تاثیر ال‌نینو-نوسان جنوبی (انسو) بر جنوب‌غرب آسیا در اقلیم آینده

The climate is an important factor in the life of humans and other living species. This is particularly the case when a region is influenced by extreme weather and climate events. The atmospheric and oceanic phenomena are the main cause of a large part of climate variability. Teleconnections are among the main factors contributing to interannual climate variability in different regions of the globe, the most important of which in the interannual timescale is the El Niño-Southern Oscillation (ENSO). The first studies about the contribution of teleconnection to climate variability of different regions were conducted by Walker in the 1920s. His studies indicated that atmospheric circulation in the tropical Pacific largely influences the South Asian monsoon. Later studies also confirmed that ENSO has global teleconnections. Indeed, ENSO largely influences agriculture, forestry, the hydrological cycle, the carbon cycle, land and ocean ecosystems, and fisheries. Thus, any substantial changes in the frequency and intensity of ENSO have large socioeconomic impacts. In recent decades, there has been growing interest in investigating changes in the frequency and intensity of El Niño and La Niña events in response to global warming.

There is growing evidence that the climate of different regions of our planet has undergone substantial changes in recent decades in response to anthropogenic global warming. For example, ENSO characteristics have changed since the 1990s, which could be caused by changes in natural or anthropogenic forcings or a combination of both. Accordingly, ENSO teleconnections have also changed in recent decades, making the prediction of weather and climate extremes a challenging task.

Most regions of Southwest Asia are characterized by a semi-arid to arid climate, implying that a substantial decrease in its water resources under future global warming would be a major threat and may hamper sustainable development plans. Indeed, among the vulnerable regions to global warming is Southwest Asia because the shortage of water supply has already been a major challenge in this region. Despite that, there has been little investigation to understand climate change impacts in Southwest Asia. In particular, the impact of changes in the characteristics of ENSO on the climate of Southwest Asia has received little attention.

The Coupled Model Intercomparison Project Phase 6 (CMIP6) provides an opportunity for a better understanding of ENSO impacts on the climate of Southwest Asia under future global warming. In this study, we investigate the impact of El Niño and La Niña on the climate of Southwest Asia in the future period (2050 to 2099) compared to the historical period (1950 to 1999). Atmospheric variables that are analyzed in Southwest Asia include sea surface temperature (SST), near-surface temperature, geopotential height at 500 hPa, and precipitation. First, based on the empirical orthogonal function (EOF) analysis, we evaluated the performance of 12 CMIP6 models in terms of their ability to diagnose El Niño and La Niña events in recent decades. The EOF analysis of the Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5) data and its comparison with the ensemble of the applied 12 CMIP6 models indicate the weak performance of the ensemble of the selected models in diagnosing El Niño and La Niña events. Among the examined models, our results indicate that the CNRM-CM6-1 model shows better performance in terms of the diagnosis of El Niño and La Niña events, while the worst performance belongs to CIESM. As such, we used the outputs of the CNRM-CM6-1 model under three scenarios of SSP1-2.6, SSP2-4.5, and SSP5-8.5 and investigated the impact of the two extreme phases of ENSO (El Niño and La Niña) on the climate of Southwest Asia. Our results show that the frequency of El Niño and La Niña will decrease under all three scenarios in the future climate. The comparison of the CNRM-CM6-1 model with the ERSSTv5 data in the historical period indicates that the frequency of both El Niño and La Niña events is about 31% less than that diagnosed based on the ERSSTv5. The analysis of the outputs of the CNRM-CM6-1 model indicates that the frequency of El Niño events in the future period under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios compared to the historical period decreases by 5, 10, and 10 percent, while the frequency of La Niña events under SSP1-2.6 and SSP2-4.5 scenarios decreases by 18 and 6 percent. The analysis of geopotential height at 500 hPa over Southwest Asia indicates the development of different patterns in El Niño and La Niña years under all three scenarios in the future period, while in the historical period, the pattern of geopotential height at 500 hPa does not significantly change from El Niño to La Niña years. We also found that the intensity of the subtropical jet stream in Southwest Asia in the historical period was higher during La Niña events compared to El Niño and neutral events. The intensified jet stream in La Niña years implies the development of a less wavy jet stream, which may have implications for changes in extreme weather events. Under the SSP2-4.5 scenario, precipitation is relatively high in the winter of El Niño years in most parts of Southwest Asia except northeastern Iran. Relatively high precipitation is also simulated in the summer of El Niño years in most parts of Southwest Asia except southeastern Iran. Our analysis indicates different patterns of near-surface temperature in Southwest Asia in El Niño and La Niña years under SSP1-2.6 and SSP5-8.5 scenarios in the future period, particularly for the latter scenario. We argue that the response of ENSO and its teleconnections to global warming is nonlinear.