| dc.description.abstract | Impacts of long-range transported biomass-burning aerosols from Indochina on regional air quality, atmospheric visibility and climate effects have been discussed extensively in the literature over the past few decades. However, the transport mechanism and regional climate feedback of Indochina biomass-burning aerosols is still not fully understood. To clarify the interaction between atmospheric dynamics and biomass-burning aerosols in the Indochina, we present results of the MERRA-2 dataset, satellite and in-situ observation in spring 2000-2019 over the 7-SEAS/BASELInE experiment region. This study finally proposes three main analysis results of the biomass-burning activities in the Indochina: (1) A 3D conceptual model of biomass-burning transport mechanism; (2) Four meteorological factors attributed to biomass-burning activities; (3) The biomass-burning aerosol effect and its regional climate feedback.
For the achievement of (1) 3D conceptual model of biomass-burning transport mechanism, we analyzed the horizontal and vertical distribution of biomass-burning aerosols and linked the synoptic-dynamic meteorology (e.g. frontal system, Low Level Jet (LLJ)) to aerosol transport. From March to April, the cold frontal system extends from Japan to the South China Sea, and the postfront accompanied a strong westerly wind belt at approximately 700 hPa (3 km), transporting biomass-burning aerosols from the source region (i.e., northern Indochina) to the sink region (i.e., Taiwan and West Pacific) via the LLJ stream. The 700 hPa LLJ may have carried the biomass-burning plumes located south of the frontal system and accompanied the upward/downward motion south/north of the frontal system. This downward motion at the north side of the frontal system brought may bring pollution down to the surface and increased surface PM concentration. The life cycle of the synoptic weather pattern is approximately 3-5 days and consistently repeatable throughout March to April.
For the achievement of (2) the four meteorological factors attributed to biomass-burning activities, we identify those factors by using long-term data with composite analysis, principal components analysis and correlation analysis. The four factors include: (1) anticyclone (Monsoon trough) in the Bay of Bengal, (2) relative anticyclone in the South China Sea, (3) Pacific subtropical high, and (4) westerlies from Indochina to Taiwan. The strong westerlies from Indochina to Taiwan positively correlates with the frequency of the postfront LLJ. When the postfront LLJ and biomass-burning transport events occur frequently in the synoptic-scale meteorology, the westerlies from Indochina to Taiwan will also increase in the climate field. The increasing anticyclone over the Bay of Bengal tends to enhance atmospheric stability, reduce precipitation, and turns to strengthen biomass-burning activities in the source region. The increasing relative anticyclone in the South China Sea can enhance westerly from Indochina to Taiwan. At the same time, the relative anticyclone can promote the stability of atmosphere, reduce the deposition efficiency of aerosol, and support the biomass-burning aerosol transport to the downwind region. The strength of the Pacific subtropical high positively correlates with the strength of the westerlies from Indochina to Taiwan. When the Pacific subtropical high northward shift, the circulation generates the southerly wind from Indochina to Taiwan, which changes the biomass-burning transport path and location. We further assess the correlation of attributed factors and different large-scale climate indexes (e.g. ENSO, Indian Ocean Dipole (IOD)). As a result, the biomass-burning activity has a stronger correlation with ENSO, but weak correlation with IOD. In the El Niño year, the increasing anticyclonic in the Bay of Bengal and the South China Sea accompanied the stronger westerlies from Indochina to Taiwan, which enhanced the biomass-burning activities and aerosol transport to the South China Sea and Taiwan.
For the achievement of (3) the biomass-burning aerosol effect and its regional climate feedback, as the aerosol frequently located from Indochina to Taiwan at approximately 700 hPa, which can increase the net radiation flux by 20% in the atmosphere as so-called the shortwave aerosol radiation effect (ARE). Those ARE play an important role in the atmospheric energy budget during biomass-burning period, and adjust the thermal and dynamic in the earth′s environmental system. Furthermore, it can be found that the summer monsoon pattern in May has linkage with biomass-burning activities from March to April. The summer monsoon will be increased when the biomass-burning emissions and transport enhance, and it will be weakened when biomass-burning accumulates in the Indochina. The correlation illustrates that the biomass-burning aerosol feedback may change the precipitation and affect the summer monsoon onset. However, the understanding of the aerosol effect is limited by only using MERRA-2 reanalysis data. To gain a better understanding of the biomass-burning aerosol effect and regional climate feedback mechanism, utilizing a numerical study would greatly benefit in the future. | en_US |