Response of the Tropical Atmosphere to a Localized Warm SST Area:
Time-Development Observed in an Aqua-Planet Ensemble Experiment

TOYODA Eizi (Grad. School of Mathematical Sciences, Univ. Tokyo)
Kensuke NAKAJIMA (Grad. School of Science, Kyushu Univ.)
Masaki ISHIWATARI (Grad. School of Environmental Earth Science, Hokkaido Univ.)
Yoshi-Yuki HAYASHI (Grad. School of Science, Hokkaido Univ.)

Efficiency of ensemble experiment is examined for the purpose of describing the time development of tropical atmospheric response to a sea surface temperature (SST) anomaly. An ensemble experiment with the test cases of 200 is carried out by the use of a simplified atmospheric general circulation model (AGCM). The surface boundary condition is idealized to have a local warm SST area placed on the equator of a sphere covered with an SST distribution symmetric around the equator and zonally uniform.

It is shown that the ensemble mean operation effectively reduce the small-scale random noise caused by precipitation activity and the noise known as intraseasonal oscillation. It is demonstrated that the time-development of planetary scale responce to the warm SST area can be clearly extracted by the enesemble mean operation. The required ensemble size for extracting the time-development of pressure field is found to be around 64.