Master thesis

Figure 3. 12 Mean surface air temperature (˚C) over ICP at ten different pentads after monsoon onset. 34

3.4 Chapter Summary This chapter presented the climatological analyses of precipitation along with atmospheric variables which known to influence precipitation distribution. The gradually increase in amount of precipitation from April is caused by several systems among which are the monsoon onset and northward migration of ITCZ starting from April and reached its highest point in August and account for more than 80% of the total precipitation range (200 mm) as the threshold to determine wet season months, the 20-year average of monthly mean precipitation over ICP. Monthly spatial climatology of precipitation over ICP during 1998-2017, results which coexistence with its annual cycle. It also supports the results of past studies where the MJJASO is the wet season over ICP, with a south and southeast to north oriented precipitation zone dominating the region. The streamlines show the presence of BoB trough or the low-pressure system during the wet season months. Its position, which, is migrated in all dimension throughout the year, sometimes it moves northward into Indian subcontinent. Another magnificent feature is the direction of the wind flow over ICP which has southwesterly characteristics during the wet season. From May to October, the wind flows from southwest to northeast over ICP while from November to April, the wind changes its direction to the northeasterly. The air temperature over ICP is observed to be less fluctuated year-round. High temperature was observed over the center dry zone and maximum temperature is in June then declined in the following month. This might due to the heavy downpours of precipitation. However, instead of this region receiving more precipitation due to instability of the atmosphere, this area received less precipitation as compared to the rest of the peninsula. The maximum of MFC flux into the ICP is during MJJASO which also consistent with the precipitation analyses. The results indicated that centered west and northwest have much more MFC than the rest, but according to precipitation analyses the area has more rainfall is south and southwest. From November the wind starts to flow northeasterly dry air from Asia continent and north PO flow into ICP and dry season is started. Using three monsoon onset techniques to quantified and defined the monsoon onset, namely normalized precipitation index, Webster-Yang circulation index and climatological pentad mean 35

rainfall index. The results showed that CPMI is the suitable one, which has mean value equal to NPWI but less deviated and has high correlation with WYI. After monsoon onset, rainfall over ICP gradually increase, on the other word, more rainfall in the following pentad. This is consistent with the low-levels wind direction over the region, which dominated by southwesterly, brought in sufficient moisture, thus heavy downpour in the following pentad. These heavy rainfall cause dropping of surface air temperature which relatively high before monsoon onset. Although the rainy season over ICP is MJJASO, further analyses will consider only 10 pentads after monsoon onset of each year since the spectrum analyses shows the oscillation of the precipitation is around 6-pentad. 36

CHAPTER 4 BAY OF BENGAL TROUGH AND ICP RAINFALL MODES 4.1 EOF Modes of Vorticity and Rainfall after Monsoon Onset 4.1.1 EOF of Vorticity at 500 hPa over the BoB To analyze further the dominant pattern of the BoB trough at 500 hPa level, the EOF analysis method is used. The spatial vectors present in Figure 4.1 and corresponding standardized time coefficient of the four EOF modes analyzed in this study. The percentage variability of the first and the second modes were 33.3% and 23.7%, respectively, which account for 57.0%, reflecting the dominant spatio- temporal variation in circulation over the BoB. The four EOF modes revealed four distinct circulation patterns. First EOF mode well displayed by negative vorticity over the center and cover all the study domain, whereas just only a very tiny spot on the northwest of the domain was displayed positive vorticity. The spatial vectors of the second EOF mode displayed positive vorticity to the north and negative vorticity to the south which is tilt west-east of study domain. Third EOF mode had a percentage variability of 10.6%, showed positive vorticity to the east and north, while negative vorticity spot is located to the southwest of domain. Fourth EOF mode (6.5%) displayed northwest-southeast oriented with negative vorticity while northeast and southwest of the study domain are captured by positive vorticity, respectively. Another interesting feature of the four EOF spatial vectors is that the uniform capturing by negative vorticity at the first EOF mode, positive to the northwest negative to the southeast in the second, positive to the east negative to the west and so on, which suggesting that the circulation has strong west-east migration. The time coefficient of EOF (principal component PC) Figure 4.2 displayed the temporal variability of the patterns observed in spatial vector of EOF 1-4. There is no such a clear oscillation period in the PC of the different modes. To further explore the factors associated with the observed spatial patterns of the EOF modes, the higher positive values from PC 1 to 4 were used for composite analysis. The data we used is according to the reconstructed pentad data and the 37

threshold data are defined in Table 4.1. And only the first two EOF modes, which represent the most dominant and majority of the percentage variability, were used for the further analyze. Figure 4. 1 Four EOF vector(a) first, (b) second, (c) third, (d) in the standardized anomaly field of the ten pentads reconstructed vorticity over BoB during 1998-2018. 38

Figure 4. 2 Four PC: (a) PC1, (b) PC2, (c) PC3, (d) PC4 in the standardized time coefficient of the ten pentads reconstructed vorticity over BoB during 1998-2018. Table 4. 1 Strong and weak pentad obtained from standardized time coefficient of the four EOF modes of BoB vorticity at 500 hPa EOF analysis. EOF Mode Threshold Strong Year (pentad) EOF 1 2 2001(2), 2003(3), 2007(1), 2010(2), 2012(1), 2013(4) EOF 2 2 1998(10),2006(9),2010(9), 2013(8), 2015(9,10), 2017(10) EOF 3 2.5 EOF 4 2 2000(7), 2006(1), 2007(5) 1999(3,10), 2003(3) 2009(6), 2013(7,8) 39

Figure 4.3 show precipitation distribution of the first (a) and second EOF (b) modes of strong pentad. The first EOF, in strong pentad, precipitation distribution over ICP is nearly uniform dry. However, a relatively high precipitation bands are observed along the coastal region, for instance, south of Myanmar, Thailand, Cambodia and Vietnam. Another precipitation band to the northeast of the peninsula appeared over northern Vietnam and center of Laos. However, these precipitations are not such a high precipitation if compare to the normal precipitation over the region. In second EOF mode, the precipitation zone is well observed over the southwest and northeast of the domain. Which Myanmar’s coastal region received heavy precipitation during this period, whereas, relative high precipitation to northeast. Both precipitation distributions captured well the observed pattern in the spatial vector of the EOF1 and EOF2, which are consistent with the northwest to southeast orientation of the ICP’s dry zone over the center of the peninsula. Figure 4. 3 Precipitation distribution over study domain for composite pentad from first EOF, (a); and second EOF (b). 40

4.1.2 EOF of Rainfall over ICP Figure 4.4 presents the spatial vectors and corresponding standardized time coefficient of the four EOF modes analyzed in this study. The percentage variability of the first and the second modes were 25.9% and 8.8%, respectively, which account for 34.7%, reflecting the dominant spatio- temporal variation in precipitation over the ICP. The four EOF modes revealed four distinct precipitation patterns. First EOF mode displayed uniform positive loading but the strongest positive loading is southwest of the study domain, which is over the Andaman Sea (AS). The spatial vector of the second EOF mode displayed north of the ICP zone positive loading, extending from south China and north Vietnam west to south-center of Myanmar. Negative loading is captured southern Vietnam, southern Laos, Cambodia, centered Thailand and AS. Third EOF (6.0%) displayed southeast-northwest oriented of negative loading cover nearly all the ICP and showed positive loading over AS and south China. Fourth EOF (4.9%) showed east-west oriented of negative loading cover center Vietnam, Laos, Cambodia, center and north Thailand and center Myanmar. And the positive loading is captured south-southwest and northeast of the study domain. Another interesting feature of four EOF spatial vectors is that the first EOF pattern well agreed with the ten pentads climatology of precipitation (Figure 3.4), suggesting that the precipitation distribution over ICP has low variability. The time coefficient of EOF (principal component PC) Figure 4.5 displayed the temporal variability of the patterns observed in spatial vector of EOF 1-4. There is no such a clear oscillation period in the PC of the different modes. To further explore the factors associated with the observed spatial patterns of the EOF modes, the higher positive values from PC 1 to 4 were used for composite analysis. The data we used is according to the reconstructed pentad data and the threshold data are defined in Table 4.2. And only the first two EOF modes, which represent the most dominant and majority of the percentage variability, were used for the further analyze. 41

Figure 4. 4 Fourth EOF vector (a) first, (b) second, (c) third, (d) in the standardized anomaly field of the ten pentads reconstructed precipitation over ICP during 1998-2018. 42

Figure 4. 5 Fourth PC: (a) PC1, (b) PC2, (c) PC3, (d) PC4 in the standardized time coefficient of the ten pentads reconstructed precipitation over ICP during 1998-2018. Table 4. 2 Strong and weak pentad obtained from standardized time coefficient of the four EOF modes of ICP rainfall EOF analysis. EOF Mode Threshold Strong Year (pentad) EOF 1 2.5 2002(6), 2012(9), 2015(10), 2016(4) EOF 2 2 1998(9), 2001(9), 2002(6), 2008(10), 2016(8) EOF 3 2.5 2015(10), 2016(8) EOF 4 2 1998(9,10), 1999(5), 2004(5), 2005(7), 2010(5) 43

The precipitation distribution according to EOF of rainfall for the extreme pentad are shown in Figure 4.6. both precipitation distribution captured well the observed pattern in spatial vector of the EOF1 and 2, which are consistent with west and southwest of ICP precipitation band. During the first EOF, west and southwest of the ICP received heavy rainfall, while northeast of Thailand received relatively less precipitation when compared to other region. On the other hand, in second mode displayed north and northeast of ICP captured more rainfall compared to first EOF and south and southwest received less rainfall. During this time southeast of the ICP, for instance, east Thailand, Cambodia and southern Vietnam experienced less rainfall also. From the first and second mode, the precipitation zone can be suggested to have shifted northeastward over ICP. Figure 4. 6 Precipitation distribution over study domain for composite pentad first EOF, (a); and second EOF (b) from ICP precipitation EOF analyses. 44

4.2 Bay of Bengal Trough in modes 4.2.1 Based on Vorticity at 500 hPa over BoB EOF The BoB trough features at three different level from two modes was presented in Figure 4.7. during the first mode, BoB presented relatively weak, retreat north and shallow. The subtropical high was observed to moved west at 850 hPa. The position of anticyclone is central to the moisture advection and also inhibition of precipitation when it moves closer to the continent. 45

Figure 4. 7 BoB trough features at different levels 850, 700 and 500 hPa based on EOF1 and EOF2 of vorticity EOF analysis. Moreover, at 700 and 500 hPa, the Mascarene high, normally located over IO, also get intensified, thus resulting less precipitation observed over ICP. During the second mode, the BoB trough get 46

intensified in all dimension, which clearly observed at low-levels. The low-pressure system over Bangladesh, is a central of dynamics influence and contributed to the high precipitation over ICP during this period. 4.2.2 Based on Rainfall over ICP EOF High-pressure or anticyclone is an atmospheric pressure distribution in which there is a high central pressure relative to surroundings. It is characterized on a synoptic chart by a system of closed isobars, generally approximately circular or oval in form, enclosing the central high- pressure. While low-pressure or cyclone is rising of air. As it rises and cools, water vapor condenses to form clouds and perhaps precipitation. Consequently, the weather in a low-pressure system is often cloudy, wet, and windy. There is usually a trough at higher levels associated with the low-pressure system. In this study, we have focused on determining BoB trough and its associated characteristics to ICP rainfall. Figure 4.8 displayed features of BoB trough at three different levels based on first and second modes of EOF analysis. Which indicating that, when the BoB trough extended southeast to AS and occupied nearly the same shape over low-levels, in other word, BoB have occupied a deeply extended. This situation caused overall of ICP experienced heavy rainfall, especially, coastal area of Myanmar as in 4.6. 47

Figure 4. 8 BoB trough features at different levels 850, 700 and 500 hPa based on EOF1 and EOF2 of rainfall EOF analysis. 48

4.3 Structures of Bay of Bengal Trough 4.3.1 Based on Vorticity at 500 hPa over BoB EOF Figure 4.9 shown vertical cross-section of specific humidity along 90°E was selected based on the position of the BoB trough during this period of time. During the EOF1, over southern BoB occupied relatively less specific humidity, on the other word, descending of dry air is central around latitude 18°N, thus suppressing the thickness of specific humidity. And then decreasing northward, indicating that, insufficient moisture played a significant role in inducing relatively dry spell over the region. On the other hand, Figure 4.9 (b) displayed relatively high specific humidity according to the second EOF analysis. Relatively high specific humidity implying that, the area receiving more moisture transported into the region, thus resulting in more rainfall observed as in Figure 4.3. Figure 4. 9 Vertical cross section of specific humidity (kg kg-1) along 90°E according to EOF1 (a) and EOF2 (b). Figure 4.10 displayed vertical cross-section of specific humidity along 95°E, indicating that, eastward into ICP the thickness of specific humidity layers increased during EOF1. Whereas, during the EOF2 opposite feature observed. 49

Figure 4. 10 Vertical cross section of specific humidity (kg kg-1) along 95°E according to EOF1 (a) and EOF2 (b). In baroclinic region, ascending air is associated with convergence at lower-level and divergence at high-level, while descending air is associated with divergence at lower-level and convergence at the high-level. In contrast, barotropic region, the convergence (divergence) at lower-level might lead to increase (decrease) column of air. Convergence at lower-level leads to vertical stretching, whereas divergence leads to vertical shrinking which suppresses convection due to air sink. The vertical velocity analyses (Figure 4.11) shows latitudinal rising and sinking air in EOF1 and 2 along 90°E. The area along latitude 10°N to 30°N were dominated by descending of air during EOF1, which consistent with specific humidity displayed in Figure 4.10. Therefore, lacking of moisture caused ICP experienced less rainfall. On the other hand, during EOF2, rising motion observed 25°N to 30°N, thus more rainfall over ICP observed (Figure 4.3). Analyzing the vertical cross-section of air temperature along 85°E from the surface up to 200 hPa during two modes are presented in Figure 4.12. The temperature profile showed the air column, both above land and ocean, had warmer temperature until higher elevations. The high surface air temperature implied that, during these periods of time the area were dominated by instability. However, it also showed that, surface air temperature reduced during second mode. 50

Figure 4. 11 Vertical velocity (Pa s-1) along 90°E according to EOF1 (a) and EOF2 (b). Figure 4. 12 Vertical cross-section of air temperature (°C) along 85 °E according to EOF1 (a) and EOF2 (b). 51

4.3.2 Based on Rainfall over ICP EOF Figure 4.13 shown vertical cross-section of specific humidity along 90°E. The relatively thick specific humidity layers observed in both two modes. It implied that the area was abundance of moisture, which resulted in more rainfall observed. Moreover, height elevations to the north was observed to played significant role like topographical barrier, thus lift up the air. Moisture air rise the water vapor condensed, thus precipitation. Figure 4.14 shown vertical cross-section of specific humidity along 95°E. The thickness of specific humidity layers decreased, in both modes, compare with the thickness observed along 90°E. On the other word, moving eastward, amount of water vapor in the air column get condensed, thus precipitation observed. Figure 4. 13 Vertical cross section of specific humidity (kg kg-1) along 90°E according to EOF1 (a) and EOF2 (b). 52

Figure 4. 14 Vertical cross section of specific humidity (kg kg-1) along 95°E according to EOF1 (a) and EOF2 (b). The vertical velocity analyses (Figure 4.14) shows latitudinal rising and sinking air in EOF1 and 2 along 90°E. during the EOF1, the area along 20°N were dominated by strong rising of air. Therefore, moist air mass rise, water vapor condenses, thus precipitation occur. Therefore, consisting with precipitation observed in 4.6, during these two modes ICP experienced heavy rainfall. Figure 4. 15 Vertical velocity (Pa s-1) along 90°E according to EOF1 (a) and EOF2 (b). 53

Analyzing the vertical cross-section of air temperature along 85°E from the surface up to 200 hPa during two modes are presented in Figure 4.16. The temperature profile showed an air column, both above land and ocean, had warmer temperature until higher elevations. The high surface air temperature implied that, during these periods of time the area were dominated by instability, thus also indicated strong rising motion of air. Figure 4. 16 Vertical cross-section of air temperature (°C) along 85 °E according to EOF1 (a) and EOF2 (b). 4.4 Chapter Summary This chapter described and analyzed the different modes of BoB trough and ICP rainfall after monsoon onset during 1998-2018, in context of structural features. The results were assessed from a structural angle in order to achieve deeper understanding of the physical mechanisms modulating the variability in precipitation and thus to explore the possibility of enhanced knowledge of the subject. For the case of EOF analyze of vorticity over BoB, the first mode and second mode described 33.3% and 23.7% variability, respectively, which agreed with precipitation pattern observed over ICP. The precipitation pattern during the first mode was relatively drier and southern part receive more rainfall while the precipitation pattern during second mode was center west and east of ICP. 54

EOF analysis of rainfall over ICP, the first mode and second mode described 25.9% and 8.8% variability, respectively. In the first mode, positive loading is observed overall the ICP, but the strongest one center around southwest of ICP, which consistent with climatological precipitation distribution observed after monsoon onset. During the second mode, positive loading is dominated over the north and negative loading dominated south of ICP. Comparing the precipitation distribution for the first and second EOF mode, resulting that during the first mode ICP received more rainfall than the second. Vertical cross-section is analyzed in order to get deep insight into the structural features of BoB based on first two EOF of vorticity over the BoB and ICP rainfall analysis. During two modes of vorticity over the BoB EOF analysis. Along 85°E, less specific humidity is observed which implied that relatively dry air mass dominated over the region. The vertical velocity displayed rising of air, however due to lacking of moisture, less precipitation observed. High surface air temperature was observed. During two modes of rainfall over ICP EOF analysis. Along 85°E, high specific humidity is observed which implied that moist air mass dominated over the region. The vertical velocity displayed rising of air, thus heavy precipitation observed. High surface air temperature was also observed and column of high air temperature extended until height elevation 850 hPa. Which meant strong rising of air and unstable of air column. 55

CHAPTER 5 THE RELATIONSHIP OF THE BAY OF BENGAL TROUGH AND ICP RAINFALL According to previous analyses, the BoB trough and precipitation over ICP, have displayed a rate of variability in time. In order to grasp the linkage between BoB trough and ICP precipitation, causation and influencing factors of BoB trough related to ICP rainfall are investigated. 5.1 Influencing factors under BoB vorticity modes 5.1.1 SST and SST anomaly Figure 5.1 exhibited SST mode over the BoB, northeast IO and adjacent sea for extreme pentad obtained from EOF 1 and 2 of EOF analysis over BoB vorticity at 500 hPa. The first SST pattern (a) displayed a positive SST anomaly over the BoB and it showed that centered of the bay was dominated by warm SST anomaly whereas negative SST anomaly is observed over China coastal region during this pentad. It seems opposite in second pattern (b), which displayed negative SST anomaly over north IO. The major difference between the two could be note over northeast IO at around latitude 8°N and 13°N where the intensity of the signal was strongly positive, whereas negative in the second EOF mode. The positive anomaly over the bay contributed to increased evaporation and therefore resulted in a moister maritime air mass over the region. Cool SST anomaly also associated to the pressure gradient oriented from the ocean toward land. This pressure gradient encouraged moisture advection toward ICP and thus favored precipitation. 56

Figure 5. 1 SST anomaly (˚C) for the composite pentad from (a) EOF1 and (b) EOF2 over northeast IO of the BoB vorticity at 500 hPa EOF analysis. Table 5. 1 The critical values for correlation test base on 2 degree of freedom and two-tailed test. Levels of significant for two- 0.10 0.05 0.02 0.01 tailed test 4 0.729 0.811 0.882 0.917 n 5 0.669 0.754 0.833 0.874 6 0.622 0.707 0.789 0.834 Table 5.1 displayed the critical values which take into consideration base on 2 degree of freedom and n sample. Therefore, in the future analysis, the critical values were applied. 57

Figure 5. 2 spatial distribution of correlation coefficient between SST and ICP rainfall during EOF1 (a) and EOF2 (b). In order to see the connection among SST over IO and ICP rainfall, Figure 5.2 displayed the correlation coefficient between two variables. During the EOF1 (a), a positive relationship observed over the center Arabian Sea and area scatter around east coast of Africa and west coast of Australia. However, during the EOF2 (b), positive relationship between the two variables are observed to be scatter over larger domain compare to the EOF1. Whereas, weak negative relationships are observed in relatively small area over IO in both times. 5.1.2 Temperature and temperature anomaly Figure 5.3 displays the air temperature anomaly (1000 hPa) over BoB and ICP. The pattern of warm temperature anomaly was captured over the continental India and ICP during the first mode. Moreover, the Andaman Sea is captured by warm air temperature anomaly whereas BoB and east coastal of India are cool anomaly. The opposite manner is observed during the EOF2. Indian 58

subcontinent and west and southwest of the ICP are captured by cool air temperature anomaly and the warm spot are moved to northeast of ICP and China. Figure 5. 3 Air temperature anomaly (˚C) for the composite pentad from (a) EOF1 and (b) EOF2 over BoB and ICP of the BoB vorticity at 500 hPa EOF analysis. The correlation coefficient analysis results shown in Figure 5.4 displayed the significance of amount of precipitation over ICP on surface air temperature. Which captured negative relationship over the northeast part during EOF1 and mainly Thailand during the EOF2, implying that the amount of rainfall had played negative impact on surface air temperature. On the other word, cooling effect of the surface air temperature by rainfall. 59

Figure 5. 4 spatial distribution of correlation coefficient between air temperature at 1000 hPa and ICP rainfall during EOF1 (a) and EOF2 (b). Comparing the actual temperature over the BoB and adjacent sea for the sea surface and atmosphere for the both EOF modes (Figure 5.5), the result showed the SST was higher than the air temperature. Thus, the system was implied to be unstable, which resulted in rising motion of the lower air and favoring for precipitation. However, less precipitation observed this might due to insufficient moisture over the region at this time. The worth mention spot during second mode is, over coastal land area of Myanmar, low surface air temperature observed, which concurrent with high rainfall observed in Figure 4.3. 60

Figure 5. 5 Sea surface temperature and air temperature at 1000 hPa (˚C) for the composite pentad from EOF1 and EOF2 over BoB, northeast IO and adjacent sea of the BoB vorticity at 500 hPa EOF analysis. 5.1.3 Geopotential height anomaly Results of geopotential height anomaly for EOF1 and EOF2 over the study domain at 850, 700 and 500 hPa are shown in Figure 5.6. During EOF1, at 850 hPa, the low-pressure system is observed, however it was relatively shallow and mostly located over northeast India and Bangladesh. There was also a high-pressure zone located at centered of BoB around 17°N. Position of anticyclones is central to the descending of dry air and caused an area to experienced dry spell when it migrates toward the ICP. At 500 hPa, the low-pressure system is very weak and migrates to northeast India and mainly over Bangladesh, which a ridge remains identified in this level over BoB. During EOF2, at 850 hPa, two major low-pressure systems were observed, one over the BoB. 61

The BoB was very strong and deep south over Sri Lanka and the trough remain visible at 500 hPa. Moreover, at high level, there was a center of high zone exist. In this circumstance, low-pressure at low-level and high-pressure at high-level, the latter may have contributed to high precipitation observed over south Asia and ICP (Figure 4.3). The strong low-pressure over east India may be caused by topography of the area, however, the high temperature also can lead to formation of low- pressure system. Thus, the low and high-pressure systems over the region play a significant role in moisture advection and precipitation over the area. Figure 5. 6 Geopotential height (m2 s-2) for the composite pentad from EOF1 and EOF2 over BoB and ICP at three different levels of the BoB vorticity at 500 hPa EOF analysis. 62

5.1.4 Wind circulation anomaly and wind divergence Wind circulation anomaly at three different level over the study domain are present (Figure 5.7) and is consistent with geopotential height anomaly observations (Figure 5.6). The important features were over the center of the BoB. During the first mode, a deep anticyclonic circulation was present over the center of the BoB at low-level 850 and 500 hPa. Whereas, at upper level 200 hPa, the cyclonic circulation existed over southern Himalaya and there was mostly southwesterly and westerly over BoB and ICP, thus leading with a strong sinking motion over the area and contributing to inhibition of precipitation. During the second mode, a cyclonic circulation was present over south of Bangladesh at the low- level 850 hPa. This result is consistent with the observation made in Figure 5.6. The position of cyclone is importance for rising air inducing precipitation over the region. At 500 hPa, there were couple of cyclonic circulation existed center over BoB, northern ICP and Cambodia, respectively. Anticyclonic circulation was also observed over equatorial IO around 85°E, which plays an important role in moisture advection toward the low. On the other hand, at 200 hPa, easterly was dominated overall the domain. However, the system within ICP also plays a significant role in determining precipitation distribution. IO is important source of moisture for ICP. 63

Figure 5. 7 Wind circulation anomaly (m s-1) for the composite pentad from EOF1 and EOF2 over BoB and ICP at three different levels of the BoB vorticity at 500 hPa EOF analysis. 64

Figure 5. 8 Wind divergence (m s-1), shaded area indicated convergence (negative) and divergence (positive) for the composite pentad from EOF1 and EOF2 over BoB and ICP of the BoB vorticity at 500 hPa EOF analysis. Divergence occurs when a stronger wind moves away from a weaker wind or when air streams move in opposite directions. When divergence occurs in the upper levels of the atmosphere it leads to rising air. The rate the air rises depends on the magnitude of the divergence and other lifting or sinking mechanisms in the atmosphere. Divergence of wind at 850 hPa showed in Figure 5.8, displayed divergence of wind to the south of the peninsula in both modes and convergence to the north. 65

5.2 Influencing factors under ICP rainfall modes 5.2.1 SST and SST anomaly Figure 5.9 exhibited below normal SST anomaly obtained from EOF1 and 2 of EOF analysis over ICP precipitation. The two SST patterns displayed nearly the same distribution for the two time periods. Its showed that northeast IO and BoB were dominated by cold SST anomalies during both EOF1 and 2. Whereas warm SST anomaly were observed over SCS, however, there was a small area of warm SST anomaly over north and northwest BoB during the second mode. The cool SST anomaly over BoB was associated to the pressure gradient oriented from ocean toward the land. This pressure gradient encouraged moisture advection toward ICP and favored precipitation. On the other hand, heavy downpour of precipitation during this time may be a cause for cold SST anomalies of BoB and adjacent sea. Figure 5. 9 SST anomaly (˚C) for the composite pentad from (a) EOF1 and (b) EOF2 over northeast IO of the ICP rainfall EOF analysis. 66

Figure 5. 10 spatial distribution of correlation coefficient between SST and ICP rainfall during EOF1 (a) and EOF2 (b). Figure 5.10 displayed the connection between IO SST and ICP rainfall. In the first mode, the SST nearly entire domain captured neutral link with ICP rainfall. However, in second mode, strong positive relationship observed over equatorial IO. Which implying that there is less significant relation between SST and ICP rainfall in the first mode. 5.2.2 Temperature and temperature anomaly The air temperature anomaly over ICP are displayed in Figure 5.11. The pattern of cool temperature anomaly for both modes displayed nearly the same spot, which is western part of the peninsula mainly Myanmar is captured by cool air temperature. An above normal air temperature spot was also observed over west BoB and east of the ICP. Cool surface air might be associated with heavy downpour of the rainfall over the region. 67

Figure 5. 11 Surface air temperature anomaly (˚C) for the composite pentad from (a) EOF1 and (b) EOF2 over study domain of the ICP rainfall EOF analysis. The correlation coefficient analysis results shown in Figure 5.12 displayed the significance of amount of precipitation over ICP on surface air temperature. Which captured strong negative relationship over south and southwest part during EOF1 and strong negative relationship is well captured in the east during the EOF2, implying that the amount of rainfall had played negative impact on surface air temperature. On the other word, cooling effect of the surface air temperature by rainfall. 68

Figure 5. 12 spatial distribution of correlation coefficient between air temperature at 1000 hPa and ICP rainfall during EOF1 (a) and EOF2 (b). 69

5.2.3 Geopotential height anomaly Figure 5. 13 Geopotential height (m2 s-2) for the composite pentad from EOF1 and EOF2 over BoB and ICP at three different levels of the ICP rainfall EOF analysis. The geopotential height anomalies for EOF1 and 2 of EOF analysis of precipitation over ICP are shown in Figure 5.13. At 850 hPa (a) and 500 hPa (b), first EOF, a strong low-pressure system was observed over the BoB and its center located around 24°N. During EOF2, at low level 850 hPa, a relatively weak low-pressure system was also observed, and its center is mainly over India deep south toward Sri Lanka. At 500 hPa, a low-pressure displayed horizontally shape extending from India to ICP. However, a high-pressure system located to the southeast of the ICP was also observed. At higher levels, the center of high moved to the southern of Himalaya, which relatively strong comparing to the EOF1. A low-pressure system was noted over BoB and ICP which favored 70

precipitation over the region. Therefore, it may have contributed to the high precipitation zone observed over BoB and west-southwest Myanmar (Figure 4.6). The strong low-pressure system in Figure 5.9 (a) may have been caused by the temperature gradient among the sea and continental land area. The position of the high-pressure system over the BoB plays a critical role in moisture transport and precipitation over ICP. 5.1.4 Wind circulation anomaly, omega, MFC and wind divergence 71

Figure 5. 14 Wind circulation anomaly (m s-1) for the composite pentad from EOF1 and EOF2 over BoB and ICP at three different levels of the ICP rainfall EOF analysis. 72

Wind circulation anomaly at three different levels over ICP and BoB for EOF analysis of precipitation over ICP are displayed in Figure 5.14. In both modes, the semi-permanent cyclonic circulation over BoB were clear visible at 850 hPa, which center of the circulation is located west of the Bangladesh and moved east during second mode. The result of wind circulation is consistent with observation made in Figure 5.13. At 500 hPa level, during the first mode, strong cyclonic circulation is observed over west of Myanmar and extend southeastward along the ICP. IO and BoB are dominated by westerly flow. In the second mode, anticyclonic circulation captured southeast of the ICP, whereas opposite situation to the north. At upper level 200 hPa, both modes easterly flow, in the other word tropical jet stream were dominant. The important features were over ICP, which during the first mode, a deep and strong cyclonic circulation was present over northern tip of the BoB, thus leading with strong rising motion over the area and contributing to high precipitation (Figure 4.6). During the second mode, a cyclonic circulation was present over north-center of ICP. This result is consistent with observation made in precipitation in (figure 4.6(b)). The positions of anticyclonic circulations are important for advection of moisture flux toward the continent. However, the systems within the ICP also play a magnificent role in determining regional precipitation. Moreover, because of the high elevation of the terrain over east and west of ICP, low-level wind was prevented from entering the ICP directly from the ocean. 73

Figure 5. 15 Moisture flux convergence (kg kg-1 m2 day-1) shaded area indicated convergence (positive) and divergence (negative) for the composite pentad from EOF1 (a) and EOF2 (b). Moisture flux convergence had also known to play significant role in amount of precipitation. Figure 5.15 displays curtain amount of moisture that transported into ICP base on EOF analysis of precipitation over ICP. Which most part of Myanmar received much more moisture than other area over ICP. This might due to the direction of wind flow toward continental land area in northern hemisphere and high mountain ranges situated along eastern border of Myanmar that deviated wind direction to northward and also lift up air mass. The vertical velocity composite analyses along 16°N result for the EOF1 and EOF2 modes are displayed in Figure 5.16. In both modes, center of ascending and descending air is nearly uniform. Rising air dominated from 95°E to 101°E and sinking air dominated 102°E to 110°E. However, a single spot of rinsing air is observed between 3°E to 5°E. The rising of air which indicating the presence of the convergence zone during that period. Therefore, agreeing with the geopotential height and wind circulation observations over the region, this area was dominated by cyclonic system, that is, an area of rising motion and favored precipitation. 74

From observing patterns of wind divergence at 850 hPa in Figure 5.17, we see that there is divergence of wind in the south of the peninsula, and convergence to the north in both EOF modes. Figure 5. 16 Vertical velocity (Pa s-1) along 16°N for the composite pentad from EOF1 (a) and EOF2 (b). Figure 5. 17 Wind divergence (m s-1), shaded area indicated convergence (negative) and divergence (positive) for the composite pentad from EOF1 and EOF2 over BoB and ICP of the ICP rainfall EOF analysis. 75

5.3 Chapter Summary This chapter described and analyzed the different modes over BoB and ICP with anomalies in their associated influencing factor after monsoon onset during 1998-2018, in context of causative circulation mechanism. The results were assessed from a dynamic angle in order to achieve deeper understanding of the physical mechanisms modulating the variability in precipitation and thus to explore the possibility of enhanced knowledge of the subject. Base on vorticity over the BoB EOF analysis, the SST anomalies in the first mode displayed warm SST anomaly, which increased in rate of evaporation over the ocean. However, during second mode, cool SST anomaly is displayed which did not indicate significant contributions. The most significant contribution came from the cyclonic and anticyclonic circulation over the region. In the first mode, the anticyclone is located over the BoB relatively close to ICP, thus contributing sinking of air and also inhibition of precipitation over ICP. On the other hand, in the second mode, there was a cyclone over IO which caused precipitation over the area. The wind pattern over BoB and ICP also explained part of the variability observed. During the first mode a deep anticyclone was dominated over BoB thus inhibition precipitation over region and its periphery area as could be observed in the precipitation pattern. In the second mode, the cyclone which is associated with convergence prevailed over IO thus explained the precipitation distribution. Based on EOF analysis of rainfall over ICP, in the first mode, positive loading is observed overall the ICP, but the strongest one center around southwest of ICP, which consistent with climatological precipitation distribution observed after monsoon onset. During the second mode, positive loading is dominated over the north and negative loading dominated south of ICP. Comparing the precipitation distribution for the first and second EOF mode, resulting that during the first mode ICP received more rainfall than the second. The SST anomalies in both modes displayed nearly uniform negative anomaly, which considered as not indicate significant contribution. Low- pressure systems were considered to have the most significant impact in contributing to heavy rainfall over the region. Wind pattern and moisture transport are also important factors. During the first and second modes, deep cyclonic circulations were dominant over BoB and ICP, thus heavy precipitation observed. Divergence over the region displayed nearly the same, divergence to the south and convergence to the north. 76

After monsoon onset, rainfall over ICP is overall increased in amount and intensity. The wettest area was recorded over AS and west coast of Myanmar, the precipitation was noted to exhibit southwest to northeast declination. When the BoB trough extended southeast to AS and occupied nearly the same shape over low-levels, in other word, BoB have occupied a deeply extended southeastward. This situation will cause east of ICP experienced heavy rainfall, Around coastal area of Vietnam and east of Laos. During this time, BoB trough have strongly southward extended, and center of low-pressure system is central BoB. At 700 and 500 hPa, the trough position moved southwest and westward extended. The ICP experienced heavy rainfall, especially, coastal area of Myanmar (AS) and Thailand and Cambodia. A very weak BoB trough are shown, and center of low-pressure system are located over northeast India. This situation will cause overall of ICP received less rainfall during this time, for instance, north Myanmar, Laos, and Vietnam. When BoB trough is intensified and located east coast of India and there is also a coexistence of one strong low-pressure system over SCS. Three heavy precipitation band throughout ICP were observed. First one located coastal area of Myanmar, second one located coastal area of Cambodia and the third one located over SCS. Although uncertainty exists regarding the distinction between the contributions of each pattern and various factors, this chapter showed the climatological pattern that prevailing during each vorticity mode, precipitation mode and after monsoon onset. Because of understanding of factors modulating the observed pattern still limited, the gaps in knowledge and the lack of understanding about the region’s climate system call for extensive research. 77

CHAPTER 6 CONCLUSION 6.1 Summary This section provides the general summary of the study in which the relationship between BoB and ICP rainfall after monsoon onset during 10 pentads of 1998-2018. it was driven by four objectives which are: to investigate the climatological feature of rainfall over ICP; To investigate the climatological feature of rainfall over ICP; To analyze the key systems, especially the BoB trough structure, evolution and relationship with the precipitation over ICP after monsoon onset, mainly focus on the fifty days period; To examine the pattern of BoB trough after monsoon onset and how it impacts on the precipitation over ICP; To investigate the influence of other environment factors such as: sea-surface temperature, air temperature, and topographical effect. Chapter 3 described on climatological study and showing that, The gradually increase in amount of precipitation from April is caused by several systems among which are the monsoon onset and northward migration of ITCZ starting from April and reached its highest point in August and account for more than 80% of the total precipitation range (200 mm) as the threshold to determine wet season months, the 20-year average of monthly mean precipitation over ICP. Monthly spatial climatology of precipitation over ICP during 1998-2017, results which coexistence with its annual cycle. It also supports the results of past studies where the MJJASO is the wet season over ICP, with a south and southeast to north oriented precipitation zone dominating the region. The streamlines show the presence of BoB trough or the low-pressure system during the wet season months. Its position, which, is migrated in all dimension throughout the year, sometimes it moves northward into Indian subcontinent. Another magnificent feature is the direction of the wind flow over ICP which has southwesterly characteristics during the wet season. From May to October, the wind flows from southwest to northeast over ICP while from November to April, the wind changes its direction to the northeasterly. The air temperature over ICP is observed to be less fluctuated year-round. High temperature was observed over the center dry zone and maximum temperature is in June then declined in the following month. This might due to the heavy downpours of precipitation. However, instead of 78

this region receiving more precipitation due to instability of the atmosphere, this area received less precipitation as compared to the rest of the peninsula. The maximum of MFC flux into the ICP is during MJJASO which also consistent with the precipitation analyses. The results indicated that centered west and northwest have much more MFC than the rest, but according to precipitation analyses the area has more rainfall is south and southwest. From November the wind starts to flow northeasterly dry air from Asia continent and north PO flow into ICP and dry season is started. Using three monsoon onset techniques to quantified and defined the monsoon onset, namely normalized precipitation index, Webster-Yang circulation index and climatological pentad mean rainfall index. The results showed that CPMI is the suitable one, which has mean value equal to NPWI but less deviated and has high correlation with WYI. After monsoon onset, rainfall over ICP gradually increase, on the other word, more rainfall in the following pentad. This is consistent with the low-levels wind direction over the region, which dominated by southwesterly, brought in sufficient moisture, thus heavy downpour in the following pentad. These heavy rainfall cause dropping of surface air temperature which relatively high before monsoon onset. Although the rainy season over ICP is MJJASO, further analyses will consider only 10 pentads after monsoon onset of each year since the spectrum analyses shows the oscillation of the precipitation is around 6-pentad. Chapter 4 described and analyzed the different modes of BoB trough and ICP rainfall after monsoon onset during 1998-2018, in context of structural features. The results were assessed from a structural angle in order to achieve deeper understanding of the physical mechanisms modulating the variability in precipitation and thus to explore the possibility of enhanced knowledge of the subject. For the case of EOF analyze of vorticity over BoB, the first mode and second mode described 33.3% and 23.7% variability, respectively, which agreed with precipitation pattern observed over ICP. The precipitation pattern during the first mode was relatively drier and southern part receive more rainfall while the precipitation pattern during second mode was center west and east of ICP. 79

EOF analysis of rainfall over ICP, the first mode and second mode described 25.9% and 8.8% variability, respectively. In the first mode, positive loading is observed overall the ICP, but the strongest one center around southwest of ICP, which consistent with climatological precipitation distribution observed after monsoon onset. During the second mode, positive loading is dominated over the north and negative loading dominated south of ICP. Comparing the precipitation distribution for the first and second EOF mode, resulting that during the first mode ICP received more rainfall than the second. Vertical cross-section is analyzed in order to get deep insight into the structural features of BoB based on first two EOF of vorticity over the BoB and ICP rainfall analysis. During two modes of vorticity over the BoB EOF analysis. Along 85°E, less specific humidity is observed which implied that relatively dry air mass dominated over the region. The vertical velocity displayed rising of air, however due to lacking of moisture, less precipitation observed. High surface air temperature was observed. During two modes of rainfall over ICP EOF analysis. Along 85°E, high specific humidity is observed which implied that moist air mass dominated over the region. The vertical velocity displayed rising of air, thus heavy precipitation observed. High surface air temperature was also observed and column of high air temperature extended until height elevation 850 hPa. Which meant strong rising of air and unstable of air column. Chapter 5 described and analyzed the different modes over BoB and ICP with anomalies in their associated influencing factor after monsoon onset during 1998-2018, in context of causative circulation mechanism. The results were assessed from a dynamic angle in order to achieve deeper understanding of the physical mechanisms modulating the variability in precipitation and thus to explore the possibility of enhanced knowledge of the subject. Base on vorticity over the BoB EOF analysis, the SST anomalies in the first mode displayed warm SST anomaly, which increased in rate of evaporation over the ocean. However, during second mode, cool SST anomaly is displayed which did not indicate significant contributions. The most significant contribution came from the cyclonic and anticyclonic circulation over the region. In the first mode, the anticyclone is located over the BoB relatively close to ICP, thus contributing sinking of air and also inhibition of precipitation over ICP. On the other hand, in the second mode, there was a cyclone over IO which caused precipitation over the area. The wind pattern over BoB and ICP also explained part of the variability observed. During the first mode a deep anticyclone was 80

dominated over BoB thus inhibition precipitation over region and its periphery area as could be observed in the precipitation pattern. In the second mode, the cyclone which is associated with convergence prevailed over IO thus explained the precipitation distribution. Based on EOF analysis of rainfall over ICP, in the first mode, positive loading is observed overall the ICP, but the strongest one center around southwest of ICP, which consistent with climatological precipitation distribution observed after monsoon onset. During the second mode, positive loading is dominated over the north and negative loading dominated south of ICP. Comparing the precipitation distribution for the first and second EOF mode, resulting that during the first mode ICP received more rainfall than the second. The SST anomalies in both modes displayed nearly uniform negative anomaly, which considered as not indicate significant contribution. Low- pressure systems were considered to have the most significant impact in contributing to heavy rainfall over the region. Wind pattern and moisture transport are also important factors. During the first and second modes, deep cyclonic circulations were dominant over BoB and ICP, thus heavy precipitation observed. Divergence over the region displayed nearly the same, divergence to the south and convergence to the north. After monsoon onset, rainfall over ICP is overall increased in amount and intensity. The wettest area was recorded over AS and west coast of Myanmar, the precipitation was noted to exhibit southwest to northeast declination. When the BoB trough extended southeast to AS and occupied nearly the same shape over low-levels, in other word, BoB have occupied a deeply extended southeastward. This situation will cause east of ICP experienced heavy rainfall, Around coastal area of Vietnam and east of Laos. During this time, BoB trough have strongly southward extended, and center of low-pressure system is central BoB. At 700 and 500 hPa, the trough position moved southwest and westward extended. The ICP experienced heavy rainfall, especially, coastal area of Myanmar (AS) and Thailand and Cambodia. A very weak BoB trough are shown, and center of low-pressure system are located over northeast India. This situation will cause overall of ICP received less rainfall during this time, for instance, north Myanmar, Laos, and Vietnam. When BoB trough is intensified and located east coast of India and there is also a coexistence of one strong low-pressure system over SCS. Three heavy precipitation band throughout ICP were observed. First one located coastal area of Myanmar, second one located coastal area of Cambodia and the third one located over SCS. 81

Although uncertainty exists regarding the distinction between the contributions of each pattern and various factors, this chapter showed the climatological pattern that prevailing during each vorticity mode, precipitation mode and after monsoon onset. Because of understanding of factors modulating the observed pattern still limited, the gaps in knowledge and the lack of understanding about the region’s climate system call for extensive research. 6.2 Conclusion Precipitation distribution over ICP displayed heavy rainfall spot scattered along the west and southwest of ICP, whereas relatively dry north and northeast. However, due to its location and structural features of BoB trough, ICP precipitation vary after monsoon onset. The onset of monsoon over ICP is varying year by year. In this study, the idea of climatological pentad mean rainfall index is utilizing to define monsoon onset dates. Climatological studies of various parameters are in support of each other. Precipitation influenced air temperature. After monsoon onset, air temperature over ICP was observed to be lower, due to the cooling effect brought by precipitation. High-pressure systems play an important role in moisture advection, therefore low- pressure systems and convergence lift up the air, thus moist air get condense, precipitation occur. However, SST over northeastern IO is considered to have less significant impact on ICP rainfall. For the relationship of BoB trough and ICP rainfall, the following conclusion can be drawn: 1) Embedded in the Indian Monsoon System, ICP has two distinct season, namely wet and dry. After monsoon onset, the wind reverse, strong southwesterly flow dominated, on the other word, abruptly increased in rainfall is observed after monsoon onset. Base on this study, the mean monsoon onset date over ICP, for 21-year duration (1998-2018), is set to be around pentad 23.81 (28 or 29 April) with 2 pentad or 10 days deviation. 2) The BoB trough hold a critical important relationship with precipitation over ICP. Physical structures and dynamical effects from the BoB trough have played significant role in ICP rainfall. On the other word, different modes of the BoB trough, shape-size and position, caused ICP to experience different rainfall patterns. The prolong south and depth extend low-levels BoB trough inducing heavy rainfall over west of ICP, on the other hand, weak, 82

shallow and retreated north BoB trough, less rainfall overall the study area. East and southeast extend of BoB trough inducing heavy rainfall over east of ICP. 3) The cyclonic circulation has also important roles in precipitation pattern over the area. It increased the surface convergence and maintained strong rising vertical motion over the area. Moreover, it intensified the southwesterly air flow and transfer more vapor and moisture toward the precipitation area. 4) The SST and Air temperature from the adjacent sea provided evidence on lack of direct influence to the ICP rainfall. Nevertheless, there is a strongly relationship between ICP rainfall and TIO SST. 6.3 Theoretical and Practical Implications of Research With the global warming and climate change, the weather and climate are more unstable and unpredictable. Therefore, the lifestyles of human are expected to change to adapt to the prevailing conditions. Recent technology allowed for better monitoring but with some limitations. This study used theoretical knowledge to investigate the relationship between the trough of the BoB and rainfall over ICP after monsoon onset. The study employed statistical and diagnostic methods to imply a relationship between the trough of the BoB and rainfall over ICP after monsoon onset in term of structural and dynamical forcing. The results implying that BoB trough has strongly influence ICP rainfall and thus, the diagnostic and investigation of BoB trough and other associated influence mechanism in precipitation patterns, in term of causative circulation are addressing. The results can be implemented to the future investigate the relationship between the trough of the BoB and rainfall over ICP after monsoon onset. 6.4 Original Contributions to Knowledge This study provided a solid statistical background which revealed the strong links between BoB trough and ICP rainfall. In obtained significant results, this study provides a baseline for future research by serving as reference information in parameterization studies. 83