Four teleconnection patterns that are possibly associated with the anomalous summer climate in Japan and the surrounding regions, were extracted by applying empirical orthogonal function and regression analyses to stream function anomalies. The two teleconnection patterns prevailing over northern Eurasia, especially in early summer, called the Europe-Japan (EJ) 1 and EJ2, are linked with the variability of the Okhotsk high. The third teleconnection pattern, called the West Asia-Japan (WJ), is a stationary wave-train pattern along the upper-level subtropical jet from West Asia to the central North Pacific, which is possibly excited by the anomalous convective heating of the Indian summer monsoon. The final teleconnection pattern is identified with the Pacific-Japan (PJ), found by Nitta. Teleconnection indices that account for the variability of those patterns are also defined on a monthly basis. The PJ and WJ patterns, which are more influential teleconnection patterns than the others, are closely related to the summer temperature anomalies, especially in northern and western Japan, respectively. EJ1 and EJ2 were amplified in several extreme summers, and they played a vital role in the cool summer of 2003, along with PJ. A combination of two or three teleconnection patterns was also responsible for the occurrence of the recent extreme summers. Monitoring the major teleconnection patterns is very useful for understanding and forecasting the anomalous summer climate in East Asia.

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A significant coupling of the Asian summer monsoon and ENSO was examined using the NCEP/NCAR reanalysis for the period . Results show that a monsoon index, which is defined as meridional gradient of summertime upper-tropospheric thickness (200-500hPa) anomalies across 20N over the Indian subcontinent, is highly correlated with Nino-3 SST anomalies in the preceding spring. This is strongly suggestive of the presence of the indirect impact of anomalous SST forcing associated with ENSO on the Asian summer monsoon.Due to attenuated Walker circulation in response to a warm episode, convection is suppressed over the northern tropical Indian Ocean and the maritime continent from the preceding winter to spring. The suppressed tropical convection in the preceding spring generates anomalous cyclonic circulation to the west of the Tibetan Plateau as a result of the Rossby-type response to convective heating off the equator. The convection-induced anomalous cyclonic circulation accompanied by large-scale ascending atmospheric motion contributes substantially to increased rainfall and greater soil moisture, thus resulting in decreased land-surface temperature over central Asia to the northwest of the Indian subcontinent. On the other hand, warm SST anomalies are initially introduced over the tropical Indian Ocean in late spring prior to the onset of the monsoon due to the changes in the surface heat flux and/or dynamic response of the ocean to wind forcing, in intimately association with pronounced in situ low-level northeasterly wind anomalies and less cloud cover. Both these different physical processes in the land and ocean areas are crucially responsible for reduced land-ocean thermal contrast (or reduced meridional tropospheric temperature gradient), eventually bringing about the weakening of the Asian summer monsoon. The reverse situation is quite true for strong monsoon years. Once the summer monsoon becomes weak (strong) at its early stage due to these processes, the initially induced warm (cool) SST anomalies over the tropical Indian Ocean are further intensified.The mechanism proposed here is valid during the period from the late 1970s to the early 1990s when weak and strong monsoon years are categorized. During that period, the unusual Nino-3 SST anomalies tend to persist from the preceding winter until summer, hence serving as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon. However, regardless of when the monsoon-ENSO coupling is prominent, both the springtime outgoing longwave radiation and low-level wind anomalies dominating over the tropical Indian Ocean, associated with anomalous Walker circulation, are still crucial factors in terms of the potential predictability of the Asian summer monsoon.

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