulting in negative precipitation anomalies. This circulation anomaly also induces the diabatic heating anomalies through allowing more solar radiation to reach the ground surface, further increasing the surface temperature anomalies. Meanwhile, the increased tropospheric temperature also raises local surface temperatures by modulating the adiabatic air expansion and compression. Ultimately, the CGT-like pattern associated with NBB–IP–NSCS precipitation anomalies sets up an atmospheric bridge by which the IOT can impact summer climate in the western United States.