Evaluating the role of South Asian High on the transition phase of the Indian summer monsoon

AbstractThe South Asian high (SAH) index was defined using the 200 hPa geopotential height for 1973–2019. Of the movements of the SAH center in the north–south, east–west, northwest-southeast, and southwest-northeast directions, the movements in the northwest-southeast direction showed the highest positive correlation with heatwave days (HWDs) in South Korea. Thirteen years with the highest values in the northwestward shift of the SAH (positive SAH years) and 13 years with the highest values in the southeastward shift of the SAH (negative SAH years) were selected from a time series of SAH indices from which the linear trend was removed, and the differences between these two groups were analyzed. An analysis of vertical meridional circulation averaged along 120°–130° E showed that in the latitude zones containing Korea, anomalous downward flows with anomalous high pressures formed in the entire troposphere and coincided with a positive anomaly of air temperature and specific humidity. An analysis of stream flows and geopotential heights showed that in the positive SAH years, anomalous anticyclones developed in Korea, the North Pacific, North America, Western Europe, and the Iranian Plateau. These anticyclones had the wavenumber-5 pattern and showed more distinct barotropic vertical structures at higher altitudes, which resembled the circumglobal teleconnection (CGT) pattern. The maintenance of CGT depends on the interaction between the CGT circulation and the Indian summer monsoon (ISM), which has a major influence on the mid-latitude atmosphere. Strengthening of the ISM results in the formation of upper-level anomalous anticyclones in the northwestern Iranian Plateau and produces continuous downstream cells along the waveguide due to the Rossby wave dispersion. When diabatic heating by Indian summer monsoon precipitation is strengthened, the SAH is strengthened to the northwest of India, and a positive CGT pattern is formed. As a result, anomalous anticyclones formed in all layers of the Korean troposphere, resulting in heatwaves, tropical nights, and droughts exacerbated in South Korea.

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Occurrence of heat wave in Korea by the displacement of South Asian High

10.21203/rs.3.rs-442109/v1 ◽

2021 ◽

Author(s):

Yumi Cha ◽

JaeWon Choi ◽

Eung-Sup Kim ◽

Joong-Bae Ahn

Keyword(s):

South Korea ◽

Heat Wave ◽

South Asian ◽

Summer Monsoon ◽

Indian Summer Monsoon ◽

Heat Waves ◽

Specific Humidity ◽

South Asian High ◽

Iranian Plateau ◽

The North

Abstract The South Asian high (SAH) index was defined using the 200 hPa geopotential height for 1973–2019. Of movements of the SAH center in the north-south, east-west, northwest-southeast, and southwest-northeast directions, movement in the northwest-southeast direction showed the highest positive correlation with heat wave days (HWDs) in South Korea. Thirteen years with the highest SAH values (positive SAH years) and 13 years with the lowest SAH values (negative SAH years) were selected from a time series of SAH indices from which the linear trend was removed, and differences between these two groups were analyzed. An analysis of vertical meridional circulation averaged along 120°-130°E showed that in latitude zones containing Korea, anomalous downward flows with anomalous high pressures formed in the entire troposphere and coincided with positive air temperature and specific humidity. An analysis of stream flows and geopotential heights showed that in positive SAH years, anomalous anticyclones developed in Korea, the North Pacific, North America, western Europe, and the Iranian Plateau. These anticyclones had the wavenumber-5 pattern and showed more distinct barotropic vertical structures at higher altitudes, which resembled the circumglobal teleconnection (CGT) pattern. The maintenance of CGT depends on the interaction between CGT circulation and the Indian summer monsoon (ISM), which has a major influence on the mid-latitude atmosphere. Strengthening of the ISM results in the formation of upper-level anomalous anticyclones in the northwestern Iranian Plateau and creates continuous downstream cells along the waveguide due to Rossby wave dispersion. When diabetic heating by Indian summer monsoon precipitation is strengthened, the SAH is strengthened to the northwest of India, and a positive CGT pattern is formed. As a result, anomalous anticyclones were formed in all layers of the Korean troposphere resulting in heat waves, tropical nights, and droughts exacerbated in South Korea.

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