Examining Trends of the Intensity of Mediterranean-Red Sea Cyclones

ABSTRACT
Among the cyclones that affect the sometimes-widespread rainfall in Iran are the merging systems of the Mediterranean and Red Seas. Therefore, it is very important to obtain the changes in the intensity of the geopotential height and the geopotential height shift of the Mediterranean-Red Sea convection patterns as one of the factors of the manifestations of these gyres, as well as the precipitation in some areas of Iran. To carry out this research, the data of geopotential height level of 1000 hectopascals related to the European Center for Medium-term Atmospheric Forecasting and ERA-Entrim version were used as a six-hour observation during the period of 1979-2018. To investigate the presence of jumps and fluctuations in the intensity of the Mediterranean-Red Sea cyclone centers during the statistical period, the Alexanderson index, known as the Standard Normal Homogeneity Test (SNHT) index, was used. A non-parametric chi-square statistic was exerted to verify and investigate the significance of the trend between geopotential height data and geopotential height tilt data. The parametric linear regression method was used to analyze and model the long-term trend. The findings of the present research indicate the increase of geopotential height in the place of the formation of the circulation centers of the Mediterranean Sea, as well as the decrease of the pressure gradient in the average annual values, which will probably lead to a decrease in instability and precipitation in the affected areas. The geopotential height shift data of the Mediterranean Sea had a significant jump in 1996, which divided the time series into two periods before and after the jump. The results indicate an upward trend in these two time periods, but the second period, with a gentler slope, has increased compared to the previous period
Extended Abstract
Introduction
Mediterranean Sea and Red Sea cyclones are a type of extratropical-tropical merge system that can influence precipitation over Iran. These combined Mediterranean-Red Sea cyclones form concurrently over the Mediterranean and Red Sea basins. They may sometimes merge as they track eastward, bringing precipitation to Iran (particularly southern and southwestern Iran). Changes in these merged cyclone systems are likely linked to shifts in Iran's precipitation climatology. Examining trends in the intensity of Mediterranean-Red Sea cyclones can thus provide insights into changes in Iran's precipitation patterns. This study investigates trends in the intensity of combined Mediterranean-Red Sea cyclonic systems and their relationship to precipitation over Iran. Cyclone intensity is assessed using geopotential height data at the 1000 hPa level over 40 years. Statistical tests, including chi-square and linear regression analysis, are applied to the geopotential height time series to detect significant trends. The focus is on examining changes in geopotential height slopes and trends that may indicate cyclone intensity changes. This research aims to improve understanding of how Mediterranean-Red Sea cyclones change and identify their impacts on Iran's precipitation climatology. The results can aid in tracking precipitation changes and projecting future climate scenarios for the region. The intensity trends may also provide broader insights into how climate change influences global cyclone behavior.
 
Materials and methods
To examine the changes in intensity of atmospheric systems and geopotential height, as well as the geopotential height shift of Mediterranean-Red Sea cyclones from 1979 to 2018, geopotential height data at the 1000 hPa level were utilized. The study area encompassed coordinates ranging from -10° E to 120° E and 0° N to 80° N, with a spatial resolution of 0.25° x 0.25°. This area consisted of 321 x 521 pixels, totaling 167,241 pixels. The Mediterranean and Red Sea cyclones, which are extratropical-tropical systems that occasionally merge and influence precipitation in Iran, were investigated. Statistical tests, such as chi-square and linear regression analysis, were conducted on the geopotential height time series for each pixel within the studied region to identify significant trends. The primary focus was analyzing changes in geopotential height slopes and trends, which could indicate cyclone intensity alterations.
 
Results and Discussion
This section presents the findings and discussion on the changes observed in monthly geopotential height intensity and geopotential height gradient of Mediterranean-Red Sea cyclones. In the Mediterranean Sea, an upward trend was observed in the geopotential height intensity, while a downward trend was observed in the geopotential height gradient. The increase in geopotential height over the circulation centers of the Mediterranean Sea and the decrease in pressure gradient are likely to result in reduced atmospheric instability and precipitation in the region. These results align with Darende's (2013) and Skleris et al. (2012) findings. Contrasting the Mediterranean Sea, the analysis of the Red Sea data revealed a downward trend in geopotential height and an upward trend in geopotential height intensity, indicating an increase in instability. This finding is consistent with the results of Asakereh and Khani (2021). No statistically significant trends were observed in the annual averages of geopotential height and geopotential height gradient in the Red Sea. However, the annual averages of both geopotential height and its gradient in the Mediterranean Sea exhibited a decreasing trend. A notable shift in the Mediterranean geopotential height occurred in 1996, dividing it into two distinct phases. Both phases showed an upward trend, albeit with a gentler slope in the second phase. The annual trend of geopotential height in the Mediterranean Sea revealed a decreasing pattern, which has been previously documented in studies by Alpert (1994, 2004). 
 
Conclusion
These studies suggest that while this reduction in geopotential height has taken place, cyclone tracks have shifted towards northern latitudes, resulting in increased drought and decreased precipitation in regions influenced by these cyclones, including Iran. The studies also acknowledge that changes in high-pressure systems near the tropics and alterations in cyclone direction contribute to variations in dry seasons and reduced precipitation. Further investigation of long-term changes in the geopotential height of the Mediterranean Sea identified three distinct phases in the time series: 1988-1979, 2005-1989, and 2006-2018. The decreasing trend in Mediterranean Sea cyclones persists until the final years of the period, indicating a potential cause for the reduction in atmospheric instability. The Kolmogorov-Smirnov statistical test was employed to determine the appropriate statistical test (parametric or non-parametric) for comparing means and variances across different periods. The parametric tests (one-sample t-test) and the one-way variance test confirmed the normal distribution of the data. Furthermore, no statistically significant trends were observed when examining the geopotential height intensity and gradient of two-day continuities of Mediterranean-Red Sea cyclones.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.