Two Types of Strong Northeast Asian Summer Monsoon

The characteristics of a strong northeast Asian summer monsoon (NEASM) with and without (A and B type, respectively) a basinwide warming in the Indian Ocean during the preceding winter are examined for the period of 1979-2006. In the case of the A type, strong El Nino-like sea surface temperature (SST) decays very rapidly from the preceding winter (December-February) to the following summer (June-August), which may be due to a feedback process of the warm Indian Ocean. In addition, the A-type strong NEASM is more associated with a weak western North Pacific summer monsoon than the B-type strong NEASM. On the other hand, for the B type an El Nino-like SST during the preceding winter is a persistent influence into the following summer. A striking difference can be found in the atmospheric teleconnection pattern from the tropics to the midlatitudes over the Indo-Pacific region, that is, the Pacific-Japan-like pattern versus a pronounced Rossby wave train pattern. This may result from the difference in location of the maximum center of rainfall anomalies over the tropical northwestern Pacific between the two types of strong NEASM. The authors argue that Indian Ocean basin warming plays a role in modifying the convective system over the subtropical western Pacific, resulting in changes in atmospheric teleconnections between the two types of strong NEASM. The weak NEASM, in which the anomalous rainfall pattern resembles that of the A-type strong NEASM except for the sign, is also discussed.