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

Zoom In Zoom Out Reset image size

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 ($- \left\langle {u{^{^{\prime}}} \cdot {\nabla _x}\bar q} \right\rangle$) and the climatological meridional advection of the anomalous meridional moisture gradient ($- \left\langle {\bar v \cdot {\nabla _y}q{^{^{\prime}}}} \right\rangle$). In comparison, both vertical convection terms ($- \left\langle {\omega {^{^{\prime}}} \cdot {\partial _p}\bar q} \right\rangle ,\, - \left\langle {\overline{\omega} \cdot {\partial _p}q{^{^{\prime}}}} \right\rangle$) make a positive contribution to the rainfall anomaly with weak strength. The anomalous meridional advection of climatological meridional moisture gradient ($- \left\langle {v{^{^{\prime}}} \cdot {\nabla _y}\bar q} \right\rangle$) makes a negligible contribution. On the contrary, the climatological zonal advection of the anomalous zonal moisture gradient ($- \left\langle {\bar u \cdot {\nabla _x}q{^{^{\prime}}}} \right\rangle$) 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.

Zoom In Zoom Out Reset image size

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 ($- \left\langle {\omega {^{^{\prime}}} \cdot {\partial _p}q{^{^{\prime}}}} \right\rangle$) is positive in the central parts of Pakistan, with maximum values in Balochistan (figure 2(b)). The anomalous vertical advection of climatological vertical moisture gradient ($- \left\langle {\omega {^{^{\prime}}} \cdot {\partial _p}\bar q} \right\rangle$) 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 (${\partial _p}q{^{^{\prime}}}$) 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). In the study area, the parameter ${\partial _p}\bar q$ 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 ($- \left\langle {\bar u \cdot {\nabla _x}q{^{^{\prime}}}} \right\rangle$ and $- \left\langle {\bar v \cdot {\nabla _y}q{^{^{\prime}}}} \right\rangle$), 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 ($- \left\langle {u{^{^{\prime}}} \cdot {\nabla _x}\bar q} \right\rangle$) 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.

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.

Zoom In Zoom Out Reset image size

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.

Zoom In Zoom Out Reset image size

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 conditions in Pakistan. The tropical Pacific experienced a rare strong La Niña event in 2022 and whether this event is able to influence the rainfall pattern in Pakistan is still an open question. The reason for SSTA warming in the northern part of the AS, which is also crucial for the anomalous rainfall in Pakistan, remains unresolved in this research.