1. Introduction
In the summer of 2022, Pakistan experienced a catastrophic flood that had impacted 33 million residents as of September 2022 (United Nations Pakistan 2022). Pakistan does not experience heavy precipitation and flooding often, and this lack of experience may increase the impact of flood disasters.
Climatologically, the spatial pattern of mean precipitation in summer shows a steep spatial gradient over South Asia. India and Bangladesh are relatively wet, but northwestern India and Pakistan are dry. In addition, although monsoon westerlies with abundant precipitation are known as the essential characteristics of the Indian monsoon, the lower-level winds are weak easterlies rather than westerlies over northwestern India and Pakistan. Thus, the strength of the monsoon westerlies may not explain the precipitation variability over northwestern India and Pakistan, and it becomes important to identify other areas in South Asia that have similar precipitation variability to that of Pakistan.
Pakistan is located in a boundary region between the tropics and midlatitudes (approximately 25°–35°N), and thus, it is influenced by both midlatitude and tropical atmospheric phenomena that can contribute to flooding events (e.g., Hong et al. 2011; Webster et al. 2011; Martius et al. 2013). Previous studies have emphasized the influence of midlatitude atmospheric phenomena on flooding. For example, during the 2010 flood in Pakistan, midlatitude troughs with high potential vorticity air intruded over and around Pakistan along the eastern edge of a significant blocking high over western and eastern Europe (Matsueda 2011), which induced strong unstable conditions over Pakistan (Hong et al. 2011; Martius et al. 2013). Similarly, significant heatwaves in 2022 brought on by a blocking high that occurred over western Europe may have affected the flooding in Pakistan.
However, it is important to consider the regional differences in heavy precipitation between the 2010 and 2022 flooding. The 2010 flooding event was induced by heavy precipitation in the northern mountainous region (Webster et al. 2011). In contrast, the 2022 flooding events occurred mainly over southern and central low-elevation regions (United Nations Pakistan 2022), suggesting that tropical influences also needed to be considered for the 2022 flooding event. Therefore, the mechanisms of precipitation variation in the 2010 and 2022 flooding events may differ significantly.
As a tropical influence, the convective activity around South Asia can be strengthened by La Niña conditions, leading to an increase in precipitation, as was the case in 2010 (e.g., Hong et al. 2011). Long-term changes and interannual variability of the seasonal drought in Pakistan can be explained mainly by El Niño–Southern Oscillation (ENSO; e.g., Ullah et al. 2022), which suggests that ENSO may be associated with precipitation variability in Pakistan. However, droughts and floods are not entirely symmetric phenomena (e.g., Takahashi et al. 2023). Because the causes of floods are more complicated than those of droughts, floods may be influenced by other additional factors.
Because Pakistan is located in the South Asian monsoon region, the abundant studies on the Indian summer monsoon may help understand the 2022 flooding. Climatologically, during the Northern Hemisphere summer months, tropical disturbances (TDs) or low pressure systems, originating from the Bay of Bengal and the north Indian Ocean, travel northwestward (e.g., Ajayamohan et al. 2010; Krishnamurthy and Ajayamohan 2010; Hatsuzuka et al. 2014; Hurley and Boos 2015). They occasionally arrive in Pakistan and may bring precipitation. These TDs generally travel westward along the South Asian monsoon trough, which is also strongly related to precipitation variability and sometimes flooding (e.g., Krishnan et al. 2000; Goswami et al. 2003; Roy and Rao 2022; Suhas et al. 2023). The importance of similar TDs related to the monsoon trough for precipitation variations over Southeast Asia was discussed (e.g., Takahashi and Yasunari 2008; Takahashi et al. 2009, 2015). More broadly, the intraseasonal oscillation of the South Asian monsoon (e.g., Gadgil and Joseph 2003; Goswami et al. 2003) may also explain the precipitation variability over Pakistan.
Understanding the atmospheric and climatological causes of flooding is necessary to mitigate similar natural disasters in the future. Because the 2022 flooding events occurred mainly over southern and central Pakistan, this study focused on tropical influences. One question that arises is whether the enhanced low-level monsoon westerlies, similar to the Indian monsoon, explain the 2022 flooding. Also, we should consider the tropical influences, such as the role of TDs, the South Asian monsoon trough, and intraseasonal oscillation of tropical Asian monsoon regions, which are essential to understand the precipitation variability in the South Asian monsoon. However, midlatitude influences may also be involved in explaining the 2022 events, as in the case of 2010. This research also considered interannual variability to compare the 2022 flooding event with other years with high precipitation in Pakistan. It is important to examine the regional characteristics of the interannual precipitation variability over Pakistan associated with the surrounding regions of the South Asian monsoon. In addition, the study assessed whether the presumable atmospheric mechanism responsible for the 2022 floods was also a major factor in the interannual precipitation variability over Pakistan or not. Therefore, this research investigated the causes of flooding using observed precipitation and atmospheric reanalysis.
The remainder of the paper is structured as follows: section 2 presents the datasets and methods used. Section 3 investigates the time–space characteristics of precipitation, atmospheric circulation, and water vapor in 2022 and other years, also focusing on the synoptic time scale. In addition, the time–space characteristics of precipitation on intraseasonal and interannual time scales are confirmed using EOF analysis. Then, the characteristics of the 2022 precipitation and interannual precipitation variability are compared. Section 4 provides the conclusions and discussion.
2. Data and methods
The analysis of spatial and temporal rainfall on a daily time scale over and around the South Asian monsoon region, including Pakistan, was conducted using the Global Satellite Mapping of Precipitation, version 6 (GSMaP; Kubota et al. 2007). While these data cannot be as accurate as quality-controlled, long-term rain gauge precipitation data on the ground, it is suitable for examining the distribution of precipitation over and around Pakistan and across South Asia in 2022, as well as interannual variability over the past more than two decades. However, there are limitations to the knowledge of the precipitation variability on a local scale and over more extended periods. To facilitate analysis and save computational memory for EOF analysis, the daily precipitation data were spatially roughened to 0.5° × 0.5° from the original horizontal resolution of 0.1° × 0.1° using an area-weighted box-average method. The GSMaP data covered from 1 April 2000 to the present. For this reason, the analysis period of this paper spanned 23 years from 2000 to 2022. To reduce the uncertainty of the GSMaP dataset, the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) V06B product (IMERG; Huffman et al. 2019) was also analyzed, including the following EOF analysis. The results were almost identical (not shown). Because both datasets are satellite-based and similar, the 2022 flood event should be analyzed using ground-based datasets in the future.
I conducted EOF analysis on the precipitation data over the South Asian domain at two different time scales to objectively determine the possible impact of shorter-than-seasonal precipitation variability on interannual variability because the accumulation of TD-related precipitation is important (see sections 3a–3c). More specifically, the study did not focus on the synoptic precipitation variations owing to the position of a single TD but focused on the variations in precipitation resulting from the accumulation of TD activity, which can vary on a time scale of longer than synoptic but shorter than monthly, which referred to as accumulated TD (ATD) time scale. To better understand the time–space precipitation variability on ATD, EOF analysis was applied to the 11-day running mean precipitation during the JJA time frame for the years 2000–22 because the 2022 flooding was associated with ATD variations (ATD; section 3e); I refer to this analysis as ATD–EOF. Note that when EOF analysis was applied to the 7-day running mean precipitation, the results were the same as those of the 11-day running mean precipitation. Additionally, EOF analysis was applied to JJA mean precipitation, focusing on the interannual variation (IAV); I refer to this analysis as IAV–EOF. To analyze the statistical spatial pattern over Pakistan and surrounding regions, namely, the Asian monsoon region, this study chose JJA as the most commonly used Asian monsoon season although the seasonal march of precipitation is different from region to region. These two sets of EOF analyses can explain whether ATD precipitation variability contributed to the JJA mean precipitation in 2022 and in other years. The score time series [principal components (PC)] of EOF was normalized by the eigenvalue.
Throughout the manuscript, the ENSO phase was determined based on the definition of the Japan Meteorological Agency (JMA; https:\\ds.data.jma.go.jp\tcc\tcc\products\elnino\ensoevents.html). This study considered the simultaneous ENSO phase, namely, JJA. Also, the study area, topography, and geographical information are shown in Fig. 1.
Topography and geographical information. The box with a solid black line is sometimes used as the area for averaging precipitation and other variables in Pakistan (Figs. 2, 4, and 7 and Figs. S2, S8, and S9). The boxes with the dashed black lines show the area for meridional averages in Figs. 5 and 6 and Fig. S3. The latitudinal average varies depending on longitudes.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
3. Results
a. Above-normal precipitation and precipitation variability in 2022
Figure 2a shows a map of the JJA precipitation anomaly in 2022 compared with the 23-yr climatological precipitation. A positive anomaly in precipitation (more than 2.5 mm day−1) was observed over Pakistan during JJA 2022. Because the summer mean precipitation is climatologically low (less than 2 mm day−1) over Pakistan (Fig. S1a), the positive anomaly value in precipitation during JJA 2022 was exceptionally high, particularly over southern and central Pakistan (Fig. 2a and Fig. S1b). Specifically, the precipitation anomaly during this period was approximately 3 times larger than the climatological precipitation and was the highest during the past 23 years (Fig. 2b). Therefore, excessive precipitation was the main explanatory factor for the 2022 flood event. In this study, 2022 and 2010 were analyzed as typical wet years, while 2012 and 2014 were analyzed as dry years, on the basis of the precipitation levels (Fig. 2b). To reduce the influence of long-term trends and focus on interannual variability, this study chooses 2012 and 2014 as the dry years. However, it has been confirmed that the anomalous patterns of IWVF and water vapor in the dry years of the 2000s were similar to those in the 2010s.
(a) Map of the JJA precipitation anomaly from the 2000–22 climatological mean. (b) Interannual time series of the area-averaged JJA precipitation over the Pakistan flooding region (66°–74°E, 26°–32°N). Asterisks denote La Niña years; for example, it is used for composite.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Over the South Asian monsoon domain, higher precipitation anomalies in Pakistan were accompanied by positive anomalies over southern and central India (Fig. 2a). In contrast, negative precipitation anomalies were observed along the Indo-Gangetic Plain. Therefore, the precipitation anomalies were similar to the zonal direction, somewhat tilting along a northwest–southeast axis. The zonal orientation of these anomalies suggested that they were associated with TDs on a synoptic time scale and with intraseasonal variability. Both time scales are known to be dominant modes of precipitation variability (Wang and Rui 1990; Annamalai and Slingo 2001; Goswami et al. 2003; Krishnamurthy and Ajayamohan 2010; Hunt and Fletcher 2019).
The spatial pattern of precipitation anomalies in 2022, with more precipitation in southern and central India and less precipitation in northern India, was similar to the active phase of the intraseasonal variability of the Indian monsoon (Gadgil and Joseph 2003; Goswami et al. 2003; Krishnamurthy and Ajayamohan 2010). This also implied that the precipitation anomalies in Pakistan in JJA 2022 can be associated with the precipitation variability in southern and central India. The time–space patterns of the precipitation variations are examined in section 3e.
When considering an entire tropical monsoon region, such as the South Asian and Southeast Asian monsoons, greater precipitation anomalies were observed. Also, high precipitation was observed over the broad area of the Maritime Continent. These precipitation anomalies, including Pakistan, could be simply explained by the La Niña oceanic influences. Indeed, the composite map of JJA precipitation based on La Niña showed a significant positive precipitation signal (Fig. 3).
Composite anomaly in JJA precipitation for the La Niña years during 23 years between 2000 and 2022. The composited years were 2007, 2010, 2020, and 2022, which were determined by the JJA SST anomaly based on JMA (see section 2). Climatological JJA precipitation was calculated for 2000–22. All plotted values were statistically significant on two sides at p < 0.05. The statistical significance of the composite was determined by a 1000-time Monte Carlo random resampling composite from 23 years (samples).
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
To understand which precipitation events were significant contributors to the flood events, Fig. 4 shows a time series of precipitation over Pakistan. In 2022, several notable precipitation events occurred in a series. However, such several notable precipitation events have never occurred continuously within a season in Pakistan in the period analyzed (Fig. S2). Climatologically, precipitation systems with cyclonic circulations generally travel westward and northwestward from the Bay of Bengal and the northern Indian Ocean via the Indian subcontinent on a synoptic time scale, and their activities are associated with the active and break phases of the intraseasonal variations of the Indian monsoon (Goswami et al. 2003; Krishnamurthy and Ajayamohan 2010). Here, the westward-traveling precipitation systems are examined in the following subsection.
Daily time series of the area-averaged daily precipitation over the Pakistan flooding region (66°–74°E, 26°–32°N) for the year 2022 from 1 Jun to 30 Sep.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
b. Westward-traveling tropical disturbances
I generated a time–longitude cross section of precipitation along the tilting longitudinal band (Fig. 1) to track the major routes of TDs (Goswami et al. 2003; Krishnamurthy and Ajayamohan 2010; Hatsuzuka et al. 2014) but with a somewhat north–south width. Climatologically, the major route of TDs is called the South Asian monsoon trough, and it is produced by the monsoon westerlies over southern and central India and weak easterlies over the Indo-Gangetic Plain (see Fig. 10).
Precipitation systems mostly traveled westward (Fig. 5), which clearly corresponded to the precipitation events in Pakistan (Fig. 4). These precipitation systems seem to be consistent with the climatological characteristics of the traveling precipitation systems. In 2022, several precipitation systems were observed over Pakistan. Particularly in mid-August and late August, a few precipitation signals swiftly traveled from 80° to 70°E within a few days, indicating westward-traveling precipitation systems. On the other hand, for precipitation events in July and early August, the precipitation signals stayed around 70°–80°E for several days. This result suggests that a precipitation system with an east–west width or multiple precipitation systems stayed there.
Time–longitude (tilting) cross section generated from daily precipitation in (a) 2022, (b) 2010, (c) 2012, and (d) 2014 on the tilting latitudinal band along the major routes of westward and northwestward TDs from the Bay of Bengal and the northern Indian Ocean. Latitudinal averaging was conducted at 50°–70°E, 70°–80°E, and 80°–95°E, averaged at 24°–32°N, 20°–28°N, and 16°–24°N, respectively (Fig. 4). The target longitude is about 68°E, shown by the solid black line. The longitudes at which the averaging latitude changes are indicated by dashed lines of 70° and 80°E.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
However, precipitation systems generally dissipate in dry years and even in 2010 before reaching Pakistan (Fig. 5). Also, in other years, most precipitation systems could not traverse the Thar Desert (Fig. S3), which is located near the border between India and Pakistan. Hence, a considerably larger-than-usual number of precipitation systems reaching Pakistan can be attributed to the above-average precipitation observed in 2022.
To link the precipitation systems to the atmospheric circulations, I show a time–longitude cross section of water vapor convergence (negative values in water vapor convergence) along the tilting longitudinal band (Fig. 6). The westward-traveling signals of water vapor convergence corresponded well with the precipitation variations and their movements (Figs. 4 and 5). These water vapor convergence events were likely due to the TDs.
Time–longitude (tilting) cross section of divergence of the vertically IWVF in (a) 2022, (b) 2010, (c) 2012, and (d) 2014 on the tilting latitudinal band along the major routes of westward and northwestward TDs from the Bay of Bengal and the northern Indian Ocean. Latitudinal averaging was conducted at 50°–70°E, 70°–80°E, and 80°–95°E, averaged at 24°–32°N, 18°–26°N, and 16°–24°N, respectively (Fig. 1). The target region located around 70°E is shown by a rectangle. The longitudes at which the averaging latitude changes are indicated by dashed lines.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Also, the signals were simultaneously observed from 70° to 85°E, which implies that the convergence signals fast-traveled or were zonally elongated from early July to early August 2022. The two westward-traveling signals in mid-August to late August 2022 were observed. The water vapor convergence in two or three events in July 2022 and one event in early August 2022 was weaker but longer than those in mid-August and late August 2022. In addition, because the precipitation amounts in the July 2022 events were similar to or higher than the precipitation amounts in the August events, it may be important to note that staying longer or moving more slowly, as well as other factors, may result in more precipitation from TDs. However, it is still being determined how the precipitation continued somewhat longer in the two or three events in July 2022 and the event in early August 2022 than those in mid-August and late August. The time sequence of TDs is examined in section 3c.
In other years, water vapor convergence also corresponded well to precipitation events (Figs. 5 and 6), again suggesting that the westward-traveling signals of the precipitation system and water vapor convergence signals were likely to be TDs. These TDs were less frequent and weakened or disappeared before reaching Pakistan. Thus, the timing of water vapor convergence could explain the precipitation events. Because the water vapor convergence could not simply explain the precipitation amount, the study examined the temporal changes in precipitable water (PW) to understand the differences in moisture conditions between 2022 and other years.
The time series of PW over Pakistan shows that higher than approximately 50-mm PW value was maintained over 1 month in July 2022 and a half month in August 2022 (Fig. 7). Although 50-mm PW does not represent a specific threshold value, heavy precipitation events over Pakistan occurred in Fig. 4, Fig. S2, and Fig. 7 during more than about 50-mm PW conditions. PW conditions are high during major precipitation events, which implies the importance of high PW conditions and a sustained water vapor supply in the background environment. In addition to this background wet condition, strong water vapor convergence owing to TDs likely resulted in a large amount of precipitation. When the same TDs arrive in Pakistan under higher and lower PW conditions, a wetter atmosphere reaches saturation earlier than the drier conditions.
Time series of area-averaged PW (mm) in (a) 2022, (b) 2010, (c) 2012, and (d) 2014 from 1 Jun to 31 Aug. Area-averaging was conducted over 66°–74°E, 26°–32°N.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
The sustained higher PW, particularly in July and August 2022, was notable compared with other years (Fig. 7). Particularly in July 2022, the high PW condition was continued for approximately 1 month. Such sustained high PW condition has never been observed. Again, very heavy precipitation was observed during the high PW periods. Even in the wet year of 2010, only a few days of high PW conditions were observed per precipitation event. In the dry years 2012 and 2014, the total duration of high PW conditions was much shorter than that of 2022, even in the wet year 2010.
On the other hand, even in dry years, it is interesting that the high PW periods corresponded almost to the timing of the TD approach, which implies that the role of the strong water vapor transports and the associated maintenance of high PW conditions may be played by the TDs. These results of correspondence between the timing of TDs and high PW conditions suggest an interaction between TD and background water vapor transports along the South Asian monsoon trough.
Hence, the occurrence of multiple TDs with heavy precipitation reaching Pakistan and the sustained high PW conditions can be attributed to the above-average precipitation observed in 2022. The following subsection shows low-level circulations during July and August 2022.
c. Synoptic disturbances and monsoon trough
Although the westward-propagating TDs over South Asia are well known as the synoptic time-scale phenomenon, some of the precipitation events over Pakistan in 2022 occurred at a longer time scale (Fig. 4). To understand the westward-propagating TDs, we show the daily sequence of spatial maps of 850-hPa winds for two cases. The first case is in mid-July. Similar features can be found in the cases of early July and early August. The latter case is in late August; similar events occurred in mid-August. Also, the manually tracked center positions of TD once a day, which were subjectively determined using 850-hPa winds, relative vorticity (not shown), and SLP (not shown), are shown (Fig. 8). The time sequence of the daily low-level circulations from late June 2022 to the end of August 2022 is provided (Figs. S4–S6).
Time sequence of daily mean 850-hPa winds (a)–(i) from 10 to 18 Jul 2022 and (j)–(o) from 19 to 24 Aug 2022. The red “x” mark in (a)–(o) is the center of TD, which was subjectively and manually determined. (p),(q) The TD tracks of (a)–(i) and (j)–(o).
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
The mid-July case showed that the TD over the western Bay of Bengal developed along the South Asian monsoon trough (Fig. 8). Another TD may be detected over the head of the Arabian Sea. The one or two TDs formed the zonally elongated cyclone circulations along the South Asian monsoon trough. In this case, the TD and cyclonic circulations tend to stay over the south coast of Pakistan and the Bay of Bengal for several days, and then, they slowly migrate northwestward and decay. The elongated cyclonic circulation has a longer time scale than the synoptic. These quasi-stationary cyclonic circulations strongly support a large amount of water vapor transports, which, in turn, leads to high precipitation, as seen in Fig. 4.
Interestingly, the monsoon trough almost simultaneously developed with TDs, suggesting their interactions. The cases of early July and early August were similar to this mid-July case. However, these features in 2022 were different from the typical monsoon lows along the monsoon troughs and the mid-August and late August cases in 2022, which will be described next.
The mid-August case showed that the distinct TDs over the Bay of Bengal migrated northwestward along the monsoon trough (Fig. S6). The other distinct TD repeatedly migrated along the monsoon trough in late August (Fig. 8). Compared with the mid-July case, the traveling speed of TDs was much faster, and they disappeared sooner. These TDs in the mid-August and late August cases are the typical TDs or low pressure systems in this region. Thus, although TD activity was not always typical, several active TDs can explain a large amount of the precipitation in the 2022 flooding.
I also confirmed the accumulated TD activity in 2022, which was measured by 850-hPa PKE. The explanation of how PKE can represent accumulated TD activity is provided in the appendix. The climatological 850-hPa PKE map shows the spatial peak along the Indo-Gangetic Plain (Fig. 9a and Fig. S7a), which is consistent with previous studies (Deoras et al. 2021; Roy and Rao 2022). A positive anomaly in 850-hPa PKE in July–August (JA) 2022 was observed over the South Asian monsoon region, particularly over Pakistan and central and southern India, which confirmed the above-normal TD activity (Fig. 9b and Fig. S7b). However, the major positive anomaly in JA 2022 was observed over central and southern India, which indicates that TDs were not active along the major TD storm tracks (Deoras et al. 2021; Roy and Rao 2022). Also, this result corresponded to the TD tracks (Fig. 8c). Thus, the TD activity in 2022 over South Asia might not be typical.
(a) Climatological mean PKE at 850 hPa in JA over 23 years from 2000 to 2022, using JRA-55 reanalysis. The PKE was calculated from the high-pass-filtered six-hourly zonal and meridional wind values, which were calculated by subtracting the 25-day running mean from the original values as a high-pass time filter (see the appendix). (b) Anomalies in PKE in JA 2022 from (a). The same figures using ERA5 are provided in Fig. S9.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Therefore, the spatial pattern of the precipitation anomaly, synoptic precipitation variability, and westward-traveling precipitation systems implied that the active TDs along the South Asian monsoon trough were associated with the 2022 flooding event. The position of the South Asian monsoon trough was located somewhat south of the climatological position. Also, the monsoon trough likely interacted with TDs, which may have resulted in somewhat more prolonged precipitation events than the synoptic time scale. Spatially, these TDs were zonally elongated along the South Asian monsoon trough.
d. Stationary moisture supply along the South Asian monsoon trough
This subsection examines the water vapor conditions during JA 2022. Climatological IWVFs can be seen in the well-known monsoon westerly IWVFs and easterly IWVFs over the Indo-Gangetic Plain, which produces the South Asian monsoon trough (Fig. 10). In JA 2022, anomalous IWVFs and 850-hPa atmospheric circulation exhibited weakened monsoon westerlies along the zonal band of 15°N and enhanced easterlies along the Indo-Gangetic Plain (Fig. 11 and Fig. S8), which indicated the enhanced monsoon trough in the South Asian monsoon region. The other wet year shows a similar pattern, and the dry years show weakened IWFTs along the Indo-Gangetic Plain. The enhanced monsoon trough can produce more TDs. Also, more active TD activity results in a deepening of the mean monsoon trough, which has been discussed in South and Southeast Asia (Goswami and Ajayamohan 2001; Takahashi et al. 2015, 2020). Although the enhancement of the monsoon westerlies can generally explain the higher precipitation over South Asia, greater precipitation over part of the Indian subcontinent and Pakistan can be more explained by the strength of the monsoon trough. In addition, the IWVF anomalies toward Pakistan resulted in a very high PW over Pakistan during JA 2022. The JA mean PW was the highest over the past 23 years (Fig. S9).
Climatological fields in PW (color) and vertically IWVFs (vectors) from the climatological mean of 2000–22 during JA.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
(a) Anomalies in PW (color) and vertically IWVFs (vectors) in (a) 2022, (b) 2010, (c) 2012, and (d) 2014 from the climatological mean of 2000–22 during JA.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
As shown in Fig. 8, TDs and the South Asian monsoon trough often formed the zonally elongated cyclonic circulations from Pakistan to the western Bay of Bengal. These Indian subcontinent–scale cyclonic circulations supported water vapor supply to Pakistan. These cyclonic circulations corresponded to the IWVF anomalies in 2022, which implies that these cyclonic circulations resulted in the high PW in JA 2022.
Next, I examined the intraseasonal variations in IWVFs, using the time sequence of 10-day IWVFs. The active phases of the South Asian intraseasonal variation spanning 2 months from late June to mid-August were observed with a short break (Fig. 12). Typically, an active/break phase of the South Asian monsoon lasts for a 15–20-day period at the most, which is half the length of the 30–40-day period of intraseasonal variation (Gadgil and Joseph 2003). As such, in 2022, the climatological and enhanced IWVFs over the Indo-Gangetic Plain continued to supply water vapor to Pakistan and maintain the water vapor necessary for the high precipitation. In particular, in July 2022, the high PW conditions (Fig. 7) were supported by these sustained water vapor supplies associated with the South Asian monsoon trough.
Time sequence of 10-day average vertically IWVFs in 2022 during periods (a) from 1 Jun 2022, (b) from 11 Jun 2022, (c) from 21 Jun 2022, (d) 1 Jul 2022, (e) from 11 Jul 2022, (g) from 1 Aug 2022, and (h) from 11 Aug 2022. Eleven-day averages are the two periods of (f) 21–31 Jul 2022 and (i) 21–31 Aug 2022.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
e. Role of TDs and monsoon trough in the 2022 flooding
This subsection discusses the kinds of ATD precipitation variability that explain the JJA mean precipitation. Although the time scale of individual TDs is synoptic, the activity of TDs coupling with the monsoon trough can result in precipitation variability on the ATD time scale. Thus, this study focuses not on the synoptic precipitation variations resulting from an individual TD, which has a shorter than several-day time scale, but on the precipitation variations resulting from the accumulation of TD activity with the monsoon trough on the ATD time scale. For this reason, I analyze the characteristics of precipitation variability with 11-day moving averages. Previous studies have examined the impact of ATD variability on interannual variability over the Asian monsoon regions (Takahashi et al. 2015). The three leading ATD–EOF and IAV–EOF modes and their PCs are shown in Figs. 13–16.
Eigenvector of accumulated TD (ATD) variation of (a) EOF1 (ATD–EOF1), (b) ATD–EOF2, and (c) ATD–EOF3.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
The normalized PCs (score time series) of ATD–EOF1, ATD–EOF2, and ATD–EOF3. Because only the JJA periods for each year are plotted for PCs of ATD–EOF1, ATD–EOF2, and ATD–EOF3, the lines are physically meaningless at the boundaries of the years.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Eigenvector of IAV of (a) EOF1 (IAV–EOF1), (b) IAV–EOF2, and (c) IAV–EOF3.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
The normalized PCs (score time series) of IAV–EOF1, IAV–EOF2, and IAV–EOF3. The background light blue color indicates whether the JJA period for that year was diagnosed as a La Niña year.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
The two leading ATD–EOF modes explained the northwest–southeast-oriented precipitation variability from Pakistan to India, where the tilting angles somewhat differed between the two (Fig. 13). The in-phase variability from Pakistan, central India, and the Bay of Bengal was found in ATD–EOF1 (explained variance ratio: 9.5%), which implied the westward-traveling TDs along the South Asian monsoon trough (Goswami et al. 2003; Krishnamurthy and Ajayamohan 2010; Hatsuzuka et al. 2014) or zonally elongated cyclonic circulations across the Indian subcontinent (Fig. 8). The PC of ATD–EOF1 showed notable negative values in 2022 (Fig. 14a), which indicated a wet phase in Pakistan. Enhanced precipitation activity with TDs from central India to Pakistan was observed (Fig. 5). In most years, the PC of ATD–EOF1 shows ATD–time scale variability.
ATD–EOF2 (explained variance ratio: 7.0%) showed the in-phase variability from Pakistan to southern India (Fig. 13b). Moreover, ATD–EOF2 indicated that an in-phase signal with Pakistan was observed over the northern Indian Ocean and the Maritime Continent. The PC of ATD–EOF2 showed high values for 2022 (Fig. 14b). Also, 2-month-long positive PC values of ATD–EOF2 were observed in high precipitation years (e.g., 2010, 2020, and 2022). ATD–EOF3 (Fig. 13c) may be the different phase of ATD–EOF1, but there were no notable signals over Pakistan. Thus, I do not discuss this mode further.
I then compared the ATD–EOF and IAV–EOF modes. IAV–EOF1 (explained variance ratio: 18.3%) showed an in-phase variability among Pakistan, southern India, and the Maritime Continent (Fig. 15a), which was very similar to that of ATD–EOF2 (Fig. 13b). The PC of IAV–EOF1 showed notably positive values in 2007, 2010, 2020, and 2022 (Fig. 16a). These years corresponded to La Niña years, which explained the positive precipitation anomalies over Pakistan during these years (also see Fig. 3). Thus, the interannual variability of JJA precipitation was partly explained by the increase in precipitation over a broad area, which was associated with La Niña conditions. The interannual precipitation variability in Pakistan can be obtained by the major leading EOF mode, which implies that the large-scale precipitation variability includes the precipitation variability of Pakistan. This is not inconsistent with La Niña controlling the precipitation variability of Pakistan. This is also confirmed in section 3f with respect to the interannual variability. IAV–EOF2 (explained variance ratio: 9.9%) showed a north–south dipole structure, but there were unclear signals in Pakistan, and the PC of IAV–EOF2 was nearly zero in 2022. Thus, this mode will not be discussed further.
The spatial pattern of IAV–EOF3 (Fig. 15c; explained variance ratio: 7.1%) was similar to that of ATD–EOF1 (Fig. 13a), which implied that ATD–EOF1 variability contributed to IAV–EOF3. It is noteworthy that IAV–EOF3 shows significant signals, particularly over Pakistan. Also, the PC of IAV–EOF3 was remarkably high in 2022 (Fig. 16c). However, the PC of IAV–EOF3 had low values in the other years even in the 2010 flood year. Therefore, ATD–EOF1 and IAV–EOF3 can explain unique features in 2022, which are essential to characterize the extreme JJA precipitation.
The PC of ATD–EOF2 mode varied intraseasonally. These northwest–southeast-oriented patterns of precipitation variation are very similar to the intraseasonal oscillation of the South Asian monsoon. Thus, this mode can be associated with the intraseasonal oscillation of the South Asian monsoon, which is shown in the analysis of water vapor transports (Fig. 12). However, the mechanism of interannual variations of intraseasonal oscillation is still unclear.
f. Interannual variability of Pakistan precipitation
The study identified the characteristics of precipitation and atmospheric circulation in JJA 2022 and how the 2022 flooding occurred. However, it is still unclear whether the mechanism of the 2022 flooding can also explain the interannual precipitation variability over Pakistan or not, which is examined, focusing on July and August, which are the primary rainy seasons of Pakistan. Note that the data from 2000 to 2022 were used in the interannual correlation analysis. In addition, because the impact of 2022 may be too strong, the same analysis was applied to 2000–21, which shows similar results. Thus, the result presented here is robust.
Positive interannual correlations with the JA precipitation over Pakistan were found for interannual precipitation variability in parts of India, the Arabian Sea, and the Maritime Continent (Fig. 17). There was also a negative correlation signal between the interannual variability of precipitation in parts of the western North Pacific and the central equatorial Pacific and precipitation in Pakistan. On the large-scale circulation, the ENSO-related precipitation anomalies can be confirmed. In particular, precipitation over Pakistan increased under the La Niña condition. The interannual correlation signals between PW and precipitation over Pakistan showed enhanced PW signals, which are consistent with the increase in precipitation.
(a) Interannual correlation between the area-averaged precipitation over Pakistan in JA and precipitation (color) and 850-hPa winds (vectors) for 2000–22. Only statistically significant values were plotted at 90% and 95% confidence intervals for precipitation and winds, respectively. (b) Interannual correlation between the same Pakistan precipitation as (a) and PW (color) and the vertically IWVFs (vector). Only statistically significant values were plotted at a 95% confidence interval. To check the robustness of the results, the correlations were also calculated without 2022. The overall results are the same.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Regional-scale circulations associated with increased JA precipitation over Pakistan on the interannual time scale showed enhanced easterlies along the Indo-Gangetic Plain, which produced an active monsoon trough over central India. The other primary signal with increased precipitation over Pakistan was an enhancement of IWVFs associated with the Somali low-level jet with an increase in PW. The two enhanced IWVFs produced cyclonic IWVF anomalies with more PW over the southern coast of Pakistan (near the Arabian Peninsula). However, the enhancements of the low-level wind signal were hardly observed over the northern Arabian Sea, the Indian subcontinent, and the Bay of Bengal. The enhanced IWVF signals over the Arabian Sea can be understood as the result of the increased PW rather than of enhanced monsoon westerlies. The interannual correlation analysis showed that the atmospheric circulation and water vapor signals were similar to the 2022 flooding events.
Also, a similar correlation map can be obtained from the simultaneous, interannual correlation between the Niño-3 SST anomaly and precipitation and atmospheric circulations, including water vapor variability (Fig. S10). Thus, interannual precipitation variation in Pakistan can be strongly correlated with the Niño-3 SST anomalies and the coupling and driven atmospheric circulations. However, accumulated TD activity measured by the 850-hPa PKE was not statistically significantly correlated with the precipitation over Pakistan (not shown). These results indicate that a combination of the enhanced, moist Somali jet and easterlies along the Indo-Gangetic Plain with the monsoon trough can induce high precipitation in Pakistan, but the activity of TDs can be determined by chance.
g. Differences between the 2010 and 2022 flooding
To identify the distinctive characteristics of the 2022 flooding event, I compared it with the 2010 flooding event, which was explained by midlatitude influences (Hong et al. 2011; Martius et al. 2013). In addition, the positive precipitation anomaly in 2010 was observed mainly over northern Pakistan (Webster et al. 2011) where midlatitude influences were more likely to occur.
However, in 2022, the lower 500-hPa geopotential height values over Pakistan, which indicated the passages of the midlatitude trough, did not correspond to the precipitation variations (Fig. S11). The correspondence between the midlatitude trough and precipitation variations was more plausible in 2010 than in 2022. In contrast, tropical influences were much more robust in 2022 than in 2010, with quantitatively noticeable anomalies in PW and IWVF. Also, in 2022, the timing of the arrival of the TDs corresponded to significant precipitation events. Nevertheless, it may be necessary to consider additional midlatitude influences on the 2022 flooding event.
4. Conclusions and discussion
I investigated the atmospheric conditions associated with the 2022 flooding event in Pakistan, using satellite-based daily precipitation and atmospheric reanalysis datasets, focusing mainly on the tropical impact. Considerable precipitation was observed over southern and central Pakistan. Compared with that during the 2022 flooding, major precipitation was observed only in the northern part of Pakistan during the 2010 severe flooding event. In addition, the total amount of JJA precipitation in Pakistan in 2022 was by far the highest in the past 23 years.
The major precipitation events of JJA 2022 were found to correspond to several westward- or northwestward-traveling precipitation systems on the synoptic time scale, which were the TDs. Climatologically, a TD travels westward on the synoptic time scale, but TDs did not always travel westward in JJA 2022 and sometimes stayed over southern Pakistan and eastern India. These stationary TDs coupled with the monsoon trough, which resulted in very high precipitation. Some TDs traveled westward or northwestward and reached Pakistan, which also brought high rainfall. Overall, the TD activity in 2022 was very vigorous. During the active TD period, a notable positive anomaly of PW was observed over Pakistan, which suggests that TDs enhance water vapor transport.
The high PW over and around Pakistan was observed from July to August 2022 with a short break, which was maintained by the enhanced easterly IWVFs over the Indo-Gangetic Plain. In addition, the PW over Pakistan in JA 2022 was the highest in the study period. The long-lasting active phase of the South Asian monsoon of the intraseasonal oscillation maintained the water vapor transports. The enhanced easterlies were often produced by the monsoon trough coupling with the TDs, suggesting that the TDs develop along the South Asian monsoon trough and that the monsoon trough is simultaneously deepened by the TDs. This circulation pattern intensifies water vapor transports along the Indo-Gangetic Plain.
Two EOF analyses were conducted to investigate whether precipitation variability on the ATD time scale contributes to the JJA mean precipitation in 2022 and other years. The results showed that two types of ATD variation modes (ATD–EOF1 and ATD–EOF2) contributed to the heavy precipitation in Pakistan in 2022. The ATD–EOF2 mode, which was associated with IAV–EOF1, could be attributed to the effects of La Niña, indicating wet years such as 2022 and 2010. This mode could explain the interannual variation in precipitation, particularly in the La Niña years, including 2022.
Notably, the ATD–EOF1 mode, associated with IAV–EOF3, could be considered to be a coupling of westward-traveling TDs and the monsoon trough across the Bay of Bengal or the northern Indian Ocean, the Indian subcontinent, and the southern coast of Pakistan. This mode was much more active in 2022 than in other wet years. In addition, it was noteworthy that the importance of TDs in the 2022 floods could be obtained by an objective method. Therefore, the 2022 floods in Pakistan were characterized by the contribution of active TDs. The 850-hPa PKE anomaly indicated that the accumulated TD activity was very active over Pakistan and southern and central India in 2022 although TDs are climatologically active over the Indo-Gangetic Plain (Deoras et al. 2021).
As a factor explaining the interannual variability of the Pakistan precipitation, the enhanced easterlies along the Indo-Gangetic Plain supported very high PW conditions. Thus, the primary factor in controlling the interannual variations in PW and precipitation was water vapor transports along the Indo-Gangetic Plain. The characteristics of IWVFs were common in other wet years. In contrast, the westerly anomalies of IWVF over the Indo-Gangetic Plain were observed in dry years. In addition, these PW and water vapor transports were simultaneously correlated with the SST anomalies over the Niño-3 region, which also corresponded to objective IAV-EOF results. However, the accumulated TD activity, measured by the 850-hPa PKE, could not explain the interannual variations of precipitation over Pakistan although TD activity was notable in 2022. Thus, I currently conclude that the impact of TD activity on precipitation over Pakistan is powerful, but TD activity can be determined by chance.
The influences of ENSO on the TD activity and the intraseasonal variability of the South Asian monsoon are still unclear, which may reflect ambiguous effects or interdecadal changes (Lawrence and Webster 2001; Hoyos and Webster 2007). The influences of ENSO on the TD activity and the intraseasonal variability of the East Asian monsoon and central–western Pacific monsoon regions have been discussed in other studies (Wang et al. 2017; Wu and Cao 2017; Jiang et al. 2018). However, TD activity and monsoon trough variation and their impact on the interannual precipitation variability are critical to understanding interannual precipitation variability, particularly over the tropical Asian monsoon regions, and need to be examined further.
Finally, the impact of midlatitude influences in 2022 was briefly examined and found to be minor. In 2022, major precipitation events occurred in southern and central Pakistan, whereas in 2010, heavy precipitation occurred in northern Pakistan. Because the dominant weather systems over Pakistan vary seasonally and can be influenced by both tropical and midlatitude systems that differ from year to year, further investigation is necessary.
The analysis of this study, which was conducted only for 23 years, may not cover the possible severe floods in Pakistan induced by different mechanisms because of the low sampling. The 2022 flood may also be associated with global climate changes; future studies may need to distinguish between long-term trends and interannual variations. Very recently, a paper on the possible impact of global climate warming has also begun to discuss and possibly suggest the impact of global warming (You et al. 2024). Moreover, the future possibility of these severe flood-induced precipitation events associated with TD activity should also be investigated. Currently, it is generally difficult to predict seasonal TD activity, which is an important future task.
Acknowledgments.
This study was supported by the Third Earth Observation Research Announcement (EORA-3) of the JAXA Precipitation Measuring Mission Science and the Japan Society for the Promotion of Science KAKENHI Grants 22H00037 and 21K18403. The author declares no conflicts of interest directly relevant to the content of this article. I thank Leonie Seabrook, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
Data availability statement.
The Global Satellite Mapping of Precipitation (GSMaP) was downloaded from the website of Japan Aerospace Exploration Agency (JAXA) Global Rainfall Watch the Earth Observation Research Center (https://sharaku.eorc.jaxa.jp/GSMaP/index.htm). The JRA-55 reanalysis datasets were downloaded from https://rda.ucar.edu/datasets/ds628.0/. Both datasets require free user registration on the website to download.
APPENDIX
Perturbation Kinetic Energy (PKE)
Also, based on focusing phenomena, time means or accumulation is applied, which can provide accumulated TD activity during the focusing event. A monthly or seasonal mean PKE can quantitatively represent the TD activity during the periods of interest, such as a month or season.
Here, it is demonstrated that PKE can represent the accumulated TD activity for the two TDs in 2022 over and around Pakistan (Fig. A1). As shown in Fig. 8, the TDs were observed over and around the Indian subcontinent moving toward Pakistan. Figure A1a shows the averaged PKE values for the period from 10 to 20 July 2022. A notable signal was found over the TD region (Fig. 8). Figure A1b is another example from 19 to 24 August 2022, which also corresponded to the time sequence and track of the TD.
PKE at 850 hPa averaged (a) from 10 to 18 Jul 2022 and (b) from 19 to 24 Aug 2022, using JRA-55 reanalysis. The PKE was calculated from the high-pass-filtered six-hourly zonal and meridional wind values, which were calculated by subtracting the 25-day running mean from the original values as a high-pass time filter.
Citation: Journal of Climate 37, 21; 10.1175/JCLI-D-23-0186.1
Since the TD track is expressed as a binary representation of the presence or absence of a TD, it is highly sensitive to the used algorithm, especially the lower limit of TD detection. For TD detection over South Asia, the commonly used algorithms with appropriate thresholds are likely to be tuned to detect well-developed TDs over the ocean and the coastal regions (e.g., Hurley and Boos 2015; Hunt et al. 2016). For the inland TD cases in this study, it might be difficult to detect the TD tracks because inland TDs are generally weakened due to the influences of surface friction and the deficit of water vapor supply. Thus, it is not easy to analyze inland TDs, such as those focused on in this study, using only TD tracks or TD frequencies. From these perspectives, we manually determined the TD track (Fig. 8) and then analyzed the TD activity using PKE (Fig. 9) after confirming its usefulness (Fig. A1).
One of the major advantages of PKE for examining TD activity is that it does not require a lower limit for TD detection. In addition, PKE includes information on TD intensity. Furthermore, because we use the high-pass-filtered wind data, no large PKE value is shown over the area where the stationary monsoon westerlies dominate (Fig. 8), for example, over the Arabian Sea, and large PKE values are seen along the TD track. However, because PKE cannot determine a TD track itself, it is not suitable for a detailed examination of the structure or dynamical analysis of each TD. Also, in the case of the slower-moving TD (Fig. A1a), the PKE value may be relatively smaller than that of the moving TD (Fig. A1b).
In summary, Figs. 8, 9, and A1 show that PKE can represent the TD activity well. Also, Fig. 9 shows the seasonal anomaly of TD activity in 2022, which is consistent with other results of this study. As the usefulness of PKE has been shown in previous studies, PKE can be used for analysis of accumulated TD activity and can also be a simple variable to illustrate TD activity.
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