1. Introduction
Today, it has been proven that changes in the intensity and frequency of extreme events will have a much more destructive effect on human health, social units, and natural systems than changes in the average climate state. Extreme weather events are phenomena that are rare in frequency and have a high degree of intensity and, when they occur, cause serious changes to the ecosystem and residents of the area. What this research seeks to find a scientific and noteworthy answer to is: What type of blocking is affecting the extreme temperatures of the southern coast of the Caspian Sea?
 
2. Methodology
The southern coast of the Caspian Sea is located between the meridians of 34°48′ to 20°56′ East longitude and the latitudes of 44°35′ to 53°38′ North latitude in the northern half of Iran. In this study, to identify the days of freezing temperatures and their relationship with blocking, first, daily maximum temperature data from 7 stations of Astara, Bandar Anzali, Rasht, Ramsar, Nowshahr, Babolsar, and Gorgan for 25 years (1986 to 2010) were received from the Iranian Meteorological Organization. The criterion for extremely warm days was also considered the 90th decile of the maximum daily temperature. The blocking criterion used in this study is a modified version of the blocking determination that has been examined in TM90 and used in various studies.
 
3. Results
Based on the geographical position of the southern Caspian Sea and previous research, four blocking regions were identified: the North Atlantic, Central Europe, Central Asia, and Siberia, located respectively within the longitude bands −30° to 0°, 0° to 30°E, 30° to 60°E, and 60° to 90°E. Calculation of the blocking index showed that out of 193 days with extreme (maximum) temperatures, 104 days exhibited no blocking pattern across these four regions, meaning that between 30°W and 90°E no blocking was detected according to the GHGS and GHGN indices and their required conditions.
Among the remaining 89 blocking days, the blocking index was detected 21 times in the North Atlantic, 44 times in Central Europe, 20 times in Central Asia, and 4 times in Siberia. Factor analysis of the 500‑hPa geopotential height field for the 193 days identified five components governing the synoptic patterns associated with extreme temperatures along the southern Caspian Sea coast. Of the 104 non‑blocking days, 38 occurred in Component 1, 34 in Component 2, 29 in Component 3, 3 in Component 4, and none in Component 5.
For North Atlantic blocking, 13, 4, and 4 days were assigned to Components 2, 4, and 5, respectively. For Central European blocking, 18 days fell into Component 1, 9 into Component 2, none into Component 3, 11 into Component 4, and 6 into Component 5. Central Asian blocking accounted for 16 days in Component 1, none in Component 2, 1 in Component 3, 3 in Component 4, and none in Component 5. Siberian blocking consisted of 1 day in Component 1 and 3 days in Component 3.
Overall, four non‑blocking synoptic patterns—namely the subtropical high ridge, the subtropical high core, the westerly ridge, and the northward shift of westerly currents—controlled extreme temperatures on 73, 29, 26, and 10 days, respectively. Blocking patterns collectively accounted for 28.5% of all extreme‑temperature days. Among them, bipolar blocking (subtropical high core), bipolar blocking (subtropical high ridge), bipolar blocking (south‑trough type), bipolar blocking (westerly ridge), cutoff‑low blocking (subtropical high core), cutoff‑low blocking (subtropical high ridge), Rex blocking (subtropical high core), Rex blocking (subtropical high ridge), Rex blocking (westerly ridge), and Omega blocking contributed 1.04%, 5.18%, 0.52%, 1.04%, 1.55%, 2.07%, 0.52%, 8.29%, 0.52%, and 7.77% of the 28.5% blocking days, respectively.
 
4. Discussion
Blocking events significantly modified the pathways of westerly flows, which normally travel west–to–east. During blocking, the jet stream is deflected northward or southward, resulting in altered climatic conditions over regions located along its usual path. Typically, ridge regions experience warmer and drier weather under blocking, while trough regions undergo increased instability, cooler temperatures, and precipitation.
In the warm half of the year, the subtropical high—especially its core and ridge—plays a dominant role either independently or in interaction with blocking systems. As midlatitude westerlies retreat poleward during summer, the subtropical high expands both zonally and meridionally toward higher latitudes. Depending on the strength and spatial extent of this expansion, the subtropical high sometimes drives extreme temperatures alone and sometimes amplifies them through synergy with various blocking types.
 
5. Conclusion
Blocking systems are an important contributor to the development of extreme temperature events along the southern coast of the Caspian Sea, accounting for nearly one‑third of all occurrences. The distribution of blocking types across the identified synoptic components highlights the diverse atmospheric configurations that can lead to extreme heat. Additionally, the prominent role of the subtropical high—both in isolated form and in interaction with blocking—underscores its central influence during the warm season. Accurate diagnosis and prediction of these circulation patterns can substantially improve early warnings of heatwaves and strengthen climate‑risk management, particularly in temperature‑sensitive agricultural regions.
 
Author Contributions
Given that the present article is derived from a doctoral dissertation, the contributions and roles of the authors were as follows: the first author was the doctoral student, the second author served as the supervisor, and the third author acted as the advisor.
 
Data Availability Statement
Data available on request from the authors.
 
Acknowledgements
We are very grateful to everyone who assisted us in conducting this research.
 
Ethical Considerations
All authors affirm that this research was conducted in accordance with ethical standards, with no data fabrication, falsification, or plagiarism.
 
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
 
Conflict of Interest
The authors declare no conflict of interest