Modulation Effect of MJO on the Precipitation over Guangdong and Its Link with the Direct Impact System in June

A distinguishing method of 500 hPa circulation systems influencing Guangdong is established, and the changes in modulation effect of MJO on the precipitation over Guangdong with the low latitude 500 hPa circulation system in June is analyzed using 30-year (1979—2008) 86-station observational daily precipitation of Guangdong and daily atmospheric data from NCEP-DOE Reanalysis 2. It is found that each phase of strong MJO may correspond to various levels of precipitation, of which the third phase has the highest probability of heavy precipitation (49.3%). The third phase is the only phase among 8 phases of MJO that strong precipitation days outnumber weak precipitation days. The 500 hPa circulation systems in low-latitude impacting Guangdong directly mainly include westerly trough, shallow westerly trough, flat westerly or border of subtropical high, subtropical high and tropical low or trough. The most strong modulation effect of MJO on the precipitation over Guangdong occurs in the case impacted by westerly trough, while the modulation effect is quite weak in the other cases. The precipitation anomaly percentage averaged over Guangdong is peak (valley) in the third phase (the sixth phase) under the case impacted by westerly trough. The changing in modulation effect of MJO on the precipitation over Guangdong with the low latitude circulation system is substantiality due to the changing of dynamic ascending motion and the water vapor transport, both of which are necessary for rainfall. In the westerly trough case, the subtropical high is strong and westward, water vapor transport to Guangdong increases significantly, and the dynamic ascending motion and high level divergence conditions are also enhanced, which results in Guangdong precipitation strengthening in the third phase of MJO. But in the sixth phase of MJO, the subtropical high is weak and eastward, water vapor transport to Guangdong decreases significantly although the dynamic ascending motion is enhanced, leading to weaker precipitation than that in the third phase of MJO. However, in the case without westerly trough, although water vapor transport to Guangdong increases in the third phase of MJO, the geopotential height anomaly is positive over East Asia and Western Pacific, therefore the dynamic ascending motion is reduced, so the precipitation is not much stronger than the other MJO phases. Therefore, the modulation of MJO on the precipitation over Guangdong needs the cooperation of westerly trough. The direct impact of westerly trough and remote correlation effect of the third phase of MJO are good prediction indicators for large domain heavy precipitation over Guangdong.