Abstract
A statistical study is made of the relationship between the Northern Hemisphere atmospheric circulation and tropical convective activity during summer on intraseasonal time scales. An emphasis is placed on differences in the tropical-midlatitude interaction over East Asia between two 50-day periods: May 26-July 14 and July 15-September 2. Each period corresponds to the East Asian rainy season or the East Asian mid-summer season, respectively. Barotropic instability of climatological monthly mean 300mb stream functions is also investigated to interpret the observational results.
It is found from correlative analysis that the tropical convection is not strongly related to the midlatitude flow over East Asia during the rainy season, and there are no definite indications that the tropical convection forces the midlatitude flow. On the other hand, during the mid-summer season convective activity around the Philippines and the Indo-China Peninsula has a large impact on the East Asian atmospheric circulation. In particular, geopotential height over the Yellow Sea is most strongly affected by the tropical convection. Active convection in the western tropical Pacific is accompanied by a wave train with zonal wavenumber 6 over the North Pacific, which originates from the western tropical Pacific. The location of the wave train does not depend strongly on the location of the convective activity.
One of the major barotropic unstable modes for the climatological 300mb flow for August resembles the wave train observed over the North Pacific during the mid-summer season. It behaves like a standing wave, and has a large amplitude around the Yellow Sea. The e-folding time of this mode is 11 days when dissipation is ignored. Barotropic unstable modes for June do not have large amplitudes over East Asia. These results are consistent with the above observational results and suggest that barotropic instability plays an important role in the tropical-midlatitude interaction over East Asia during the mid-summer season.
