انجمن ملی ژئوفیزیک ایران مجله ژئوفیزیک ایران 2008-0336 19 3 2025 07 23 The Siberian high: changes in strength and its relationship with the Mediterranean cyclones from 1970 to 2020 The Siberian high: changes in strength and its relationship with the Mediterranean cyclones from 1970 to 2020 25 46 195901 10.30499/ijg.2024.436293.1569 FA Saeid Karbasi M.Sc. Student, Institute of Geophysics, University of Tehran, Tehran, Iran 0000-0003-0809-6038 Farhang Ahmadi-Givi Associate Professor, Space Physics Department, Institute of Geophysics, University of Tehran, Tehran, Iran 0000-0002-9487-4862 Alireza Mohebalhojeh Professor, Space Physics Department, Institute of Geophysics, University of Tehran, Tehran, Iran 0000-0002-5906-8486 Journal Article 2024 01 21 As a large semi-permanent pressure system, the Siberian High (SH) has the potential to make impact on atmospheric circulation in vast areas of the Northern Hemisphere. This study aims to investigate the changes of the SH as well as the effects of SH on the characteristics of the Mediterranean cyclones in the period 1970–2020 using the Japanese 55-year Reanalysis (JRA-55) dataset. The method used is based on computing an index for the activity of the SH during winter season, analyzing the change points in the resulting time series, defining two subsets of years with high and low activity of the SH, and obtaining cumulative behavior of cyclones over four subareas of the Mediterranean.       Results establish that the activity of the SH has undergone a substantial weakening in the last two decades of the 20th century, followed by a resurgence of the SH since the early 21^st century. The higher prevalence of cyclones during the subset of high SH activity years, particularly between 0˚ and 30˚ E longitudes, is found to be related to the changes in the winter mean tropospheric circulation, which can be interpreted as changes in the Northern Hemisphere stationary wave due to changes in the lower boundary induced by the SH. During the high-activity years, the main trough associated with the stationary wave over the Mediterranean undergoes a westward displacement by about 15˚ relative to the low-activity years. This is found to be a main factor in making the cyclonic activity different over the subareas considered. As a large semi-permanent pressure system, the Siberian High (SH) has the potential to make impact on atmospheric circulation in vast areas of the Northern Hemisphere. This study aims to investigate the changes of the SH as well as the effects of SH on the characteristics of the Mediterranean cyclones in the period 1970–2020 using the Japanese 55-year Reanalysis (JRA-55) dataset. The method used is based on computing an index for the activity of the SH during winter season, analyzing the change points in the resulting time series, defining two subsets of years with high and low activity of the SH, and obtaining cumulative behavior of cyclones over four subareas of the Mediterranean.       Results establish that the activity of the SH has undergone a substantial weakening in the last two decades of the 20th century, followed by a resurgence of the SH since the early 21^st century. The higher prevalence of cyclones during the subset of high SH activity years, particularly between 0˚ and 30˚ E longitudes, is found to be related to the changes in the winter mean tropospheric circulation, which can be interpreted as changes in the Northern Hemisphere stationary wave due to changes in the lower boundary induced by the SH. During the high-activity years, the main trough associated with the stationary wave over the Mediterranean undergoes a westward displacement by about 15˚ relative to the low-activity years. This is found to be a main factor in making the cyclonic activity different over the subareas considered. https://www.ijgeophysics.ir/article_195901_5472674a0a6af7afab079d2a1c10f1f2.pdf