Assessing the Impact of Global Meteorological signals on Drought occurrence in Iran

This study investigated the complex interplay between global meteorological signals—specifically the Southern Oscillation Index (SOI) and the North Atlantic Oscillation (NAO)—and drought occurrence in Iran. Utilizing the Standardized Precipitation Index (SPI), a widely recognized metric for characterizing drought conditions, the research analyzed rainfall data from 79 synoptic stations across Iran, spanning a three-decade period from 1988 to 2017. The primary objective was to provide a detailed understanding of the spatial and temporal patterns of drought variability nationwide and to elucidate the varying degrees of correlation between these global climatic drivers and regional drought dynamics.

The analysis revealed intricate spatiotemporal drought patterns across Iran. Generally, the country experienced mild to moderate droughts, with a notable exception: the Lake Urmia basin, which exhibited relatively wetter conditions throughout the study period. Conversely, the Central Plateau and Eastern Border regions emerged as particularly vulnerable to drought, a vulnerability that was acutely observed during the 1998-2007 period. A deeper dive into the data, using minimum SPI analysis, suggested a country-wide susceptibility to drought. However, the mean SPI analysis highlighted the eastern border region as being consistently affected, while milder drought conditions were observed in the Central Plateau and Qarah-Qom regions.

The study employed composite indices of SPI coupled with SOI and NAO, revealing distinct regional patterns of drought severity and sensitivity to these global climate signals. The SPI-SOI index indicated that the Caspian Sea sub-basin experienced the least severe drought, whereas the Qaraqum sub-basin suffered the most. Similarly, the SPI-NAO index showed the Lake Urmia sub-basin with the lowest drought severity and the Eastern Border sub-basin with the highest. These regional differences underscore the varied ways in which different parts of Iran respond to large-scale atmospheric forcing. Periodic indices for each decade further emphasized these regional and temporal variations, consistently showing higher drought severity in the Eastern Border and Qaraqum sub-basins, particularly during the 1998-2007 and 2008-2017 periods, which experienced more intense drought compared to 1988-1997.

Temporal variations in drought severity were a key finding, highlighting the dynamic nature of drought and the imperative for continuous monitoring. The study identified a general trend of shifting drought patterns across the three decades. Initially (1988-1997), mild droughts were prevalent, with the Central Plateau experiencing the most severe conditions. This was followed by a significant intensification of drought (1998-2007), impacting the entire country, with the Central Plateau once again being the most affected. Subsequently, a decrease in severity was observed (2008-2017), leading to a return to mild drought conditions in most regions. This observed shift aligns with findings from other studies in the Mediterranean region, which also reported an increase in drought periods during the late 1990s and early 2000s, consistent with work by Hoerling et al. (2012) and Spinoni et al. (2015). The study period notably encompasses two of the most extensive and devastating droughts in the Mediterranean basin over the past 40 years (1999-2001 and 2007-2012), as referenced by Mathbout et al. (2021). The extended observational period to the end of 2017 in this study, while maintaining overall consistent trends, allowed for a more comprehensive temporal analysis.

The analysis of mean SPI changes further reinforced the trend of increasing drought, particularly in the eastern border region. During the second and third decades (1998-2007 and 2008-2017), this region experienced significant drought, with the Central Plateau and Qaraqum also showing signs of mild drought. This eastward expansion of drought raises considerable concerns regarding long-term water security in Iran. Conversely, the earlier period (1988-1997) saw only mild drought in the Eastern Iranian border basin, with other regions experiencing near-normal to slightly wet conditions, suggesting a potential influence of climate change on regional drought patterns. The results, revealing a decrease in SPI values during winter, wet months, and on an annual scale, and an increase in SPI values during summer, confirm a tendency towards decreasing winter and annual precipitation and increasing summer precipitation identified in several regions of Iran (Caloiero and Veltri, 2019).

Regarding the specific influence of global meteorological signals, the Southern Oscillation Index (SOI), which reflects the El Niño-Southern Oscillation (ENSO), was found to significantly exacerbate drought conditions, particularly in southern and southeastern Iran. The North Atlantic Oscillation (NAO), however, exhibited a more complex and regionally varying influence, with a less pronounced overall impact. These findings underscore the importance of understanding the individual and combined effects of these climate signals for accurate drought prediction and mitigation. The study's results corroborate previous research in the Mediterranean area, which often acknowledges the NAO as a primary driver of drought periods in the region (e.g., Vicente-Serrano et al., 2011). Strong positive phases of the NAO are typically associated with below-normal temperatures and precipitation in the study area, while negative phases are linked to opposite patterns (Caloiero et al., 2011). The observed dry conditions at the beginning of this century, corresponding to a positive phase of the NAO, further support this link. While ENSO's influence on the Calabria region's rainfall has been noted as weak in prior studies (Caloiero et al., 2011), its significant role as a drought driver in other regions like Turkey and China is recognized. Beyond SOI and NAO, the study also revealed a strong influence of the Mediterranean Oscillation (MO), consistent with findings from Mathbout et al. (2021).