The impact of tropical sea surface temperature on extreme precipitation in Pakistan during the summer of 2022

In August 2022, Pakistan experienced an unprecedented precipitation event that caused significant damage. Analysis of the observations reveals that this extreme rainfall is primarily driven by anomalous atmospheric zonal advection, resulting in an anomalous water vapor concentration in Pakistan. The climatological meridional advection also contributes to this flooding. Anomalous easterly winds and low-level vertical convection combine to be critical factors contributing to the moisture concentration over the country. Further investigation identifies the air-sea interaction over the tropical Indian Ocean and abnormal warming over the Arabian Sea are crucial factors influencing this extreme flooding event. The concurrent occurrence of a negative Indian Ocean Dipole event and the warming sea surface temperature anomalies in the northern Arabian Sea intensifies the easterly winds over Pakistan, helping to transfer the anomalous water vapor from the remote region into Pakistan, ultimately contributing to the extreme flooding in 2022.


Introduction
In 2022, Pakistan experienced severe flooding due to heavy monsoon rains.The multi-day anomalously high rainfall began in July 2022, resulting in approximately double (180%) the long-term average rainfall for that month in Pakistan (Ma et al 2023, Qamer et al 2023).Subsequently, in August 2022, rainfall continued to increase, exceeding 500% of its climatology average, ultimately leading to extreme floods in Pakistan (Malik et al 2023).Heavy rains in Pakistan have caused nearly one-third of the country to be submerged, leading to the loss of 1700 lives and the displacement of 8 million people.This has affected over 33 million individuals, roughly 15% of the country's population, according to the National Disaster Management Authority of Pakistan (Bhutto et al 2022, UN News 2022).
Pakistan is located in the western part of the Indian subcontinental, and its complex topography undergoes substantial changes in altitude.The unique geography and topography of Pakistan have played an essential role in shaping Pakistan's climate.Pakistan experiences rainfall primarily during two distinct seasons: the boreal summer and winter (Khan 1993, 2002, Kureshy 1998).During the summer months from July to September bring significant monsoon rains originating from the east and northeast (Luo andLin 1999, Salma et al 2012).Southern Pakistan, including regions like Sindh and southern Balochistan, experiences relatively weak rainfall, with precipitation often occurring due to tropical cyclones (Fatima and Safdar 2011, Wu et al 2022).
The Arabian Sea (AS) and the Bay of Bengal (BOB) have been identified as crucial sources of moisture for summer precipitation in Pakistan.Numerous studies have demonstrated that during the monsoon season, moisture from the BOB can join forces with low-pressure systems in the AS or abnormally high pressure on the Tibetan Plateau to generate substantial rainfall in Pakistan, occasionally resulting in severe flooding (Ullah and Gao 2012, Rasmussen et al 2015, Bibi et al 2020).The monsoonal depression formed in BOB could bring rainfall to the areas along its path, including Pakistan.Due to the influence of large-scale circulation, the trajectory of the monsoonal depression is not fixed.The Pakistan Meteorological Department (PMD 2022) divides the trajectory from BOB to Pakistan into three mechanisms.In two instances, the depressions from BOB through the Gangetic plains over India reach Pakistan, promoting rainfall in central and northern Pakistan.In the third scenario, as the depression passes through the Arabian Sea, moisture from AS and BOB converges, accompanied by upper atmospheric circulation such as jet streams, resulting in precipitation over Pakistan (Otto et al 2023).Besides, the AS could also adjust the rainfall in Pakistan by changing the atmospheric circulation (Ullah and Gao 2012, Li et al 2022, Mallapaty 2022, Nanditha et al 2023).
The monsoon that affects summer rainfall in Pakistan is part of the South Asian Summer Monsoon (SASM) system.Strong evidence indicates a growing trend in extreme precipitation in South Asia (Seneviratne et al 2021), along with an increase in the intensity and westward movement of the monsoon over Pakistan (Hanif et al 2013).The latest research suggests that warming land surface temperature in the Middle East region during spring contributes to summer precipitation in monsoon edge regions, such as Pakistan (Li et al 2023).Recent studies have shown that monsoon precipitation in Pakistan exhibits high spatial and temporal variability (Gadiwala andBurke 2013, Hanif et al 2013).Tropical air-sea interaction events, such as the ENSO and the IOD, can influence precipitation in Pakistan through both local and remote effects (Liu et al 2011, 2014, 2017, Yang et al 2019).The strong La Niña could enhance convection and circulation over BOB, and in conjunction with the moisture transport in AS, causing persistent moist convective activity over the Indo-Pakistan region (Mujumdar et al 2012).In addition, ENSO may affect the onset of the summer monsoon in Pakistan (Ali et al 2019).In the analysis of the relationship between IOD and precipitation in Pakistan, researchers hold various opinions.Hussain et al (2017) suggest that rainfall could increase in coastal and southwestern regions of Pakistan during the positive IOD (pIOD) from 1958 to 2010.In contrast, Iqbal and Hassan (2016) conclude that Pakistan experienced its most significant proportion of annual floods after 1970 during El Niño, non-ENSO, and pIOD years.
It is not the first time Pakistan has suffered such destructive floods in 2022.In the summer of 2010, the country experienced striking floods due to excessive rainfall (Hong et al 2011, Galarneau et al 2012, Paulikas and Rahman 2015, Nie et al 2016).The heavy rainfall in 2010 mainly affected the northern region, whereas the anomalous rainfall in 2022 was concentrated in southern Pakistan.The rainfall in August 2022 exceeded 726% in Sindh and 590% in Balochistan, exceeding the record set in 2010, making it the most intense precipitation event on record (Chughtai 2022, PMD 2022).Previous studies on extreme rainfall in Pakistan have primarily examined the high-frequency time scale and the northern part of the country (Hong et al 2011, Hunt et al 2018, Battula et al 2021).Few studies have investigated the extreme rainfall events in the densely populated southern part of Pakistan.Abnormal changes in tropical sea surface temperature (SST), with a strong La Niña event and a negative IOD (nIOD) event occurring in the Pacific Ocean and the Indian Ocean, respectively, in 2022, which may present impact on the rainfall of Pakistan.The study by Ma et al (2023) does not account for the teleconnection between the weather in mid-latitude areas, such as Pakistan, and the sea surface temperature anomalies (SSTA) in tropical oceans.He et al (2023) suggest that there is a lack of quantitative description of the impact of SSTA in the tropical Indian Ocean.This study aims to use the moisture budget to quantitatively reveal the influence from SSTA in the Indo-Pacific region on the rainfall of Pakistan.The rest of the paper is organized as follows.The descriptions of the data and methodology are presented in section 2. Section 3 presents the results and the discussion.Finally, a summary is provided in section 4.

Data and methodology
The monthly datasets used for the moisture budget analysis mainly come from the ECMWF Reanalysis v5 monthly data (ERA5) (Hersbach et al 2020).These datasets include single-level data such as precipitation and evaporation, as well as multi-level data comprising horizontal and vertical winds, specific humidity, and geopotential height across 23 pressure levels ranging from 200 to 1000 hPa).To accurately depict the rainfall patterns in the study area, the monthly mean precipitation data for climatological analysis is downloaded from the global precipitation climatology project (GPCP) (Adler et al 2018).The monthly mean SST data is obtained from NOAA extended reconstructed SST V5 (Huang et al 2017).
The ERA5 reanalysis data is on a grid of 0.25 • × 0.25 • , whereas the SST and GPCP data are on a grid of 2.5 • × 2.5 • .The 44 year average from 1979 to 2022 is defined as climatology, and the anomalous field is the deviation of its mean from the long-term climatology.The anomalous field was selected in August 2022.
The moisture budget analysis is conducted to uncover the mechanisms behind precipitation anomalies in Pakistan.The vertically integrated moisture equation is as follows: Overbars represent the climatological monthly mean, and primes indicate departures from the climatology, ⟨ ⟩ indicates vertical integration from the surface (p = 1000 hPa) to the top (p = 200 hPa) of the atmosphere; subscript h represents the horizontal direction; p (hPa) represents pressure; q (g/kg) denotes specific humidity; ⃗ u (m/s) denotes meridional and zonal winds; ω (Pa/s) denotes vertical velocity; P (mm/day) and E (mm/day) is precipitation and evaporation, respectively (Trenberth and Guillemot 1995).R represents the residual term introduced to balance the equation, accounting for errors in the calculation process and nonlinear terms (such as the anomalous zonal winds advection of the anomalous zonal moisture gradient, − ⟨u ′ • ∇ x q ′ ⟩) (Dominguez et al 2006, Li et al 2010, Minallah and Steiner 2021).
The second and fourth terms on the righthand side (− ) are associated with changes in water vapor, mainly induced by temperature changes, so they are referred to as thermodynamic contributions.Similarly, the two terms −

Anomalous rainfall in 2022
In August 2022, Pakistan experienced extreme flooding with a strong rain belt expanding from the northern part of the AS to the north boundary of Pakistan (figure 1(b)).The maximum rainfall is located in the central and southern parts of the country, while a portion of the eastern region witnesses an abnormal drought event.In the northern part of this country, moderate rainfall is observed.The anomalous rainfall distribution is the opposite of the typical climatological distribution.At climatological status, the central and northern parts of Pakistan are in drought, and the rainfall area mainly stays in the northeastern of the country.The abnormal rainfall significantly influenced the total precipitation of the country during the summer, reaching a 50 year record high (figure 1(c)).Significant interannual variability is evident in figure 1(c), indicating that rainfall variation has been quite complex over the past decades.

Moisture budget analysis
The results of the moisture budget analysis for August 2022 in Pakistan are presented in figure 2. Quantitatively, the horizontal advection movements play crucial roles in the abnormal precipitation in the study area, while the vertical convection makes a secondary contribution to the rainfall anomaly due to the weak amplitude associated with each term.The primary factor contributing to rainfall anomaly is the anomalous zonal advection of the climatological zonal moisture gradient (− ⟨u ′ • ∇ x q⟩) and the climatological meridional advection of the anomalous meridional moisture gradient (− ) make a positive contribution to the rainfall anomaly with weak strength.The anomalous meridional advection of climatological meridional moisture gradient (− ⟨ v ′ • ∇ y q⟩ ) makes a negligible contribution.On the contrary, the climatological zonal advection of the anomalous zonal moisture gradient (− ⟨ū • ∇ x q ′ ⟩) presents a negative contribution to the precipitation during the same period.In summary, the influence of horizontal advection significantly contributes to the positive precipitation, resulting in the increased rainfall in Pakistan in August 2022.
The spatial distribution of convection and advection terms are listed in figure 2 to illustrate the different contributions to the moisture concentration.In terms of vertical vapor transportation, the anomalous vertical moisture gradient convected by the climatological vertical advection (− ).In the study area, the parameter ∂ p q is positive, and the anomalous vertical velocity is upward.This results in a positive anomalous  vertical advection, which significantly contributes to the moisture concentration and leads to positive rainfall in Pakistan.
In the spatial distribution diagrams of the anomalous horizontal moisture gradients advected by climatological horizontal advection (− ⟨ū • ∇ x q ′ ⟩ and − ⟨ v • ∇ y q ′ ⟩ ), both terms contribute oppositely to the rainfall.The zonal advection leads to a significant reduction in rainfall across the entire country, with a more noticeable decrease in eastern Balochistan.The meridional advection plays an essential role in the positive rainfall over most of the country, especially over the northern boundary regions (figures 2(d) and (e)).In addition to climatological advection, anomalous zonal advection presents a dominant role in moisture concentration.The zonal gradient of q advected by the anomalous zonal advection (− ⟨u ′ • ∇ x q⟩) makes the dominant contribution to the local rainfall, which brings substantial positive precipitation over most of Pakistan (figure 2(f)).Since annual precipitation in the eastern area is greater than in the western area, the zonal gradient of climatological q is positive.Considering the anomalous easterly wind pattern across most of Pakistan in August 2022, the combined effect has a positive impact on the overall moisture concentration.The contribution to the total moisture from the anomalous meridional advection can be ignored due to its weak value (figure 2(g)).
Due to the requirement of meteorological data ranging from 1000 hPa to 200 hPa for moisture budget analysis, the results of this method may be influenced by the local terrain distribution.The eastern parts of Pakistan, especially in the regions of Sindh and Balochistan, which were severely affected by the heavy rainfall, have a lower altitude than the western part.Despite the value of R, the conclusion of this article is supported by the analysis results.

Discussion
The climatological moisture zonal gradient advected by anomalous zonal advection plays a dominant role in the precipitation of the country.Since the moisture gradient is climatological, the abnormal contribution to the moisture concentration mainly comes from the zonal advection anomaly.The horizontal wind anomalies at the top (200 hPa) and bottom (850 hPa) of the troposphere in August 2022 over the northern Indian Ocean are listed in figures 3(a) and (b), associated with the SSTA in the same diagrams.During the study period, a significant anomalous easterly wind pattern extends throughout Pakistan, from the bottom to the top of the troposphere.This pattern is the primary contributor to the rainfall anomaly in the country.The mechanism that caused the easterly wind anomalies in 2022 needs to be addressed.
The spatial distribution SSTA demonstrates a typical nIOD event in August 2022 and an east-west seesaw distribution with positive SSTA in the east and negative SSTA in the west.During the same period, a significant positive SSTA region exists at the top of the AS.At the lower level, the anomalous easterly winds prevail from India to Pakistan, with a maximum speed beyond 4 m s −1 (figure 3(a)).A cyclonic circulation dominates the northern part of the AS, expanding from 12 • N to Sindh in southern Pakistan, with its center at 70 • E 18 • N. Accordingly, another cyclonic circulation occurs in the southern part of the BOB.Yu (2005) points out that IOD events could induce cyclonic circulation along equatorial regions by modulating the local atmospheric circulation.During the nIOD event in August 2022, a cyclonic circulation prevails in the southern part of the BOB, leading to easterly wind anomalies in the central BOB and the Indian Peninsula and bringing abundant water vapor from the BOB to the Indian Peninsula.
At the same time, the positive SSTA at the top of the AS modulates the local atmospheric circulation and favors the intensification and maintenance of anomalous easterly winds on the eastern side of the warming SSTA, which strengthens the easterly winds expanding from the Indian into Pakistan.Such anomalous easterly winds are carrying water vapor from the BOB into the AS and Pakistan, inducing extreme rainfall in Pakistan in 2022.The anomalous easterly winds, vital in Sindh and Balochistan, diminish rapidly in Punjab and eventually dissipate.This low-level circulation contributes to a convergent upward flow in the AS, which promotes the enhancement and persistence of vertical upward convection.The anomalous easterly winds over the southern part of Pakistan could bring sufficient moisture from the BOB into the AS, contributing to the unprecedented heavy rainfall in Sindh and southern Balochistan during the study period.The air-sea interaction in the East Indian Ocean and the warming SSTA at the top of the AS modulate the significant easterly wind anomalies over Pakistan, bringing sufficient water vapor into Pakistan and inducing heavy rainfall in the country.The related mechanisms are summarized in figure 4.During the nIOD event, a significant anomalous cyclone circulation remains over the BOB, which facilitates the prevalence of easterly winds crossing the Indian subcontinent into Pakistan, leading to the water vapor transportation from the BOB to Pakistan, favoring the extreme flooding in August 2022.
Many studies indicate that the massive moisture transport from the BOB is the main aspect that caused the extreme precipitation in Pakistan in August 2022.However, the abnormal precipitation may be linked to abnormal land or SST with supporting physical mechanisms argued by both sides (Otto et al 2023, Nanditha et al 2023).During the summer of 2022, researchers observed that extraordinary long-term heat waves hit Russia, southwestern China, and Pakistan (Mallapaty 2022, Rosane 2022, He et al 2023).Thus, the increase in land surface temperature strengthens the low-pressure system over land, leading to enhanced water vapor convergence.The mid-latitude atmospheric circulation pattern could affect Pakistan's rainfall to a certain extent.In addition, the abnormal rainfall as a non-adiabatic heating forcing may generate feedback on anomalies in the upper and lower tropospheric circulation (Lu et al 2020, Hsu et al 2022).The extreme rainfall event in 2022 is comparable to the one in 2010, both coinciding with the presence of La Niña in the tropical Pacific Ocean (Hong et al 2011, Di Capua et al 2021, Aziz 2022).Priya et al (2015) believe that La Niña could enhance Walker circulation and the east-west circulation of the Indian Ocean, leading to intense rainfall in northern Pakistan.The SST magnitude of La Niña in 2010 was more intense than in 2022, while La Niña in 2022 is accompanied by nIOD, which indicates abnormal warming of the sea surface in the East Indian Ocean (Otto et al 2023).Moreover, as mentioned by Martius et al (2013), the warming SSTA may increase evaporation and water vapor content in the troposphere, accompanied by increased convective instability, favoring a large amount of moisture over Pakistan (Ullah et al 2021).The strength and location of the Western Pacific subtropical changed dramatically by La Niña also favors this process (Qiao et al 2023).
Previous studies have shown that La Niña has a negative impact on rainfall in Pakistan (Khan 2004) and the pIOD can increase rainfall in the coastal and southwestern regions of this country (Hussain et al 2017).How about the conditions when both La Niña and nIOD occur in the same year?The composite map illustrates that the rainfall anomaly within Pakistan is negative in the years of both La Niña and nIOD (figure 1(d)), which is in contrast to the condition in 2022.We speculate that the strong La Niña events may play some roles in the abnormal con- warming in the northern part of the AS, which is also crucial for the anomalous rainfall in Pakistan, remains unresolved in this research.

Conclusions
Pakistan is modulated by the monsoon system, with the dominant rainy season taking place during the summer months due to the impact of the South Asian monsoon.However, the country has experienced numerous meteorological disasters that have had a severe impact on its society and economy.These disasters, combined with the country's intricate topography and economic underdevelopment, have presented numerous challenges for social activities.In August 2022, an unprecedented precipitation occurred in this country.The research conducted in this study aims to investigate the causes of the flooding that occurs during the event.To accomplish this goal, we utilized data from oceanic and atmospheric reanalysis to conduct an analysis revealing the mechanisms responsible for this extreme flooding event.
The analysis conducted in this study demonstrates that anomalous zonal advection plays the most significant role in the concentration of local water vapor, as indicated in the moisture budget analysis.Another contributing factor is the climatological meridional advection.In contrast, the climatological zonal advection has a negative effect on the moisture concentration.Further investigations reveal that a combination of anomalous easterly winds and anomalous vertical convection in the low troposphere play critical roles in contributing to the moisture concentration.
Anomalous convection over the Pakistan region is closely linked to the local cyclonic circulation in the northern part of the AS.Analysis of wind fields suggests that significant anti-cyclonic circulation around the periphery of the upper troposphere facilitated the formation and dispersion of convection.The lower troposphere in southern Pakistan displays apparent anomalous easterly winds that dissipated quickly towards the north.The significant cyclonic circulation in the adjacent southern AS also contributes to maintaining the easterly winds over Pakistan.In 2022, a significant nIOD event occurred in the tropical Indian Ocean region, resulting in prevailing westerly winds over the eastern tropical Indian Ocean.This abnormal event aids the formation of cyclonic circulation in the southern part of the BOB, strengthening the easterly wind anomalies over the central BOB, which extends into the AS and Pakistan region.The anomalous easterly winds carry a continuous stream of moisture from the BOB to the AS, contributing to convergence and upward movement in Pakistan.As a result, it facilitates the occurrence of extreme rainfall in August 2022.
to changes in anomalous advection are the dynamic component(Chou and Lan 2012, Chou et al 2013).
the central parts of Pakistan, with maximum values in Balochistan (figure 2(b)).The anomalous vertical advection of climatological vertical moisture gradient (− ⟨ ω ′ • ∂ p q⟩ ) also contributes to the significant water vapor concentration in the central of Pakistan, associated with the negative water vapor concentration over the northern part of the country (figure 2(c)).During the study period, the anomalous relative humidity vertical gradient (∂ p q ′ ) shows an east-west seesaw distribution, with positive values in the east and negative values in the west.Under climatic conditions, upwelling prevails in August, except for small areas in Khyber-Pakhtunkhwa.The vertical water vapor change mainly depends on the local water vapor gradient, which is related to the evaporation process from the surface.Therefore, the anomalous moisture vertical gradient convected by climatological vertical advection reveals that this positive precipitation anomaly is more closely related to the water vapor change associated with global warming and increased aerosol emissions(Zhang et al 2020, Allan and Douville 2023, Kotz et al 2023, Martinez and Neelin 2023

Figure 1 .
Figure 1.(a) Climatological precipitation in August.(b) Anomalous distribution of precipitation in August 2022.The location of the red box is 23 • N-37 • N, 60 • E-80 • E. (c) Time series of averaged Pakistan rainfall in the summer season.The red points indicate the years of negative IOD and La Niña.(d) The composite map of precipitation anomaly in August season during La Niña and negative IOD events.

Figure 3 .
Figure 3. Spatial distributions of anomalous wind fields (vector) at (a) 850 hPa/(b) 200 hPa and climatological wind fields at (c) 850 hPa/(d) 200 hPa in August 2022.The shadings in (a) and (b) are the SSTA in August 2022.The shading in (c) and (d) are climatological q distributions at each level.

Figure 4 .
Figure 4. Schematic map of the mechanism responsible for the flooding in Pakistan.In the upper layer (lower troposphere), the shading indicates the water vapor transportation in August 2022 (g • cm −1 • hPa −1 • s −1 ), the red box is the study region, and the black solid vectors denote atmospheric circulation.In the bottom layer (ocean), the shading denotes SSTA in August 2022 ( • C), and the dashed line indicates the atmospheric circulation is stimulated with the local SSTA distribution.