Stagnation of the Baiu front and the Yellow Sea High
Figure 1 shows the time–latitude cross section of the GSMaP rainfall intensity averaged between 125 and 130°E and the meridional positions of the Baiu front retrieved from the JMA weather chart at 130°E for July 2020. The date of Baiu withdrawal in 2020 (southern Kyushu: 28 July; northern Kyushu: 30 July) was approximately 2 weeks later than the climatological withdrawal date in the middle of July. Very high rainfall intensities exceeding 100 mm/day (indicated by the red color) continued over Kyushu (30 − 35°N). Tracking the meridional positions of the Baiu front at 130°E, the total period during which the Baiu front remained between 30 and 35°N was 20 days (3 − 9, 11–14, 16, 18 − 19, 22 − 24, 26 − 27, and 29 July), and there was no typhoon around Kyushu in July 2020. Notably, the July 2020 heavy rainfall event was found to be directly induced by the long-term stagnation of the Baiu front.
Table 1 indicates the number of total days with synoptic fronts (stationary, cold, and warm fronts) between 30 and 35°N at 130°E in July from 2002 to 2021. The largest number of total days with synoptic fronts (20 days) was observed over the 20-year period. Remarkably, the YSH, which contributes to Baiu front stagnation, appeared 7 times (on 1, 2, 4, 8, 14, 15, and 21 July 2020) in the JMA weather chart.
Table 1
Number of total days with synoptic fronts (stationary, cold, and warm fronts) between 30 and 35°N at 130°E in July from 2002 to 2021. The rightmost columns provide the averages and standard deviations over 20 years.
yy | '02 | '03 | '04 | '05 | '06 | '07 | '08 | '09 | '10 | '11 | '12 | '13 | '14 | '15 | '16 | '17 | '18 | '19 | '20 | '21 | ave | std |
day | 5 | 19 | 2 | 11 | 17 | 16 | 3 | 11 | 13 | 3 | 6 | 2 | 9 | 7 | 9 | 3 | 5 | 10 | 20 | 5 | 8.8 | 5.7 |
Figure 2 shows the time–latitude cross section of the surface potential temperature, SST, and YSH index for June, July, and August 2020. The YSH index is defined as a positive SLP anomaly from the zonal mean between 105 and 115°E (Moteki and Manda 2013). The YSH occurs in areas with a positive SLP anomaly and a surface atmospheric temperature (SAT) below 24°C. The SAT threshold of 24°C is based on the fact that the SST of the Kuroshio Current exceeds 24°C. The conditions needed for YSH development are lost as the SST over the Yellow Sea increases and exceeds 24°C, and the SST difference between the Kuroshio Current and Yellow Sea disappears. The YSH index can be employed to detect the presence of cold anticyclones related not only to remarkable YSH occurrences, as indicated by closed SLP contours, but also to cold SLP ridges extending to the east. The positive SLP anomaly of the YSH index shown in Fig. 2 continued within the latitudinal zone between 30 and 38°N throughout July 2020. The SST contour representing 24°C varied between 30 and 33°N, and a notable surface potential temperature contrast was maintained between 30 and 35°N throughout July 2020. The meridional positions of the synoptic front based on the JMA weather chart correspond to the notable surface potential temperature contrast at the southern edge of the positive YSH index.
Figure 3 shows the same information as Fig. 2 but for the surface sensible heat flux retrieved from the JRA-55 data. Atmospheric cooling due to a negative sensible heat flux continues in association with very low SSTs below 22°C between 31 and 38°N over the Yellow Sea. This is consistent with the fact that the YSH is maintained until the end of July 2020. The atmospheric warming due to a positive sensible heat flux that intermittently emerges in association with the passage of an extratropical cyclone and typhoon (Bavi) reaches approximately 10 W/m2 over the Yellow Sea.
Figure 4 shows the same information as Fig. 2 but for the SST anomaly based on the 30-year climatological mean of NOAA OIv2.1 data and the surface wind speed of 5 m/s retrieved from the JRA-55 data. The minimum and average SST anomalies over the Yellow Sea were approximately − 4°C and − 1.5°C, respectively. In ordinary years, the SSTs over the southern Yellow Sea increase during the latter half of July, thereby eventually exceeding 24°C, and the YSH occurrence frequency significantly decreases (Moteki and Manda 2013). However, in 2020, the SSTs over the whole Yellow Sea remained quite low, i.e., below 24°C, until the end of July. The extended presence of the YSH, which remained for longer than that in typical years, could be due to these significant negative SST anomalies. The long-term stagnation of the Baiu front is thought to be attributable to YSH maintenance associated with negative SST anomalies. Since the SST over the Yellow Sea, which should normally increase in July, remained quite low (less than 24°C), the YSH was maintained for a long time, and the Baiu front could not migrate north of 35°N.
Moreover, the SSTs over the Yellow Sea in June were found to be dominated by positive anomalies ranging from 0.4–2°C. After the strong surface southerly winds associated with the passage of a strong extratropical cyclone emerged on 29 June, the positive SST anomalies were drastically reversed into negative anomalies between 32 and 37°N. During the June–August period in 2020, 12 extratropical cyclones and Typhoon Bavi passed over the Yellow Sea (11, 18, 24, and 29 June; 3, 10, 13, 19, and 24 July; and 6, 9, 13, and 23–27 August), and the SSTs were suggested to decrease in association with the strong surface winds.
Influence of extratropical cyclones on the SST over the Yellow Sea
Here, the influence of the extratropical cyclone on 29 June was suggested as a cause of the remarkable negative SST anomalies observed in July. The SSTs over the Yellow Sea could drastically decrease due to vertical mixing in the ocean surface layer associated with the strong winds of the extratropical cyclone.
Figure 5 shows the same information as Fig. 2 but for the ocean mixed layer depth obtained from the ECCO2 data. During the first half of June, the depth of the ocean mixed layer was very shallow (~ 10 m), and the SST gradually increased under seasonal migration. During the latter half of June, the observed ocean mixed layer deepening by more than 15 m corresponded to low SSTs (21–22°C) as each extratropical cyclone passed over the Yellow Sea. In July, the ocean mixed layer remained deeper than 15 m because of the passage of the above 5 extratropical cyclones, which inhibited the increase in the SST associated with seasonal migration. In August, the ocean mixed layer became shallower (~ 10 m), and the SST gradually increased again under seasonal migration.
Figure 6 shows the same information as Fig. 2 but for the differential ocean mixed layer depth retrieved from the ECCO2 data. As extratropical cyclones passed on 18, 24, and 29 June and 13, 19, and 24 July and Typhoon Bavi passed from 23–27 August over the Yellow Sea, the 1–5 m/day ocean mixed layer deepening was found to correspond well to the 0.1°C/day SST decrease.
Figure 7 shows the horizontal distributions of the differential water temperature retrieved from the ECCO2 data and the differential SST for the extratropical cyclone passing over the Yellow Sea from 28–30 June. As shown in Fig. 5, the negative SST anomalies in the Yellow Sea during July were likely triggered by the passage of the extratropical cyclone at the end of June. As shown in Fig. 5, the negative SST anomalies in the Yellow Sea in July were triggered by the passage of a low-pressure system at the end of June. As shown in Fig. 7a, 1 day before extratropical cyclone passage, there was almost no variation in both the ocean mixed layer depth and SST under calm conditions. As shown in Fig. 7b, on June 29, when the extratropical cyclone passed through the Yellow Sea, the SST decreased in areas where the ocean mixed layer deepened throughout the entire Yellow Sea. On June 30 (Fig. 7c), the developing extratropical cyclone moved into the Sea of Japan, and the ocean mixed layer deepened under the influence of strong winds within a broader area. The SST significantly decreased in the eastern Yellow Sea, along the north side of the Kuroshio Current, off the east coast of mainland China and southwest of Kyushu. In summary, the ocean mixed layer deepened by 5 to 15 m, and the SST decreased by 0.5 to 1.5°C over the Yellow Sea from June 28 to 30 as this extratropical cyclone passed.
Figure 8 shows the vertical section of the differential water temperature obtained from the ECCO2 data and the differential SST averaged between 122 and 125°E on 29 June. When the extratropical cyclone passed at 34°N and 124°E, the ocean surface layer of the Yellow Sea indicated that the water temperature structure was significantly impacted in response to the surface wind maximum. As shown in Fig. 8a, the peak of the surface wind speed (9–11 m/s) at approximately 34°N corresponded very well with the observed 1°C SST decreases and 1°C negative SST anomalies. As shown in Fig. 8b, the SST decrease was consistent with the decrease in the water temperature within the ocean surface layer (5–15 m depth) and the increase in the thermocline (15–35 m depth). The structure of the water temperature fluctuations suggests that vertical mixing caused the water temperature in the mixed layer to decrease and that in the thermocline to increase as the extratropical cyclone passed. Between 30 and 34°N, where the meridional gradient of the SST is high, vertical mixing could easily occur in response to surface wind forcing. Ekman upwelling associated with horizontal divergence in the ocean surface layer with a 200–300 km scale was locally confirmed in the Yellow Sea and could partially contribute to the observed SST reduction (not shown). However, because the stratification in the ocean surface layer was highly notable in the Yellow Sea, deepening of the ocean mixed layer was the most reasonable process for the broad-area SST decrease.
In contrast, to the south of 30°N, positive SST anomalies were observed despite strong southwesterly winds exceeding 10 m/s. Thus, the sensitivity of the oceanic response to atmospheric forcing is very different between the Yellow Sea and Kuroshio Current because of the difference in the corresponding ocean mixed layer depths. As a result, high SSTs greater than 24°C were maintained over the Kuroshio Current, while the SST over the Yellow Sea was significantly reduced under extratropical cyclone passage, and the meridional gradient of the SST was enhanced, which was considered to contribute to the prolonged stagnation of the Baiu front.