Summer circulation of the Northern Hemisphere atmosphere in periods of strong and weak East Asian monsoon

Abstract

The composite analysis of monthly average pressure and wind fields in years of strong and weak East Asian summer monsoon (EASM) was performed using ECMWF ERA-40 and ERA-Interim data. The study showed that strong EASM periods are characterized by increased meridional flows over East Asia, especially in the area of interaction of monsoon and midlatitude circulation, where an increased cyclonicity zone appears. The atmospheric pressure is markedly larger in weak than strong EASM periods over the most part of Asia. In the weak EASM periods, the polar vortex is weak; however, troughs over the Kara Sea and the Sea of Okhotsk, as well as over Hudson Bay, are well developed, and the pressure is lowered in the Aleutian Low area. The strongest and significant long-term changes in the pressure field, indicating weakening of the meridional exchange in recent decades, are characteristic for areas of convergence of the meridional flows of the East Asian and African monsoons.