Fluid Dynamics in Earth and Planetary Sciences (FDEPS) Second FDEPS Workshop Dec 04 - Dec 08, 2000 Graduate School of Mathematical Sciences, University of Tokyo
On the existence of subtropical anticyclone with equivalent barotropic
structure over Japan in August.
Enomoto, T. (Department of Earth and Planetary Science, Univ. Tokyo)
The Tibetan anticyclone is a most notable feature in the upper-troposphere during the Northern Hemisphere summer, which introduces a zonal asymmetry in the flow. Embedded in this planetary-scale feature, there are synoptic-scale quasi-stationary anomalies . In particular, the Ogasawara high develops over Japan in August. The vertical structure of this anticyclone is different from the Pacific high. The Pacific high is the most intense near the surface and there is a cyclone spanning mid- to upper-troposphere. By contrast, the Ogasawara high is very deep. It extends from the upper- to lower-troposphere and sometimes reaches the surface. This deep anticyclone, which brings hot and relatively dry climate, is absent during the rainy season of June and July. Although the equivalent-barotropic (vertically in-phase) structure of the Ogasawara high has been known for many years, its cause is not yet fully explained. We examine the generation and maintenace of the Ogasawara high with a simple GCM by comparing responses to various heatings and vorticity forcings. Numerical results show that the anticyclone over Japan is a result of an interaction between the subtropical heating and mid-latitude jet. First of all, descent appears to the northwest as a primary response to the subtropical heating. Adiabatic and diabatic processes localise the descent in regions over the Aral Sea and East Mediterranean Sea. Stationary Rossby waves are effectively induced because the descents coincide with the jet. Accordingly, Rossby waves are induced near the jet entrance as a secondary response to the heating. The vertical structure of these waves become equivalent-barotropic as they propagate downstream. This wave propagation along the jet (we call it `the Silk Road Pattern') results in an equivalent-barotropic anticyclone over Japan. It should be noted, however, that the Silk Road Pattern is affected by the zonally-varying flow due to the Tibetan high. Thus we suggest a novel hypothesis on the formation of the equivalent-barotropic anticyclone over Japan. (collaboration with Matsuda, Y. (Graduate School of Science, Univ. Tokyo), Hoskins, B.J. (Reading U., U.K.) )