Tracks of the consecutive northward typhoons in 2020 and the concurrent changes in O3
The temporal proximity of the consecutive northward super typhoons was noteworthy, and both typhoons avoided direct landfall in eastern China. Typhoon Maysak formed on August 28, and Typhoon Haishen formed on August 31. They moved over the ocean to the east of China and followed a northward track along the periphery of the WPSH (Fig. 1a). Despite Typhoon Haishen’s track being further east than Maysak’s in their early lifetime, their tracks largely overlapped after making landfall (Fig. 1a). Maysak and Haishen intensified into super typhoons on September 1 and September 4, respectively. Maysak made landfall in the southern part of the Korean Peninsula on September 3, followed by Haishen just about four days later on September 7. They then both continued their northward track into northeastern China before dissipating. Such moving paths, markedly distinct from the landfalling typhoons that bring about significant winds and precipitation6,15, led to eastern China experiencing the continuous influence of the typhoons’ periphery for around 12 days. Such prolonged exposure to the periphery of the typhoons, unlike the more transient effects of direct landfall, often resulted in sustained O3 pollution3,10,11.
During the CTYT, eastern China experienced drawn-out and severe O3 pollution, covering a large area including the NCP, Shandong-Henan (SD-HN), and YRD regions (Fig. 1a), owing to the impacts of Typhoon Maysak’s and Typhoon Haishen’s periphery. Specifically, the average MDA8 O3 levels in these areas were observed to rise from 115 μg·m-3 in PRE-CTYT to 150 μg·m-3 during the CTYT, with increase rates of 46%, 32%, and 6% in the SD-HN, YRD, and NCP regions, respectively (Fig. 1b). The increase is more dramatic than typhoon-induced O3 pollution reported in autumn 201823. Most of the nine key cities recorded maximum MDA8 O3 exceeding 200 μg·m-3 during the CTYT period (Table S4), with Jinan and Shanghai reaching up to 240 μg·m-3. Numerous cities, including Shijiazhuang, Zhengzhou, Jinan, Shanghai, and Hangzhou, experienced as many as 8 days with O3 pollution during the 12-day CTYT period surpassing the Grade II thresholds of the Chinese National Ambient Air Quality Standard (i.e., 160 μg·m-3) (Table S4). In comparison, they experienced much less severe O3 pollution during the PRE-CTYT and AFT-CTYT, underscoring the significant influence of the consecutive northward super typhoons on elevating O3 concentrations in eastern China (Fig. 1b).
In addition to increasing O3 concentrations, typhoons may also change the location of air pollution3. The meridional location of the high-value O3 pollution zone in eastern China changed three times as these two typhoons passed by (Fig. 1c). The changes all occurred when two typhoons were between 30°–40°N. High O3 concentrations were mainly concentrated in the southern NCP and the northern SD-HN (35°–41°N, August 28–September 2). After Typhoon Maysak passed 35°N and made landfall in the Korean Peninsula, the high-value O3 zone moved to the YRD area (around 31°N, September 3–4). As Typhoon Haishen moved toward the Korean Peninsula (September 5–7), multiple peak MDA8 O3 centers (over 220 μg·m-3) were observed again in the NCP and SD-HN regions (35°–41°N). At the same time, in the YRD region (around 31°N), O3 concentrations increased and the range of the O3 pollution zone also expanded. Eventually, the area of high O3 values shifted from the NCP and SD-HN regions to YRD after Typhoon Haishen landed on the Korean Peninsula (passed 35°N, September 7–8).
Typhoons-induced changes in local circulation and thermodynamics
The large-scale background circulation in East Asia, combined with the influence of consecutive northward-moving typhoons, produced a circulation pattern favorable for O3 generation. During the CTYT, the WPSH showed an unusual increase in strength and a northward expansion (Fig. 2a). The intensity of the WPSH peaked at 592 dagpm (Fig. 2a), and the western ridge point of the WPSH shifted approximately 5° northward from its climatology of August–September (Fig. 1a). Additionally, Typhoon Maysak and Typhoon Haishen moved northward through similar locations just four days apart, which contributed to the persistence of the East Asian deep trough over eastern China. This circulation pattern featuring a stable East Asian deep trough and a northward shift of the WPSH favors the O3 generation in eastern China during the summer season1.
Under this circulation pattern and the influence of the two typhoons periphery, the O3 favorable meteorological conditions including high temperature, low RH, and strong radiation, were formed and maintained in eastern China during the CTYT. After September 2, the RH was below 60%, the UVB radiation was strong, and the air temperature remained above 30°C at 14:00 local time in most parts of eastern China (Fig. 2b). These meteorological conditions were closely related to the two typhoons moving northward towards the Korean Peninsula and corresponded to a period of significant increase in O3 concentrations. The interaction of the dry northwesterly descending airflow from inland behind the East Asian deep trough and typhoons' moisture accumulation24 reduced RH and inhibited cloud formation over eastern China. These factors further lead to intensified solar radiation during the daytime. Although Typhoons Maysak and Haishen did not directly traverse eastern China, their peripheral circulation affected eastern China for a considerable period of time. Furthermore, typhoons indirectly contribute to elevated temperatures in their surrounding areas by triggering anomalous descending motion25,26. Consequently, the temperature in eastern China was more than 1°C higher than the climatic average in August and September (Fig. 2a, Anomalies). In addition, the sea surface temperatures (SST) in the coastal ocean of eastern China remained relatively high (Fig. S3), because the typhoons did not pass over these areas. In contrast, the SST significantly decreased over the ocean south of the Korean Peninsula, which was along the typhoons’ trajectories. High SST in coastal areas leads to the reduction of the land-sea breezes, hindering the effective dispersion of local pollutants in coastal cities27,28. This was another meteorological condition of northward typhoons affecting air quality in eastern China.
Compared to meteorological conditions, the impact of the consecutive northward-moving typhoons did not lead to significant fluctuations in O3 precursors. During the CTYT, the column number density of HCHO and NO2 in eastern China remained stable between 8–121015 molec·cm-2 and 10–151015 molec·cm-2, respectively (Figs. S4 and S5). The column number density of HCHO and NO2 closely aligned with the average column concentrations observed by satellites over eastern China in recent years29-31. This was different from the O3 increase caused by enhanced emissions from biological sources in southern China before typhoons made landfall3. The temperature rise caused by consecutive northward typhoons was further north (Fig. 2a), and there was no large area of dense vegetation as in southern China.
Repeated cross-regional O3 transport and backflow during the CTYT
The movement of Typhoons Maysak and Haishen induced a cross-regional transport of O3, which is a significant factor contributing to regional O3 pollution3,9,10. The positions of the typhoons played a crucial role in large-scale circulation patterns and synoptic weather6,32, thereby affecting the transmission and diffusion of O39. Differing from the three phases of O3 accumulation, transport, and removal that occur under the influence of individual typhoons3,6,20, the processes induced by consecutive typhoons manifested in a pattern of initial increase, subsequent decrease, and rapid increase in O3 content in key cities within the NCP and SD-HN and a pattern of continuous upward trend in O3 in the YRD cities during the CTYT (Fig. S6). Specifically, the cross-regional transport involved the following sequential processes: Phase 1. Typhoon Maysak bolstered the WPSH, and O3 accumulation in NCP and SD-HN; Phase 2. Southward O3 cross-regional transport on the west side of Typhoon Maysak; Phase 3. Northward backflow and O3 accumulation in NCP and SD-HN between two typhoons; Phase 4. Secondary southward O3 cross-regional transport on the western side of Typhoon Haishen; Phase 5. Northward backflow in YRD after Typhoon Haishen moved north.
In Phase 1, Typhoon Maysak bolstered the WPSH as it was in 20°–25°N. The WPSH controlled Eastern China (Fig. S7, August 28–31), fostering a distinctive abnormal anticyclonic circulation band that dominated a substantial area from central-eastern China to the Pacific Ocean east of Japan in the middle and lower troposphere26. This atmospheric configuration resulted in stagnant weather conditions, and O3 accumulation commenced in NCP and SD-HN (Figs. 3, S8, andS9, before August 31). Meanwhile, when Typhoon Maysak moved into 25°–30°N, the wind on the west side of Typhoon Maysak brought clean air from the ocean to YRD (Fig. 3) from August 31 to September 2, maintaining low O3 levels in YRD. However, the areas where O3 accumulated were not affected.
In Phase 2, southward cross-regional O3 transport occurred on September 2–3 (Fig. 3). After making landfall on the Korean Peninsula and merging with a cold vortex, Typhoon Maysak facilitated the transition of entire eastern China to a weather pattern dominated by the East Asian trough (Fig. S7, September 2–3). Eastern China was behind the East Asian trough and on the southwest side of Typhoon Maysak, surface wind direction shifted to a consistent northwest flow (Fig. 3, September 2–3) which rapidly transferred the O3 that accumulated in NCP and SD-HN to the YRD (Fig. S10). Thus, MDA8 O3 levels in YRD begin to exceed 160 μg·m-3 (Fig 3).
Phase 3 occurred under the re-establishing of stagnant weather conditions between two typhoons. The combined influence of Maysak merging with a cold vortex and Haishen moving toward the Korean Peninsula (September 4–6) exhibited sparse contour lines in 500hPa (Fig. S7) over eastern China. This is similar to rising O3 in the north YRD during the Meiyu season with a northeast cold vortex33. Meanwhile, a weak anticyclone formed on the surface in eastern China between two typhoons (Fig. 3), playing a crucial role in elevated O333. This was a unique circulation pattern that only occurred between two consecutive typhoons. Under this weak anticyclone influence, weak northward backflow formed successively from YRD to NCP and SD-HN regions (Figs. 3, and, S8–S10, September 4–6), and led to further O3 accumulation in the YRD region following the transport and resulted in a substantial elevation of O3 levels in the NCP and SD-HN regions (Fig. S6, September 4–7), indicating by a significant increase in Horizontal advection (HADV) levels in NCP cities after September 4 (Fig. S6).
Phase 4 was the second time southward cross-regional O3 transport which occurred on September 7 (Fig. 3) as Haishen moved northward in a similar latitude to Maysak's on September 3 (Fig. 1a). The same circulation pattern in Phase 2 occurred again (Fig. S7, September 3 and 7). Thus, a shift in wind direction to the northwest reoccurred (Fig. 3, September 6–8). Under the northwest winds, the high MDA8 O3 center (more than 200 μg·m-3) relocated from the southern part of Hebei to the western part of Shandong and the northern part of Henan (Figs. 3, S8, and S9). In the YRD region, the high MDA8 O3 region (more than 200 μg·m-3) moved from Anhui and northern Jiangsu to southern Jiangsu, Shanghai, and northern Zhejiang (Figs. 3, and S10).
In Phase 5, following the transport of O3 to the YRD area, the second northward backflow took place in the YRD region. When Typhoon Haishen moved northward into northeastern China on September 8, the WPSH regained control over southern China (Fig. S7, September 8) which caused the northward backflow in the YRD area. Northward backflow on the surface hindered O3 diffusion, similar to that of September 4 (Figs. 3 and S10, September 8). These conditions occurred after O3 was transported to the southern part of eastern China, causing high O3 levels in YRD (more than 200 μg·m-3) at the end of the CTYT.