3.1 Comparison of climate characteristics and synthesized three-dimensional structures
Following Hu et al. (2019), the 0–20°N, 100–160°E region was defined as the active zone of the East Asian tropical monsoon. The MLPSs passing through this region (with at least one point in its track point falling in this zone) were defined as East Asian MLPSs. East Asian MLPSs with different intensities were selected from the VB2020 for May-October 1979–2018. Figure 1 shows the density distribution of the different intensities of the MLPSs formed. The monsoon lows formed were evenly distributed in the South China Sea and Pacific region, with areas of higher distribution density found in the south and close to the Equator (Figure 1a). The monsoon depressions were mainly formed in the South China Sea region, but a small number were formed in the Northwest Pacific region. Compared to monsoon lows, they are generally formed farther to the north (Figure 1b). The formation density of the deep depressions was similar to that of the monsoon depressions in the north-south direction, but the former was more concentrated in the Northwest Pacific region than in the South China Sea (Figure 1c). The above density distribution pattern for the MLPSs formed agrees with the results obtained from the HB2015 (Hu et al., 2019).
The VB2020 identified 267 EAMDs during the 40-year period from 1979 to 2018, while the HB2015 identified 313 during the 34-year period from 1979 to 2012. A significantly higher number of monsoon depressions was identified using the HB2015. This could be due to two reasons. (1) A more stringent set of identification criteria is adopted by the VB2020, resulting in some monsoon depressions not being identified using this dataset. (2) The change in the MLPS classification standard causes some systems classified as monsoon depressions by the HB2015 to be labeled as monsoon lows or deep depressions by the VB2020. As is indicated by the monthly count of the EAMDs formed in May-October identified by the VB2020 (Figure 2), fewer monsoon depressions formed in May-June than in July-October. This is consistent with the results of Hu et al. (2019). The lifetimes of the East Asian MLPSs with different intensities identified by the VB2020 were also statistically analyzed (Figure 3). The results show that most of the EAMDs had lifetimes of 3–9 days, with an average of about 6 days. This deviates significantly from the 10-day average lifetime given by the HB2015 (Hu et al., 2019). This may be due to the inclusion of closed contours as one of the identification criteria of the VB2020, which causes the identified monsoon depression’s genesis to lag behind. The VB2020 also limits the activity of the monsoon depressions to the 35°S–35°N zone. This may lead to an early ending of the monsoon depressions identified. These two factors together result in the shorter lifetimes of the monsoon depressions identified by the VB2020.
The movement paths of all 267 EAMDs identified by the VB2020 for May-October 1979–2018 were analyzed (Figure 4). Most of the monsoon depressions moved westward. Some of them were not able to move onto land and were extinguished at sea right after they were formed there. Those that eventually reached land had an impact on Southern China and the Indochina Peninsula. A few very exceptional ones reached India after passing over the Indochina Peninsula and the Bay of Bengal. Because of the limitations imposed by the VB2020 regarding the region where MLPSs can occur (35°S–35°N), the northernmost point reached by the EAMDs in their movement paths was only around 35°N (Figure 4), while the monsoon depressions identified by the HB2015 reached as far north as 50°N (Hu et al., 2019). This could lead to the premature ending of the tracking in the VB2020 and could also be one of the reasons for the shorter movement paths and lifetimes of the EAMDs identified by the VB2020. Using the classification standard of Hu et al. (2019) for the movement paths of EMADs, the paths of such systems generated by the VB2020 were classified. Compared to the results given by the HB2015, a significantly higher number of depressions identified by the VB2020 exhibited westward movement, with fewer following other movement paths. A large decrease in the number of eastward moving depressions (EMD) and those that changed direction (CDD) was observed. The number of depressions with unclassified paths (UCD) also increased (Table 1). The primary reason for this is the shorter movement paths of the EAMDs generated by the VB2020. Some systems did not meet the classification standard because of their short movement paths, and some of the northwestward moving depressions (NWMD) and those that changed direction were categorized as westward moving depressions (WMD) because their movement paths were cut off in the middle.
Table 1
Comparison of the numbers of EAMDs with different movement paths
| EMD | WMD | NWMD | CDD | UCD |
VB2020 | 4 | 97 | 33 | 18 | 115 |
HB2015 | 32 | 64 | 51 | 53 | 60 |
Dynamic synthesis was performed on the EAMDs identified by the VB2020. The synthesized vertical structures of the monsoon depressions were obtained (Figure 5). As is shown in Figure 5, the relative vorticity is symmetrically distributed about the central axis of the monsoon depressions, with almost no skew. The positive vorticity extends upward to 200 hPa, and the maximum vorticity occurs at 850 hPa. The specific humidity exhibits a positive anomaly above the centers of the EAMDs, with the maximum occurring at 700–800 hPa (left column in Figure 5). The temperature increases with the geopotential height, with a warm zone above 850 hPa and a cold zone below. The center of the warm zone is located at 400 hPa, and that of the cold zone is located near the ground. The center of the relative vorticity is located at the intersection of the cold and warm zone. Two zones with large potential vorticities are present in the potential vorticity field. The one near 500 hPa is the strongest, and it is located below the center of the warm zone The potential vorticity at 750 hPa is the second strongest (middle column in Figure 5). These results are consistent with those obtained by Hu et al. (2019) using the HB2015. The divergence field shows convergence in the lower level and divergence in the upper level. The convergence center is located at 950 hPa, and the divergence center is located at 150 hPa. This is different from the conclusions of Hu et al. (2019); that is, the convergence center is symmetrical about the depression center. The convergence centers of the EAMDs generated by the VB2020 are located to the southwest of the depressions’ centers. Vertical upward movement primarily occurs in the depression center, while weak subsidence occurs around it, which is consistent with the divergence field. The maximum upward movement also occurs to the southwest of the depression’s center (right column in Figure 5). This is different from the symmetry in vertical motion the about the depression center observed by Hu et al (2019).
The above comparisons show that the EAMDs identified by the two sets of MLPS track data share similarities in many aspects, such as their genesis locations, counts, movement paths, and synthesized three-dimensional structures. However, differences also occur, and they are likely to affect their credibility when the two datasets are applied to the East Asian tropical monsoon. To validate their credibility and understand the causes of any possible differences, further comparison and analysis are needed.
3.2 Case study of a typical year
Figure 6 shows the annual variation in the cumulative number of EAMDs during May-October generated by the HB2015 and VB2020. Substantial differences were observed in the annual variations in the numbers of EAMDs given by the HB2015 and VB2020. The HB2015 generally gives a higher number of monsoon depressions than the VB2020, and the difference is quite large for some years. As can be seen from Figure 6, the number of EAMDs identified by the HB2015 was the highest in 1986 and 1992 (14 depressions). For the VB2020, the numbers of EAMDs in these two years were 9 and 5, respectively. To ensure a sufficient number of cases was used, 1986 was chosen as the typical year for the comparative analysis of the EAMDs.
Figure 7 shows the genesis locations of the EAMDs given by both datasets for 1986. Similar patterns were found in the genesis location distributions of the monsoon depressions identified using both datasets. The EAMDs mainly formed over the South China Sea and the tropical West Pacific, with scattered occurrences along the east coast of the Bay of Bengal. Figure 8 shows the movement paths of the EAMDs given by the two datasets for 1986. After careful comparison, the paths of 8 depressions were found to overlap to some degree (depressions 1–8 in Table 2). However, their paths were not entirely identical. In most cases, the movement paths of the monsoon depressions identified by the HB2015 were longer than those identified by the VB2020. For the other 6 monsoon depressions identified by the HB2015 (depressions 9–14 in Table 2), some were classified as other types of MLPSs (2 lows and 1 deep depression) by the VB2020, and the others were not classified (3 systems). The remaining one monsoon depression identified by the VB2020 (#15 in Table 2) was classified as a monsoon low by the HB2015. In terms of the lifetimes of the EAMDs in 1986, the average lifetime given by the HB2015 was 10.54 days, while that given by the VB2020 was 6.75 days. This is consistent with the results obtained in the previous section on EAMDs during the entire study period.
Table 2
Comparison of the EAMDs generated by the two datasets for 1986 (shown in Universal Time)
No. | HB2015 | VB2020 |
Start time | End time | Path | Start time | End time | Path |
1 | 00Z05May | 12Z15May | EMD | 06Z08May | 12Z10May | UCD |
2 | 12Z17May | 12Z29May | EMD | 12Z15May | 18Z27May | CDD |
3 | 18Z30Jul | 18Z03Aug | UCD | 06Z30Jul | 12Z03Aug | CDD |
4 | 18Z05Aug | 12Z14Aug | UCD | 06Z08Aug | 06Z12Aug | WMD |
5 | 06Z16Aug | 00Z02Sep | CDD | 18Z17Aug | 06Z26Aug | UCD |
6 | 18Z11Sep | 12Z23Sep | WMD | 06Z20Sep | 00Z23Sep | UCD |
7 | 06Z02Oct | 18Z08Oct | CDD | 18Z27Sep | 00Z07Oct | NWMD |
8 | 06Z11Oct | 00Z25Oct | NWMD | 18Z13Oct | 12Z23Oct | NWMD |
9 | 06Z25Jun | 12Z04Jul | CDD | | | |
10 | 18Z08Jul | 06Z14Jul | WMD | | | |
11 | 12Z09Jul | 18Z27Jul | UCD | | | |
12 | 18Z11Jul | 12Z24Jul | CDD | | | |
13 | 18Z08Aug | 12Z13Aug | UCD | | | |
14 | 18Z13Oct | 12Z23Oct | CDD | | | |
15 | | | | 18Z03Oct | 12Z09Oct | WMD |
The movement paths of the monsoon depressions in East Asia are rather complex. The movement paths of the monsoon depressions identified by the VB2020 and HB2015 were classified using the standard of Hu (2019). As can be seen from Table 2, large differences occur in the movement paths of the EAMDs identified by the two datasets. To understand the causes of these differences and to determine the reliabilities of both datasets, the relative vorticity, stream function, and wind field at 850 hPa were compared and analyzed for the EAMDs identified by the two datasets at different times in 1986 along the movement paths of the depressions.
Two eastward moving monsoon depressions (#1–2) identified by the HB2015 are presented in Table 2, while the VB2020 did not identify any eastward moving monsoon depressions. Through careful comparison, it was found that monsoon depression 1 formed at 00:00, May 5 (Universal Time, same below) according to the HB2015. This depression overlaps with the one formed at 06:00, May 8 according to the VB2020. Figure 9a shows the relative vorticity and stream function at 850 hPa for the monsoon depressions identified by the HB2015 at the time of their genesis (00:00, May 5). The identified depression center (blue dot) is located in a zone of high relative vorticity and exceeds the specified threshold (0.5×10−5 s−1). Although closed stream function contours can be observed near the center, this system does not meet the classification criteria of the VB2020 for eastward moving depressions. Distinguishable cyclonic circulations are also absent near the depression’s center in the 850 hPa flow field at the same time point (Figure 9b). At 06:00, May 8 (Figure 9c), the monsoon depression was identified by the VB2020. At this time, the locations of the depression centers generated by the two datasets were slightly different, but nevertheless they were both located in the zone of high relative vorticity and the area enclosed by the closed stream function contours. Concurrently, discernable cyclonic circulations appeared at the depression centers (Figure 9d). The monsoon depression identified by the VB2020 only lasted for a short period of time, and its movement was essentially northward. Hence, it did not meet the criteria for an eastward moving depression. This depression reached the Indochina Peninsula at the end of its movement (Figure 9e, 06:00, May 10), and cyclonic circulations were still observed (Figure 9f). Figure 9g shows the relative vorticity and stream function at one time step (12:00, May 10) after the end of the monsoon depression’s movement identified by the VB2020. The relative vorticity near the center of the depression identified by the HB2015 still met the identification criteria, but the stream function around it no longer exhibited closed contours. In the corresponding 850 hPa flow field, the closed circulations had transformed to a trough (Figure 9h).
Another monsoon depression identified as eastward moving by the HB2015 (#2 in Table 2) was determined to have undergone changes in its path direction by the VB2020. Compared to the HB2015, the VB2020 gave an earlier genesis time for this depression. This monsoon depression formed in the Northwest Pacific monsoon trough. Initially, the West Pacific subtropical high (WPSH) strengthened and extended westward, and this depression moved to the west with the easterly flow present to the south of the WPSH and increased its strength. Later, when the WPSH weakened and retreated to the east, this depression shifted northeastward with the Southwest Monsoon. The cyclonic circulations eventually weakened and dissipated, but the vorticity was maintained at a value above the threshold of the HB2015. In addition, the path of this depression ended early in the VB2020 (figure not included). The other six monsoon depressions with overlapping paths (#3–8 in Table 2) all formed in July–October (the paths of the depressions generated by both datasets for June do not overlap) in the South China Sea and Northwest Pacific region. Depressions 3–5 formed in the Northwest Pacific monsoon trough during July–August. Depression 3 had a short lifetime, which is consistent across both datasets. Its movement paths given by the two datasets are also identical. It is labeled as unclassified by the HB2015 and as having experienced direction changes by the VB2020. The VB2020 recognized its genesis 12 h before the HB2015 did, while both datasets recognized its extinction simultaneously. The tracking of depressions 4–5 started earlier and ended later in the HB2015 than in the VB2020. The paths of the depressions given by the VB2020 coincide with those given by the HB2015 when the depressions were strong. Figure 10 shows the movement path of depression 5 in mid-August and its circulation conditions at typical times. As is shown in Figure 9, errors exist in the identification of the monsoon depression’s genesis by the HB2015, i.e., the relative vorticity near the center reaches the threshold, but a closed center is not present in the stream function (Figure 10a), nor are cyclonic circulations present as is the case for actual monsoon depressions (Figure 10b). The monsoon depressions identified by the VB2020 all feature centers with closed stream function contours around them at their genesis (Figure 10c) and extinction (Figure 10e). Clear cyclonic circulations were also found in the 850 hPa wind field (Figures 10d, f). At the conclusion of tracking in the VB2020, the depression centers tracked by the HB2015 no longer have closed stream function contours in their proximity (Figure 10g), and the cyclonic circulations in the 850 hPa wind field are also dissipating (Figure 10h).
Depressions 6–8 all formed in the tropical West Pacific region. They were generated to the south of the WPSH in September–October. In this case, the tracking again started earlier and ended later for the HB2015 than the VB2020. Figure 11 shows the evolution of a unique monsoon depression (#7 in Table 2). It is labeled as northward moving (green line) by the VB2020 and as two separate MLPSs by the HB2015: a monsoon low (purple curve) in the early stage and a depression with a path direction change (blue line) in the late stage. At 18:00 on September 27 (Figures 11a, b), the system was recognized by the VB2020 as a monsoon depression, with its center (green dot) located in a zone of relatively high vorticity and surrounded by closed stream function contours. In the 850 hPa wind field, cyclonic circulations were also observed in the vicinity of the depression center. In this stage, the depression center and its subsequent movement path corresponded to a monsoon low (purple line) identified by the HB2015 earlier. In addition, at the time of the monsoon low’s genesis as identified by the HB2015, no closed circulation was present at the center in the 850 hPa wind field. Thus, this monsoon low could be a mis-identification by the HB2015. Figure 11c shows the distribution of the relative vorticity and the stream function at the onset of the monsoon depression tracking by the HB2015. Compared to Figures 11a and b, the depression had increased in intensity, and the cyclonic circulations at its center had become more prominent (Figure 11d). By comparing the distribution of the relative vorticity and the stream function at 850 hPa at the conclusion of the depression tracking by the VB2020 (Figure 11e) with those one time step (6hr) later (Figure 11g), it was found that this dataset sometimes fails to give a full accounting of the depression’s movement path. This could be due to the sudden acceleration in the depression’s movement. The distance traveled by the depression in the northward direction in 6 h exceeded the limiting condition set forth in the search for the next candidate point by the VB2020, i.e., within a 3° great-circle distance of the depression center. The fact that the cyclonic circulations were maintained and strengthened at the center of the 850 hPa wind field also confirms the on-going presence of the monsoon depression (Figures 11f, h).
In addition to the above monsoon depressions with overlapping centers and paths, several MLPSs were identified by both the HB2015 and VB2020, but were classified into different intensity categories. For example, two systems identified by the HB2015 as monsoon depressions (#9 and 10 in Table 2) were labeled as lows by the VB2020, and one depression (#13 in Table 2) was labeled as a deep depression. A system identified by the VB2020 as a monsoon depression (#15 in Table 2) was categorized as a monsoon low by the HB2015. This is largely due to the differences in the methods adopted by both datasets for calculating the mean sea level pressure anomaly in their classification of the MLPS intensity. The HB2015 uses the maximum sea level pressure anomaly within 500 km of the MLPS’s center (relative to the 21-day moving average), which is a comparison made with respect to time. In the VB2020, the sea level pressure anomaly is defined as the pressure difference between the MLPS’s center and the closed contour of the sea level pressure field within a 3° great-circle distance from the center, which is a comparison made with respect to space. Sometimes large differences occurred between the two methods. For example, at 12:00 on June 29, 1986, depression 9 in Table 2 has developed to its highest strength. The sea level pressure anomaly in the vicinity of the MLPS’s center was above 5 hPa, and this system was identified as a monsoon depression by the HB2015 (Figure 12a). However, the sea level pressure anomaly within a 3° great-circle distance from the MLPS’s center was less than 3 hPa and did not meet the criteria for a depression. Thus, it was classified as a monsoon low by the VB2020 (Figure 12b). In addition, three systems (11, 12, and 14 in Table 2) identified as monsoon depressions by the HB2015 failed to be classified by the VB2020. This is primarily because the stream function did not form a closed contour or the closed contour formed outside of the latitude range specified in the VB2020, i.e., the MLPS’s center was at a latitude of >35°N (figure not included).
In the above comparisons of the monsoon depressions that occurred in 1986, it was found that the monsoon depressions identified by the HB2015 had longer lifetimes and movement paths than those identified by the VB2020. There are two main reasons for this. (1) The HB2015 used a less scrutinized set of conditions to identify monsoon depression formation, and it sometimes identifies a trough as an MLPS when the cyclonic circulations have yet to form. This leads to early onset of the tracking. (2) The HB2015 often cannot make an accurate judgement as to whether an MLPS has weakened to a trough. This results in a late ending of the tracking in the HB2015 compared to the VB2020. Generally speaking, the VB2020 uses a stricter set of identification criteria for MLPS genesis and extinction and a more rigorous tracking algorithm. The monsoon depressions identified are almost guaranteed to have cyclonic circulations near their centers as they continue their movement, which is a feature observed in actual monsoon depressions. In this sense, the VB2020 is a more reliable dataset. However, the tracking performed with this dataset sometimes ends prematurely.