تبیین سازوکارهای همدیدی مؤثر بر ناهنجاری‌های سرد فصل گرم در نواحی ساحلی جنوب ایران مبتنی بر تحلیل عاملی

Explaining the Synoptic Mechanisms Affecting Warm‑Season Cold Anomalies in the Southern Coastal Regions of Iran Based on Factor Analysis



ABSTRACT

Cold anomalies can cause ecological disruptions in many regions; however, in the southern coastal areas of Iran—where the warm season is characterized by extreme heat—temperatures below normal provide an opportunity to improve thermal comfort. This study aims to explain the synoptic mechanisms affecting warm-season cold anomalies using factor analysis. The warm season was defined as April to October, and daily minimum temperature data from coastal and marine stations for a 33-year period (1986–2019) were obtained from the Iran Meteorological Organization. Extreme cold events were identified using the standardized Z‑score, and cases were selected based on three criteria: Z < –1.8, a minimum duration of three consecutive cold days, and involvement of at least 50% of the stations. In total, 31 cold or cool waves comprising 113 days were identified. Results indicated that 12 events were associated with the Migratory Anticyclone, 11 events were caused by the Siberian High, and the remaining events were linked to the Polar Vortex pattern, the African Anticyclone, and a combined Siberian–Migratory pattern. In the dominant Migratory Anticyclone pattern, the positioning of the anticyclonic core from the Black Sea to the Caspian region and the advection of cold air from higher latitudes led to a significant temperature decrease over the study area.



Keywords: Cold anomalies, Warm season, Synoptic patterns, Factor analysis, Migratory anticyclone, Siberian High.



Extended Abstract

Introduction

Warm-season cold anomalies in the southern coastal regions of Iran represent exceptional climatic events that interrupt the prevailing hot and dry subtropical regime. These anomalies hold considerable environmental and socioeconomic importance due to their impacts on thermal comfort, building energy demand, regional circulation patterns, and even marine conditions. While strong subtropical anticyclones typically inhibit the incursion of cold air masses during the warm season, occasional cold outbreaks illustrate that under specific synoptic configurations, this dominant structure can be disrupted. The aim of this study is to identify and explain the synoptic mechanisms responsible for these rare cold anomalies and determine the dominant circulation patterns driving them.



Methodology

Daily maximum and minimum temperatures for the period 1986–2019 were examined for the warm-season months (April–October). Cold waves were identified based on three strict criteria: temperatures falling below the 10th percentile, a drop of at least 12°C below climatological averages, and persistence for a minimum of two days. A total of 31 valid cold waves were detected. Atmospheric circulation patterns were investigated using sea-level pressure and geopotential height fields at 1000, 850, 700, and 500 hPa extracted from the NCEP/NCAR reanalysis. Normality of the dataset was confirmed using the Anderson–Darling test in Minitab. Factor analysis in S-mode using the principal component method was then applied; the KMO value of 0.89 indicated high sampling adequacy. Out of 15 initial components explaining 94% of total variance, five dominant factors were selected for synoptic classification. Composite circulation maps were plotted using GRADS to reveal multi-level atmospheric structures associated with each event.



Results and discussion

Results revealed three dominant circulation patterns: the Siberian High (11 events), the Migratory Anticyclone (12 events), and the African Anticyclone (3 events). Together, the Siberian and Migratory patterns accounted for approximately 70% of all warm-season cold anomalies.

The Siberian High pattern is characterized by an anticyclonic core positioned between Balkhash–Baikal or Aral–Baikal, extending southwestward toward Iran. Due to the strong barrier effect of the Zagros Mountains, the cooling is partially limited in southwestern regions; however, substantial drops (≈15°C) were recorded in Abadan and Mahshahr. Upper-level analysis showed deep trough development over eastern Iran, strengthened by the northern ridge of the Arabian anticyclone, which enhanced cold advection.

The Migratory Anticyclone pattern was the most frequent. At 1000 hPa, it typically centered over the Black Sea–Caucasus region, directing a north–south cold tongue into Iran. At higher levels (850–700 hPa), the Arabian anticyclone prevailed, forming a deep trough over eastern Iran. The 13 April 1997 cold wave exemplified this pattern, producing unusually low temperatures of 15.6°C and 15°C in Abadan and Mahshahr, respectively, and involving a cut-off cold pool over Afghanistan that intensified cold advection.

The African Anticyclone pattern, though less frequent, also generated notable cold anomalies. When a strong anticyclone formed over Egypt and Libya and extended northward toward central Europe, enhanced northerly flow penetrated the Middle East. Cooling under this pattern was strongest in southwestern Iran, with temperature drops of 7.4°C in Abadan and 8.8°C in Mahshahr.



Conclusion

The study demonstrates that warm-season cold anomalies in southern Iran arise from complex multi-layer interactions between surface and mid-tropospheric circulation systems. Key mechanisms include the orientation of high-pressure ridges, the formation of deep troughs over eastern Iran, and the role of orographic barriers such as the Zagros Mountains. The dominance of the Siberian and Migratory patterns highlights their importance in regional atmospheric dynamics. These findings contribute to improved forecasting and energy management strategies in hot coastal regions and provide new insights into the synoptic behavior of the warm-season atmosphere over the Middle East.