Regulation of the East Asian Summer Monsoon by the Himalayas 周明達 吳奇樺 馮語涵 中央大學 大氣科學系

Asian summer monsoon comprises two major systems: East Asian summer monsoon South Asian summer monsoon Summer monsoon period: May – September Onset of Asian summer monsoon: Indochina in early May to mid-May

Why is Indochina the most favorable region for the earliest Asian monsoon onset?

Indochina is most favorable for the earliest Asian summer monsoon onset for the following reasons: A large-sized land in the tropics where solar heating in April and early May is strong. Located in the vicinity of the Maritime Continent convergence zone, where the center of the planetary-scale upper tropospheric high pressure system in April resides – susceptible to deep convections. Located near the end of the western stretch of the NP high, where the control of the NP high on convections is weak – susceptible to convections when forced by solar heating of the land.

Vertically Integrated Heating Precipitation + Radiation Indochina Bay of Bengal Early Mei-yu South China Sea India Mei-yu/Baiu Himalayas Bangladesh Units: W m -2

FIG. 2. (Continued) Gangetic Plain Monsoon Retreating Baiu ends Typhoon Season

Zonal Wind Speed at 850-hPa Red – Arabia Sea Blue – Bay of Bengal Green – Indian Ocean The westerlies strengthen rapidly in May and June, attains a maximum in July, and weakens in August. We expect the South Asian summer monsoon expands and strengthens north- and northwest-ward from Indochina in early May to northern India and the foothills of the Himalayas in late June and early July Developing Mature Retreating

Longitudinal variations of the zonal wind at 850-hPa averaged over the latitude band 10-20°N. Phase 1 (15 May – 15 June)Developing phase Phase 2 (15 June – 24 July)Mature phase Phase 3 (24 July – 31 August)Retreating phase Why does the westerlies remain nearly unchanged during Phase 2?

Compared to the lower tropospheric circulation, the upper tropospheric circulation is rather smooth. It reflects the overall heating of a large domain below.

Streamlines and GPH at 200 hPa Black dots:Center of the Asian high Star * :Center of the previous 10-day period Blue dots:Center of other earlier 10-day periods

Streamlines and GPH at 200 hPa Black dots:Center of the Asian high Star * :Center of the previous 10-day period Blue dots:Center of other earlier 10-day periods

The latitude (upper panel) and longitude (lower panel) of the center of the Asian high at 200 hPa. The two vertical lines separate the three monsoon phases. Phase 1Phase 2 Phase 3 In Phase 2, the northern front of the summer monsoon has pushed to the foothills of the Himalayas, and the Asian high center is constrained to the latitudes of ~27-30°N. The speed of the northward movement of the Asian high center is twice as fast in Phase 1 as in Phase 2.

Vertically integrated total heating in the Himalaya region (25 – 30°N; 80 – 100°E). Phase 1 Phase 2 Phase 3

Mean annual variation ( ) of the precipitation in Taiwan at 8 Central Weather Bureau stations. Phase 1 Phase 2 Phase 3 Typhoon Season

Streamlines and the GPH at 700 hPa of the 3 phases of the East Asian summer monsoon: Phase 1 (15 May – 15 June) Phase 2 (15 June – 24 July) Phase 3 (24 July – 31 August) These 3 phases are identical in time with that of the South Asian summer monsoon. Phase 2 is Mei-yu/Baiu/Changma rainy season with stagnant frontal systems extending from Yangtze valley to Japan and Korea. What causes the frontal systems to remain nearly stagnant in Phase 2?

We postulate that there is indirect connection between the South Asian and East Asian monsoon systems through the upper tropospheric circulation. It is based on the following reasoning: Regions of high pressure in the upper troposphere are susceptible to deep convections. On the other hand, regions with deep convections induce high pressure in the upper troposphere. In Phases 1 and 2 of the East Asian summer monsoon, the rain band corresponds to the frontal system which is located on the northwestern flank of the NP high ridge. Thus, in the EA-WNP region, the upper tropospheric high ridge, which is an eastern extension of the Asian high ridge, is located to the north of the NP high ridge. Since the Asian high is strongly controlled by the South Asian summer monsoon, the South Asian summer monsoon has an effect of constraining the location of the NP high and, hence, the East Asian summer monsoon.

The GPH at 200 hPa (left) and 850 hPa (right) averaged over the longitude band °E. The dots represent the seasonal variation of the location of the maximum GPH.

Seasonal variations of the location of the maximum GPH at 200 hPa (red line) and 850 hPa (green line) averaged over the longitude band E. The two vertical lines separate the three monsoon phases. Phase 1 Phase 2 Phase 3

Conclusions The ridge of the upper and lower tropospheric pressure in the East Asian – western NP are coupled, with the former located to the north of the latter. Thus, the evolution of the NP high is constrained by the evolution of the Asian high, and the East Asian summer monsoon is connected to the South Asian summer monsoon. It is concluded that the slow northward movement of the mei-yu/baiu/changma rain band in 15 June – 25 July is caused by the blocking of the South Asian monsoon by the Himalayas.