Analysis on the Multi-center Structure of Summer South Asia High and Its Thermal Influence Factors

The multi-center characteristic of the summer South Asia high (SSAH) is analyzed based on circulation fields and diabatic heating fields extracted from the NCEP/NCAR I, the NCEP/DOE II and the JRA-55 (Japanese 55-year Reanalysis Project) reanalysis datasets. Moreover, the vertical dynamic and thermodynamic structures of the SSAH centers in different areas and their relationships with heat sources at different regions are studied. Results show that: (1) The SSAH demonstrates an obvious multi-center feature. It can have up to five to six centers simultaneously. Among all the multi-center cases, two-center and three-center cases account for the largest proportion (about 70%-80%) of the total, while one-center and four-center cases only account for about 10% of the total cases. (2) No matter how many centers the SSAH has, the dynamic and thermodynamic features of these SSAH centers show different characteristics in different regions. These regions can be divided into three areas, i.e. 20°-70°E, 80°-120°E and 120°-160°E. For the SSAH centers over the Iranian plateau and its west region within 20°-70°E, the middle levels of the SSAH centers correspond to northeastern Iranian subtropical high and the lower levels correspond to northwestern India-Burma trough, where a strong descending motion occupies the entire troposphere. For the SSAH centers located from the Tibetan Plateau to eastern China within 80°-120°E region, their lower levels correspond to central-northern India-Burma trough, where a strong ascending motion occupies the entire troposphere with anticyclonic circulation in the upper levels and cyclonic circulation in the lower levels. For the SSAH centers over the western Pacific within 120°-160°E region, their middle and lower levels correspond to the western Pacific subtropical high, where an ascending motion is significant with anticyclonic circulation throughout the entire troposphere. (3) All the SSAH centers in the three regions display a warm-high structure. Subsidence heating is the main reason for the formation and maintenance of the SSAH centers within 20°-70°E region, while the deep convective heating is the main reason for the formation and maintenance of the SSAH centers within 80°-120°E and 120°-160°E regions. (4) The enhanced SSAH has significant impacts on local circulation, deep convection and precipitation in all the three areas, i.e. 20°-70°E, 80°-120°E and 120°-160°E, where the SSAH centers are located.