Probing the Baiu Dynamics
A. Kuwano-Yoshida and B. Taguchi (JAMSTEC), and S.P. Xie (IPRC)
Observed
CFES mini
AFES
Monthly mean rainfall in mm/day (shading) and SST (contours) in observations, CFES and AFES runs.
The Baiu is a summer rain band of great importance for one billion people in East Asia but poorly
simulated in models. Baiu dynamics are investigated with the Atmospheric and Coupled GCMs on the Earth
Simulator (AFES and CFES). Despite sharing the same atmospheric model, AFES and CFES produce very
different Baiu. During July, Baiu remains vigorous in CFES as in observations, but has dissipated in AFES.
Our analysis shows that the mid-tropospheric westerly jet and its thermal advection cause this difference
in simulated Baiu. In AFES, deep convection in the subtropical Northwest Pacific sets in prematurely,
displacing the westerly Asian jet northward over cold ocean surface. The mid-tropospheric thermal
advection is similar in magnitude in AFES and CFES, but sea surface evaporation is much suppressed under
the Asian jet in AFES as a result of cold ocean temperature and surface moist advection from the south.
These results reveal important processes for Baiu: the mid-tropospheric thermal advection by the Asian jet
induces ascending motion, which is affected in turn by subtropical convection to the south; surface
evaporation east of Japan is an important contributor to Baiu rainfall.