The East Asian Subtropical Jet, East Asian Polarfront Jet, and Transient Activities in FGOALS

Abstract

We have assessed upper-level jet streams over East Asia as simulated by FGOALS-s2 and FGOALS-g2. The results demonstrate that both the intensity and meridional location of the East Asian subtropical jet (EASJ) in boreal winter were reproduced well by FGOALS-g2. However, the EASJ simulated by FGOALS-s2 for both summer and winter was weaker than that indicated by NCEP2. Moreover, the geographical border between the EASJ and the East Asian polarfront jet (EAPJ) was not evident in FGOALS-s2 simulations. FGOALS-g2 also produced reasonable simulations of meridional temperature gradient and Ertel’s potential vorticity, although its simulation of synoptic-scale transient eddy activity (STEA) and transient heat flux were weaker. To identify the causes of the mean state bias in FGOALS-s2, the dynamical and thermal forcings of STEA on mean flow were examined, focusing particularly on the boreal winter. The results suggest that the failure of FGOALS-s2 to reproduce the tripolar pattern of the divergence of the extended Eliassen–Palm flux (E) over the jet regions likely led to the unsuccessful separation of the EASJ and EAPJ, whereas dynamical forcing contributed less to the weaker EASJ. In contrast, the weaker transient heat flux was found to partly explain the weaker EASJ over the ocean.