3.1 Validation of the hourly CTs from ERA5
Fig.1 shows the input patterns from the 9 retained components as classified from ERA5. The congruence coefficients from Table 1 between the patterns from ERA5 and NCEP-NCAR shows a one to one correspondence and were generally greater than 0.95, indicating that the input patterns presented by the scores, were well reproduced in each case. The CTs are designated by the mean SLP map in Fig. 2. Classification of the CTs in the study region at higher horizontal resolutions and with other reanalysis products and climate models resulted in input patterns with the same ordering as in Fig. 1 (not shown) except that type 7 preceded type 6 in some cases, and also the spatial structure of 8+ shows less stability, otherwise the CTs are stable in all cases even with a one to one correspondence suggesting that they are actual synoptic situations in the study region.
Table 1: Congruence coefficients between scores from ERA5 and corresponding scores from NCEP-NCAR
Component
|
Congruence coefficient
|
1
|
0.99
|
2
|
0.99
|
3
|
0.99
|
4
|
0.99
|
5
|
0.96
|
6
|
0.95
|
7
|
0.98
|
8
|
0.93
|
9
|
0.93
|
Fig. 3 shows the relative frequency of occurrence of the CTs from the hourly ERA5 classification and the daily classification from NCEP-NCAR. It can be seen that regardless of the choice of the reanalysis product, temporal resolution, and classification period, the relative frequency of occurrence of the CTs shows satisfactory stability. 1+, 2+, 3+, and 4+ have a relatively higher probability to occur. They can be understood as mean patterns based on their persistence for a longer time. 1+ is the most frequent and the climatology of atmospheric circulation in the region. The annual occurrence of the CTs (not shown) indicated that 1+ is dominant in austral winter, whereas 3+ is specifically the austral summer climatology since it is the most frequent austral summer pattern. 2+ dominates almost in all seasons and 4+ also is dominant in austral summer. The input patterns of type 3 and type 4 (Fig. 1) indicate two major scenarios of variability in the semi-permanent high-pressure systems during austral summer. Type 3 (i.e. 3+ in this case) shows the west to east movement of the semi-permanent high, which ridges through the Agulhas current towards eastern South Africa. Type 4 (i.e. 4+ in this case) shows a scenario where the western portion of the Mascarene high weakens and the mid-latitude disturbances track further north, allowing the cyclonic system from the Mozambique Channel to move further southwest since atmospheric blocking by the western branch of the Mascarene high is diminished.
3.2 Selections of CTs with tropical cyclone characteristics
TC in the southern hemisphere is usual in austral summer and early austral autumn. Considering the high heat capacity of ocean water, the SST threshold (about 26°C-28°C) necessary for the development of TC can be slowly attained, resulting in why early austral autumn might be the period favorable for TC development in the Mozambique Channel. From the input patterns in Fig. 1, type 9 clearly shows a strong negative anomaly in the Mozambique Channel, apparently blocked by a positive anomaly positioning at the western branch of the Mascarene high. From Fig. 2, 9+, a strong cyclonic anomaly subjected to atmospheric blocking by the Mascarene high is evident. Based on the composites from precipitable water (PW), wind vector at 850 hPa (Fig. 4), and relative vorticity (not shown), 9+ is the major CT that presents a synoptic state favorable for TC in the Mozambique Channel. According to Jury and Pathack (1991) during TC season in the Mozambique Channel, lower level winds are westerly at about 15°S northward. This is evident in 9+ from Fig. 4; since the strong cyclonic anomaly adjusts the cross-equatorial easterly winds to become predominantly westerly towards the northern part of the Channel. Concerning the strong correlation between SST, SLP, and PW and also that the transfer of energy that strengthens the radial circulation within a TC mainly comes through the evaporation of water into the atmosphere, PW is an important field in the monitoring and prediction of the intensity and track of TC. Fig. 4 shows that for 9+ PW is relatively enhanced in the Channel and at the east coast of Madagascar. An enhancement of PW implies a drop in SLP and an increase in SST. At the upper levels, PW is equally related to the mass of clouds, thus even though this analysis has not explicitly incorporated more upper-level variables in the selection of CTs with TC characteristics, PW might still be sufficient to approximate an atmospheric condition under which TC might develop. From Fig. 5, the annual occurrence of 9+ dominates from February to April, which corresponds to the TC season in the southern hemisphere.
6+ is another CT selected to have TC characteristics though with a lesser intensity of the low-pressure system in the Mozambique Channel compared to 9+, and also with a different track altogether. Its input pattern from Fig. 1 shows a strong anomaly at the southern landmasses, extending eastward into the southwest Indian Ocean. Unlike in 9+, the western branch of the Mascarene high is weakened allowing the cyclonic system to progress further southwest towards the Agulhas current. Its dominating period from Fig. 5 is in the peak of austral summer when continental heating is highest; suggesting that diabatic heating might play a role in generating the positive SST anomalies in the southwest Indian Ocean. It is equally evident that under 6+ cross-equatorial easterly winds are well expressed, transporting moisture from the tropical Indian Ocean into the Channel. PW under 6+ extends also southwest, following the weakening of the western branch of the Mascarene high.
3+ and 4+ though not explicitly selected as TC types should be noted. Since they are austral summer mean patterns, their persistence for a longer time, relative to the winter dominant CTs is crucial in creating the long-term atmospheric condition under which TC might develop in the Channel. Moreover, the persistence of their signal will aid in enhancing the TC characteristics of the CTs designated to have TC features, whereas the occurrence of winter dominant CTs such as 4- and 6-, associated with the dominance of high-pressure system over the southwest Indian Ocean might have a buffering effect on the tropical cyclonic characteristics of the TC types - related to drier air which can inhibit moist convection. 5- presents a synoptic state similar to 6+, concerning a weak state of the western branch of the Mascarene high, thus allowing warm SST in the southwest Indian Ocean. However, Fig. 4 shows that PW is higher north of Madagascar suggesting that TC might develop mainly towards the north of the Mozambique Channel under this synoptic state. 7- is also similar to 9+ in terms of the strength of the circulation in the south Indian Ocean high pressure, though the circulation appears relatively stronger under this synoptic state, with the cyclonic system in the Mozambique Channel being weaker. The strong easterly wind associated with a stronger south Indian Ocean high-pressure might aid in steering TCs westward towards southern Africa. However, as shown in the case of 9+ (Fig. 4) should the intensity of the cyclone in the Mozambique Channel be high, easterly trade winds might be adjusted to westerly rather than steering the cyclonic system to move westward. Hence the mean wind in the Channel will decide if the TC can be steered by either the southeast or northeast trade winds. Moreover, the convergence of the easterly winds in the Channel can influence the storm development through the vorticity it induces. To this end, 9+ and 6+ are the major CTs with synoptic states favorable for the development of TC in the Mozambique Channel, whereas 3+ and 4+ are vital for creating the long-term atmospheric conditions for both the development of the TC and the occurrence of the TC types, whereas 5- and 7- might play vital roles in the development of TC north of the Channel and in its track. Generally, 9+, 7- and 3+ commonly enhance blocking of the cyclone from moving further south; on the other hand, 6+, 5- and 4+ commonly feature the weakening of atmospheric blocking by the Mascarene high, allowing the TC to move further south.
Correlation analysis between the time series of the patterns and teleconnection indices such as the Nino3.4 index, SAM index, IOD index, and the SIOD index revealed that 5- and 6+ are related to ENSO (R=0.48 and R=0.31 respectively); 9+ is related to the IOD (R=0.42); 7- is related to SAM (R=0.25) and 6+ is related to the SIOD (R=0.33). This shows that the CTs might as well be constrained by other low-frequency modes of variability. Specifically, the strong relationship between ENSO and type 5 (i.e. 5- and 5+) reveals that during the different phases of ENSO the western branch of the Mascarene high can be weakened (as in 5-) or strengthened (as in 5+). The different phases of the IOD can be related to type 9 through SST anomalies in the Mozambique Channel. The positive and negative phases of the SAM can be related to type 7 through suppression of the easterly circulation of the Masacerene high when the polar fronts track northward (as in 7+) and enhancement of the circulation at the Mascarene high when the polar fronts track poleward (as in 7-). Finally, the SIOD is related to type 6 since in its positive phase the southwest Indian Ocean is anomalously warm, enhancing cyclonic activity over there as in 6+, while in its negative phase the reverse can be inferred as in 6-. Thus the CTs are related to these teleconnections directly through the enhancement and weakening of the easterly anomalies at the Mascarene high and SST anomalies at the southwest Indian Ocean.
3.3 2019 TC season in the Mozambique Channel and influence of the selected TC circulation types
In this section, it will be investigated if the hourly occurrence of CTs, according to the classification output, influences the time development and track of TCs in the Mozambique Channel. For this purpose, the 2019 TC events of TC Idai and TC Kenneth were analyzed. In each case, the hourly occurrence of the CTs for two days before the TC reached its first maximum intensity, the day the TC reached its first maximum intensity, and a day after the TC reached its first maximum intensity were investigated. Table 2 shows the hourly occurrence of CTs, according to the classification output and for the aforementioned days. It can be seen that the CTs designated as mean patterns persist for a longer period and overlap with other CTs, and also generally, the hourly occurrence of the CTs reveal that the CTs do not readily change at an hourly scale, thus a 3 or 6 hourly classification can still be optimal while equally reducing computation time. It should be noted that a change in pressure pattern (and wind pattern) at a 3 hourly scale is realistic and might be of predictive skill in the direction TC systems will likely travel – TCs will likely move to regions where the pressure drops rapidly. Also, a large drop in pressure ahead of a cyclone might imply that the cyclone is deepening. This is the advantage that analyzing a sub-daily occurrence of the CTs can present.
Table 2: Hourly occurrence of the CTs from the ERA5 classification output for the selected days of the 2019 TC season of TC Idai and TC Kenneth
Time (UTC)
|
9 March
|
10 March
|
11 March
|
12 March
|
23 April
|
24 April
|
25 April
|
26 April
|
0:00
|
1+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
2+,3+,9+
|
1-,2+,3+
|
2+,3+
|
1+,3+,6+
|
1+,3+,4+,
|
1:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
2+,3+,9+
|
1-,2+,3+
|
2+,3+
|
1+,3+,6+
|
1+,3+,4+,
|
2:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
2+,3+,9+
|
1-,2+,3+
|
2+,3+
|
1+,3+,6+
|
1+,3+,4+,
|
3:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-
|
2+,3+,9+
|
2+,3+
|
2+,3+
|
1+,3+,4-,6+
|
1+,3+,4+,
|
4:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-
|
2+,3+,9+
|
2+,3+
|
2+,3+
|
1+,3+,4-,6+
|
1+,3+,4+,
|
5:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-
|
2+,3+,6+,9+
|
2+,3+
|
2+,3+
|
1+,3+,4-,6+
|
1+,3+,4+,
|
6:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-
|
2+,3+,4-,6+,+9+
|
2+,3+
|
2+,3+
|
1+,3+,4-,6+
|
1+,3+,4+,
|
7:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-,9+
|
2+,3+,4-,6+,+9+
|
2+,3+
|
2+,3+
|
1+,3+,4-,6+
|
1+,2+,3+,4+,
|
8:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-,9+
|
2+,3+,4-,6+,+9+
|
2+,3+
|
2+,3+
|
1+,3+
|
1+,2+,3+,4+,
|
9:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,7-,9+
|
3+,4-,6+,+9+
|
2+,3+
|
2+,3+
|
1+,3+,5-
|
1+,2+,3+,4+,9+
|
10:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+,9+
|
3+,6+,+9+
|
2+,3+
|
2+,3+
|
1+,3+,5-
|
1+,2+,3+,4+,9+
|
11:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
3+,6+
|
2+,3+
|
2+,3+
|
1+,3+,5-
|
1+,2+,3+,4+,5-
|
12:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
1+,2+,3+,4+
|
3+,6+
|
2+,3+
|
2+,3+
|
1+,3+,5-
|
1+,2+,3+,4+
|
13:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
3+,6+
|
2+,3+
|
2+,3+
|
1+,3+,5-,8-
|
1+,2+,3+,4+
|
14:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,4+
|
3+
|
2+,3+
|
2+,3+
|
1+,3+
|
1+,2+,3+,4+
|
15:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+
|
3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,8-
|
1+,2+,3+,4+
|
16:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+
|
3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,4+,
|
1+,2+,3+,4+
|
17:00
|
3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,4+,
|
1+,2+,3+,4+
|
18:00
|
2+,3+,4+,5-
|
2+,3+,4+
|
2+,3+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,4+,
|
1+,2+,3+,4+
|
19:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,4+,
|
1+,2+,3+,4+
|
20:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-,7-
|
2+,3+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+
|
1+,3+,4+,
|
1+,2+,3+,4+
|
21:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-,7-
|
2+,3+,9+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+,6+
|
1+,3+,4+,
|
2+,3+,4+
|
22:00
|
2+,3+,4+,5-
|
2+,3+,4+,5-
|
2+,3+,9+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+,6+
|
1+,3+,4+,5-
|
2+,3+,4+
|
23:00
|
2+,3+,4+,5-
|
2+,3+,4+
|
2+,3+,9+
|
1+,2-,3+
|
2+,3+
|
1+,2+,3+,6+
|
1+,3+,4+,5-
|
2+,3+,4+
|
Fig. 6 shows the mean SLP and vector wind field at 850 hPa for the selected days and Table 3 presents the field correlation between the mean SLP patterns for each day and the mean SLP pattern for the selected CTs with TC characteristics. The analyzed days for TC Idai are from 9 March to 12 March. From 9 March to 10 March 19:00 UTC, long persistence of 2+, 3+, 4+, and 5- can be seen (Table 2). Recall that 3+ and 4+ are austral summer mean patterns, and 4+, similar to 5-, presents a weaker state of the western branch of the Mascarene high. Hence the persistence of the combination of 4+ and 5- might suggest a synoptic state associated with a weaker circulation at the western branch of the Mascarene high. This was evident from Fig. 6 on 9 March - the semi-permanent high-pressure system appears weakened over the southwest Indian Ocean and relatively more southward, similar to the mean pattern of 5- in Fig. 2. A TC can be seen also towards the east coast of northern Mozambique. Recall that 5- favors the development of TC north of the Channel. On 10 March between 20:00 UTC and 21:00 UTC, 7- associated with stronger circulation at the south Indian Ocean high-pressure occurred and this might have contributed to why the Mascarene high moved a bit northward and strengthened; following a possible strengthening of convergence by the ITCZ as a result of stronger southeast trade winds penetrating the Mozambique Channel, the TC appears to develop further. Table 3 shows that on 9 March and 10 March, the SLP signal of 5- tends to be relatively dominant. On 11 March 9+ persisted from 7:00 UTC to 9:00 UTC; Table 3 shows that its signal dominated the others on this day. The SLP and wind vector pattern of 11 March shows equally a strengthening of the south Indian Ocean high-pressure and convergence of easterly winds (adjusted to westerly) in the Channel. The meteorological history of TC Idai notes that it reached its first peak intensity by 12:00 UTC on 11 March (Meteo France La Reunion 2019). Also between 21:00 UTC of 11 March and 10:00 UTC, of 12 March, 9+ persisted again so that from Table 3 its SLP signal dominates again on 12 March. Furthermore, the mean SLP and wind vector pattern on 12 March shows that the south India Ocean high-pressure strengthened more, further blocking the TC from progressing southward. On the same day (i.e. 12 March) between 5:00 UTC and 10:00 UTC, 6+ occurred together with 9+, but from Table 3, the signal of 9+ tends to persist relatively. It is also remarkable that 4- occurred from 6:00 UTC to 9:00 UTC. 4- is a winter dominant pattern and Fig. 2 suggests that through the subsiding motion (enhanced anticyclone) it brings in the southwest Indian Ocean, it might inhibit deep convection in the basin. Its occurrence implies that the TC might further weaken, and from the meteorological history of TC Idai (Meteo France La Reunion 2019), it was reported that relative to its strength on 11 March, Idai weakened on the 12 March at the same time (6:00 UTC) the classification output noted that 4- started to occur. According to Oguejiofor and Abiodun (2019), an increase in surface pressure over a TC basin can be related to a decrease in SST, maximum precipitation rate, and wind speed. In a scenario that the classification is solely focused on the TC season, the occurrence of a winter type will not be captured, and this might be a further example of the pitfalls following the oversimplification of synoptic classifications.
Table 3: A measure of the signal of the CTs with TC characteristics during the selected days of the 2019 TC season of TC Idai and TC Kenneth
CT
|
9 March
|
10 March
|
11 March
|
12 March
|
23 April
|
24 April
|
25 April
|
26 April
|
5-
|
0.68
|
0.74
|
0.54
|
0.47
|
0.55
|
0.63
|
0.81
|
0.80
|
6+
|
0.44
|
0.58
|
0.59
|
0.56
|
0.55
|
0.73
|
0.78
|
0.75
|
7-
|
0.46
|
0.66
|
0.66
|
0.46
|
0.57
|
0.67
|
0.67
|
0.76
|
9+
|
0.61
|
0.74
|
0.73
|
0.69
|
0.72
|
0.78
|
0.75
|
0.74
|
Coming to TC Kenneth, on 23 April and 24 April the SLP and wind vector field from Fig. 6 shows that the TC was situated north of Madagascar with the Mascarene high northward and strengthened. The classification output showed that 2+ and 3+ which are both associated with the west to east movement of the semi-permanent high-pressure system, ridging through the Agulhas region to eastern South Africa, exceptionally persisted from the onset of 23 April till 21:00 UTC on 24 April when 6+ co-occurred with them. 6+ persisted for the rest of day till 7:00 UTC of 25 April. Recall that 6+ implies a weakening of the western branch of the Mascarene high, enabling the TC to progress further southwest. Also, 5- with a similar feature as 6+ persisted for 5 hours on the same day between 9:00 UTC and 13:00 UTC. 8-, from Fig. 2, equally supports weakening of semi-permanent high-pressure system; from Table 2 it can be seen to have occurred at some points on 25 April. Also between 22.00 UTC and 23:00 UTC 5- occurred again. The implication of the persistence of the CTs that favors the weakening of the western branch of the Mascarene high is reflected in the mean map for 25 April (Fig. 6). Cross equatorial easterly winds are stronger, penetrating further southwest and steering the TC towards the east coast of northern Mozambique. Southeast winds are weakened with the Mascarene high situated further southeast and cyclonic circulation tends to enhance southwest towards the Agulhas region. The meteorological history of TC Kenneth reported that it reached its first maximum intensity on 25 April (Meteo France La Reunion 2019). From Table 3, the signal of 6+ and 5- dominates on this day. Hence it can be inferred that unlike TC Idai, the reason why TC Kenneth remained most active at the northern Mozambique Channel during its maximum intensity on 25 April can be partly as a result of the stronger signal of 5-, followed by 6+ which both favors the buildup of TC towards the north of the Channel, basin-wide warming of the southwest India Ocean and enhancement of the cross-equatorial northeast trade winds steering the TC southwestward. On 26 April 5- occurred only at 11:00 UTC, whereas the occurrence of 2+, 3+, and 9+ which are all in favor of the stronger west to east progression of the semi-permanent high-pressure system can counter the warm SST in the southwest Indian Ocean induced from the previous day synoptic states. Though from Table 3, 26 April, the signal of 5- dominates, the SLP field of 26 April (Fig. 6) clearly shows the low-pressure system located further south away from the Channel, where it will be possibly weakened by the eastward-moving high-pressure systems.