In boreal summer of 2022, Pakistan experienced a sequence of unusually intense monsoon rainfall surges that struck from early July to late August. During these surges, strong southerly flows originating in the Arabian Sea reached Pakistan and converged with the anomalous southward-penetrating northerly flow from the extratropics. The monsoon surges led to a record accumulated rainfall (450-mm), which was approximately four times higher than the climatological average and twice as much as the previous record set in 2010. The flooding across one-third of the country left over 30 million people homeless and resulted in 1000 deaths. At the same time, heatwaves persisted over central China and northeastern Europe. The simultaneous occurrence of these extreme events suggested a possible remote relationship between the three extreme events. In this study, we present the causes for the record rainfall and explain how the Pakistan flooding linked to the China and Europe heatwaves (Fig. 1).

Fig. 1. (a) Bars and lines represent daily and accumulated Pakistan rainfall (black box in b), respectively. (b) July–August averaged anomalous precipitation (PR, shading; unit: mm·day^-1) and 850-hPa horizontal winds (UV850, vectors; unit: m·s^-1; minimum vector: 1). Black box indicates the region for Pakistan rainfall index (c) The shading and black contour lines represent the near surface temperature (T2m; unit: °C) and 500-hPa geopotential height (H500; unit: m; interval: 15) anomalies, respectively.

The year 2022 witnessed a triple-dip La Niña that began in 2020. Previous studies have indicated that a La Niña summer tends to exhibit a strong western North Pacific Subtropical High (WNPSH) and Indian monsoon flow. The influence of La Niña-induced changes in large-scale circulation has been identified as a crucial factor in causing the 2010 extreme flooding event², a similar impact was observed in 2022³. The La Niña in 2022 was moderate, with Niño3.4 (Niño 4) index much smaller than that in 2010. However, the total rainfall in 2022 was nearly double that of the strong La Niña year in 2010. This suggests that La Niña alone cannot fully explain the anomalous rainfall in Pakistan. Additionally, extreme rainfall events did not occur in other La Niña summers, such as 1998 and 1999.

Extreme events have been reported to occur when different influencing factors synchronized. Figure 2a summarizes the compounding factors leading to the 2022 flooding. The unusually strong easterly anomaly induced by La Niña over the northern Indian subcontinent, along with an enhanced southerly flow from the Arabian Sea (which is characterized by an enhancing trends in the past few decades), resulting in a strong convergence of moisture flux, promoting the development of intense convection and heavy rainfall over mountainous Pakistan. The third and prolonged surge in August was particularly influenced by the interaction between an unusually warm-moist southerly flow from the Arabian Sea and an extratropical cold-dry northerly anomaly associated with upstream Europe blocking. Furthermore, the tropical–extratropical interaction played a significant role, contributing approximately 50% of the total rainfall during the third surge. It is important to note that these contributing factors generally occur independently and rarely coincide simultaneously, which explains the infrequency of such extreme events. We argue that only when these compounding factors occur concurrently can they lead to the manifestation of extreme events. As demonstrated in Figure 2b, the proposed factors synchronized and reached unusually high values in 2022, setting it apart from other years. This was also the case in the 2010 Pakistan flooding event, as well as in extremely dry 1987 when all factors synchronized in negative phase. The compounding effects of influencing factors were similar to those identified for the 2010 Pakistan flooding¹. It could be viewed as an intensified manifestation of the 2010 Pakistan flooding event under a warming trend.

Fig. 2. (a) Schematic diagram presents the main results of this study. (b) Relive contribution of multiple influencing factors. The stacked bars (refer to left y-axis) are normalized indices of 500-hPa geopotential height (H500) averaged over Central China (CC; 25°–35°N, 90°–120°E; yellow), H500 averaged over Northeastern Europe (NEU; 55°–70°N, 35°–60°E; orange), 850-hPa meridional moisture flux (Vq850) over Arabian Sea (AS; 5°–20°N, 55°–70°E; light gray), zonal wind (U850) over northern Indian subcontinent (NI; 20°–30°N, 70°–90°E; blue), and sea surface temperature (SST) in Niño3.4 region (N34; 5°S–5°N, 170°–120°W; deep gray). Note that NI and N34 indices were multiplied by “-1”.

Wave activity flux and regression analyses unveiled a distinct stationary Rossby wave-like pattern connecting the flooding in Pakistan and heatwaves in Europe and China. Positive streamfunction anomalies (i.e., blocking or ridges) were associated with heatwaves in Europe and China, while negative anomalies (i.e., deepened troughs) were associated with third monsoon surge, contributing approximately half of Pakistan rainfall (Figure 3). This pattern, an emerging teleconnection pattern in recent decade, exhibited substantial differences from the reported teleconnection patterns, such as Scandinavia, circumglobal teleconnection, or British-Okhotsk Corridor pattern⁴. The nature of teleconnected extreme events is a newly emerging area of research that warrants further investigation.

The concurrence of flooding in Pakistan and the heatwave in central China suggests a potential mutual enhancement between wet and dry extremes. Our analysis supports this hypothesis by revealing a potential positive feedback between the two extremes. Specifically, the ascending motion associated with flooding in Pakistan and the descending motion associated with the heatwave in China were coupled. Numerical experiments indicate that the anomalous rainfall in Pakistan may trigger an upper-level anticyclone downstream, strengthening the anticyclone associated with the China heatwave. Conversely, the upper-level anticyclonic anomaly associated with the heatwave exhibited an equivalent barotropic structure, with an associated easterly anomaly extending from the upper troposphere to the lower troposphere. This easterly anomaly was suggested to be correlated with the ascending motion in Pakistan⁵. However, the specific feedback processes between the China heatwave and Pakistan rainfall remain unclear and require further investigation.

It has been reported that weather and climate extreme events are expected to increase in frequency and intensity as a result of a warming climate. Our findings suggest that the record rainfall observed in Pakistan in 2022 may be a footprint of a warming climate. The enhanced southwesterly flow over the Arabian Sea, which played a critical role in facilitating strong moisture flux convergence, and this enhancement can be attributed to recent trends. Under an extreme warming climate, further investigation is needed to understand the potential changes in the southwesterly flow over the Arabian Sea and its possible impact on tropical–extratropical interaction and extreme weather events in these regions.

Find the paper at: https://www.nature.com/articles/s41612-023-00492-2

Reference:

1. Hong, C.-C., Hsu, H.-H., Lin, N.-H. & Chiu, H. Roles of European blocking and tropical-extratropical interaction in the 2010 Pakistan flooding. Geophys. Res. Lett. 38, L13806 (2011).