Fig. 1 shows the distribution of mean ASR in the UTLS and associated meteorological variables at 100 hPa during the ASM period for the years 2012 to 2018. It shows the pronounced mean anticyclonic (high pressure) and water vapor center in the UTLS, with strong easterlies in the tropics and westerlies in the extra-tropics, in conjunction with a cold center over the southern TP. Significant deep convection occurs over the west coast of India, the Bay of Bengal (BoB), northeastern India, and occasionally over the southern TP. These are well-known climatological features associated with the Asian summer monsoon (46-50). Pronounced enhancement in ASR with a magnitude ranging from 1.05-1.22 is found over the southern anticyclone, spanning the Arabian Peninsula, the Himalayan-Gangetic Plain (HGP) and the China South Sea, with the highest ASR appearing over the northern BoB, East India and Bangladesh. The regions with the maximum ASR largely overlap with those of the minimum tropopause temperature and deep convection, with the former being located to the north of the latter, where pollutants from nearby emission sources are abundant and can be transported upward by deep convection associated with the ASM (51). The most noteworthy aspect of the ASR is the confined local maximum over the BoB.
Note that in most years, except for 2012 and 2018, the ASR and deep convection maxima show notable differences with respect to the horizontal locations. The pronounced ASR enhancement is located north of the deep convection region. There is a pattern of strong enhancement of ASR between 15-25°N and 80-100°E, just over the northern BoB and Bangladesh, located at the northwestern edge of the extensive areas of the strongest convection. Compared to southeastern Asia and central India, higher AODs are found around the northern BoB and adjacent coastal areas (Fig. S1), where strong convection occurs as well. Therefore, the locations of largest aerosol loading in the PBL and deep convection indicate that the major pathway for the transport of aerosols and precursors to the tropopause is located at the northern BoB and adjacent land areas. The deep convection over the BoB triggers low-level convergence of polluted air from the Indian subcontinent. It appears that the aerosols transported to the UTLS over the northern BoB and adjacent land areas originate from anthropogenic sources since carbon monoxide, which is generally considered as a pollution tracer, takes on a similar regional distribution pattern (Fig. S2).
As shown in Fig. 1, the enhancements of ASR in the UTLS during the ASM period show large inter-annual differences from 2012 to 2018, with the ASR varying from a minimum of 1.058 in 2013 to a maximum of 1.075 in 2012, averaged over the ATAL region (15-45°N; 5-105° E) defined by Vernier et al. (1, 3). Since pollution emissions are not expected to be highly variable, changes in the activity of deep convection are likely responsible for the inter-annual variability in the ASR during the ASM period. It should be noted that the geographical location and range of strong anticyclonic circulation varied from year to year (see Fig. 1). The inter-annual variations in the intensity of the ATAL compared to that of the ASM were investigated based on the relationship between the regionally-averaged ASR in the ATAL and the QBO index for different ASM periods (see Fig. 2). The inter-annual fluctuations in the ASR appear to be synchronized with inter-annual QBO index variations. Higher ASR corresponds to the eastward (negative) phase of the QBO and lower value matches westward (positive) phase. The highest ASR corresponds to the lowest QBO index in 2012, and vice versa in 2013. The ASR and QBO show an anti-phase relationship with a correlation coefficient of -0.78. The ASM intensity is affected by the QBO, and the correlation coefficient between the QBO index and the ASM intensity Index (ASMI) was calculated to be as high as 0.91 for the years 2012 to 2018 (see Fig. S3).
Previous work has shown that the QBO-induced secondary circulation is associated with an increase in upwelling during the easterly shear phase and a suppression of the upwelling during the westerly phase (52), and large anomalies of annual cycle variations in water vapor and other trace gases are due to the QBO disruption (44). The regionally averaged ASR in 2013, when the QBO index was in the westerly phase characterized by suppression of upwelling, is relatively lower than that in 2012, 2014 and 2018, when the negative QBO index was in the easterly shear phase. This result is consistent with the conclusion about the variation in water vapor during the 2015–16 QBO disruption by Tweedy et al. (53). We also investigated the relationship between the regionally averaged CO mixing ratio and the QBO index for different ASM periods, and the result also shows a correlation, although not as significant as that between the ASR and the QBO (Fig. 2). Atmospheric CO levels over South Asia are significantly influenced by biomass burning, and a close relationship between the CO mixing ratio in the UTLS region and the carbon emission flux from biomass burning is found (R=0.79, Fig. S4). The increasing trends of ASR with CO mixing ratio underscore the influence of anthropogenic pollution on the formation of the ATAL. As mentioned above, we applied the latitude and longitude range of 15-45°N and 5-105°E to average the ASRs over the ATAL region. Sensitivity tests show that small differences in selected latitude and longitude range does not substantially change the inverse relationship between the ASR and the QBO (see Table S1).
The increase in tropopause ASR with decreasing QBO index can be attributed to dynamic processes, i.e., the intensification of the secondary circulation driven by the QBO. Fig. 2 also shows that deep convective activity increases with decreasing QBO index, being strongest between 10 and 20°N during the extreme easterly shear phase. Deep convective activity is gradually suppressed with an increasing QBO index. Collimore et al. (41) investigated the mechanisms linking the QBO with deep convection and found that the QBO modulation of tropopause height can allow convection to penetrate deeper. Therefore, deeper convection favors the transport of aerosols and gaseous precursors from the PBL to the tropopause.
We further explored the inter-annual variations of the three-dimensional structure of the vertical velocity with the QBO. The latitude-height cross-sections of the vertical velocities and the difference between 2012 and 2013 at the 90°E cross section are shown in Fig. S5. These two years, characterized by the extreme QBO index during the easterly and westerly phase, respectively, are particularly suitable to demonstrate the difference in vertical velocity induced by the QBO secondary circulation. Three outstanding columns with stronger upwelling motion, reaching the UTLS and capped near 100-90 hPa, are identified at about 18°N, 25°N and 32°N during the easterly phase in the year 2012. This QBO-associated secondary circulation anomaly produces upwelling by -0.05 Pa s−1 with a peak of -0.15 Pa s−1 at the southern flank of the TP, which facilitates the entry of tropospheric constituents into the subtropical lower stratosphere. The increased subtropical upwelling also supports the transport of aerosols from the upper troposphere to the lower stratosphere. As found in previous studies (e.g., refs. 27, 42), this instability develops in association with the QBO-derived secondary circulation near the UTLS. Interestingly, the descending motion over the region north of the TP is suppressed during the easterly phase of the QBO in 2012. This configuration of atmospheric vertical motion helps maintaining a balance of the ATAL intensity.
A most striking feature of the ATAL is the asymmetry in the regional distribution of enhanced aerosol levels even within the anticyclone. Relatively high ASR on the southeast of the anticyclone coincides with the strong convection between 40°E to 120°E and 10°N to 30°N. To the west of this strong convection region, there is a gradual decrease of ASR associated with downward transport and westward advection from the hot spot over the BoB, where the largest enhancement of ASR has been found (see Fig. 1). We analyzed this asymmetry distribution of ASR together with the vertical velocity from the ERA5 data, and noticed that the spatial distribution of enhanced aerosol in the UTLS was closely related to the horizontal cycle and vertical motion. During the ASM period, westerlies and descending motion prevail over the Middle East and northeastern Africa (Fig. 1). This overlap between the main descent region and the low ASR indicates that aerosol particles have been transported into the upper troposphere region below 12 km altitude within the ASM anticyclone. This finding can explain the significant reduction of aerosols in the northern ASM region. Past studies showed that the aerosols transported into the lower stratosphere by the monsoon convection are confined within the strong anticyclonic circulation until breakup of the associated ASM anticyclone followed by recirculation in the lower stratosphere (12, 13, 27, 54). The asymmetry distribution of enhanced aerosols reveals a significant sink within the ASM region.
We selected a vertical cross section at 0-60°E and 90°E of the western anticyclone to demonstrate the role of downward transport in this area, which plays a key role in balancing the ATAL intensity. For quantitative comparison, we calculated the average extinction coefficients in the upper troposphere (6-12 km) and the vertical velocity at 100 hPa, and the ASR in the UTLS at the vertical cross section. As shown in Fig. 3b, there is a clear relationship between the vertical velocity and the upper tropospheric extinction coefficient. The load of UTLS aerosols in the western anticyclone can also impact the concentration levels further downward through descending motion in the upper troposphere, as indicated by the similar inter-annual variation trends between the extinction coefficient and ASR for the years 2015 to 2018. However, their relationship is not significant for the period before 2015. Statistical analysis indicates that the correlation of the extinction coefficient with ASR does not meet the 95% confidence level of student t test (see Table S2). A possible reason for this asynchronic variation is that the transport capacity of aerosols by descending motion approaches saturation for the abundant aerosol loading in the UTLS. But it is evident that the descending motion can play a role in the dissipation of the ATAL.
To summarize, Fig. 4 illustrates the horizontal movement of the aerosols within the anticyclone and the vertical transport pathways in the troposphere that balance the intensity of the ATAL during the ASM season. Deep convection over the BoB plays a dominant role in the vertical transport of aerosols from the polluted boundary layer to the UTLS within the anticyclone, where they are transported westward by the equatorial easterly jet. At approximately 70°E the aerosol particles and precursor gases are partially removed from the UTLS by the large scale descending motion, resulting in a concentration reduction in the western part of the ASM anticyclone. Our analysis indicates that in addition to the large-scale ascending circulation inside and eddy shedding at the margins of the anticyclone (8, 35, 39), downward transport in the western part of the ASM anticyclone provides an important dynamical aerosol sink in the UTLS, which maintains a balance in the intensity of the ATAL.