Effects of Combined Teleconnection Patterns on the East Asian Summer Monsoon Circulation: Remote Forcing from Low- and High-Latitude Regions

摘要

Using the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40) data, we examined two summertime teleconnection patterns, the Europe-Japan (EJ) and the Pacific-Japan (PJ) patterns, which prevail over northern Eurasia and the western North Pacific, respectively, and highlighted the effects of the combination of the two on the East Asian summer monsoon variability on a high-frequency (HF) component (sub-monthly timescales) and a low-frequency (LF) component (longer than about one month). The combination of positive EJ type 2 (EJ2(+)), which particularly accounts for the variability of the Okhotsk high among the two types of EJ pattern (EJ type 1 and EJ type 2), and negative PJ (PJ(-)) patterns establishes a noticeable tripole structure in the mid-tropospheric geopotential height field of East Asia that is characterized by two positive anomalies over the Okhotsk Sea and the western North Pacific to the north of the Philippines and a negative anomaly over Japan. It is found that the combination of both patterns, EJ2(+)/PJ(-), leads to a larger surface temperature decrease in the northeastern part of East Asia than a single teleconnection pattern alone, having a substantial impact on the East Asian summer monsoon circulation. The LF-combination also induces significant cool sea surface temperature anomalies in the vicinity of Japan. In the HF-combined case, the time evolution of a positive height anomaly over the Okhotsk Sea considerably differs from that of the LF-combined one. The HF-anomaly is extremely displaced eastward to the Aleutian Islands a few days after the peak day and occupies part of a newly organized wavetrain pattern across the central North Pacific. Further development of the wavetrain pattern may be attributed to anomalous convective heating related to enhanced synoptic-scale disturbances to the east of Japan. Both the LF-EJ2(+) and LF-PJ(-) patterns tend to appear frequently during the post-1980 period, and, as a result, the number of LF-combined cases may have increased in the last two decades. Exceptionally cool summers with significant amounts of rainfall, particularly in Japan, such as the 1993 and 2003 summers, often arise from such an LF-combination.