cyclogenesis

The MJO is the dominant mode of sub-seasonal variability in tropical and subtropical regions and plays a crucial role in the atmosphere-ocean circulation system. Numerous studies have investigated the effects of the Madden-Julian Oscillation on precipitation and temperature. In Iran, this phenomenon has also attracted attention, and several studies have been conducted to assess its impact on the country's climate variations. However, historical reviews show that the predictive use of this phenomenon has been less emphasized. Moreover, the MJO significantly affects the behavior of cyclones in the region, and the annual phase changes of the MJO have substantial impacts on the distribution, movement tracking, and intensity of cyclones entering Iran. In this study, to examine the cyclogenesis conditions during different phases of the Madden-Julian Oscillation (MJO) in the Mediterranean region during the warm season (March, April, and May), MJO, OLR index data and Mean sea level pressure data from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 section between 1989 and 2020 were utilized. Cyclogenesis characteristics in three aspects—frequency, depth, and cyclone tracking—were analyzed using the Melbourne University Cyclone Detection and Tracking Algorithm. In this analysis, days with MJO occurrences having an amplitude of 0.75 or higher, and phases one through eight where this amplitude was consecutively maintained, were extracted as the study period. Due to the limited number of selected periods based on the mentioned conditions, phases that missed one cycle were also included, resulting in 34 selected periods for the warm season overall. Filters were applied to the extracted data using the Melbourne University algorithm, including: a) Only cyclones located within the defined spatial and temporal boundaries (10-55°E / 10-45°N, during 1989-2020) were considered. b) Cyclones lasting less than 24 hours (four time steps: 00:00, 06:00, 12:00, 18:00) were excluded. c) Cyclones identified over multiple time steps (assigned identical numbers) were reviewed, and only the instance with the lowest recorded pressure was considered. d) The median pressure during selected days of different phases was calculated, with the maximum surface pressure of 1005.8 hPa for the warm season. The study of OLR indicates that in phases 1, 2 and 7, the most negative OLR anomaly occurrs over the Middle East and especially over Iran, which indicates the occurrence of convective currents with cloudiness in different regions of Iran. The study of the characteristics of the cyclones in the region showed that phase 8 with more than 17% has the most and phases 1 and 4 with nearly 9.36% have the lowest cyclogenesis area in the study region. In terms of distribution, out of the 25 cases of minimum pressure in warn season, 17, 6, and 2 cases are related to April, March, and May, respectively. The tracking of the cyclogenesis indicated that in phase 1, most of the cyclogenesis were on the Mediterranean Sea and very few cases were formed in its castern area. At the same time, in phase 2, along with the formation of cyclones in the eastern Mediterranean, some of them have entered Iran. In phases 3 and 4 in the north and center of Iraq, the cyclones have traveled their path, and in phases 5 and 6, the majority of the cyclones were over the Mediterranean, and in phases 7, they reached their peak, and the majority of the cyclogenesis were in the central to southern regions of Iraq. The formed cyclone in phase 8 over the Mediterranean Sea have a different behavior and at the same time they have traveled longer paths than other cyclones in different phases. The results of examining the cyclogenesis characteristics during the warm season in different phases of the Madden-Julian Oscillation (MJO) over the Mediterranean Sea reveal that the highest negative anomaly of the OLR index occurs in phase 1 of this oscillation, with its primary focus on Iran. The study of cyclogenesis areas indicates that cyclonic activity during the warm period of the year is asymmetrically distributed. In various phases of the MJO, most cyclones during the warm season are concentrated over Iraq and the western and northwestern regions of Iran, while other cyclonic cores form over the southern of the Persian Gulf and the Arabian Peninsula. The findings from the number of cyclogenesis centers show that out of 25 cases of minimum pressure during the warm season, 17 cases occurred in April, 6 in March, and 2 in May. Cyclone tracking in the region revealed that cyclones formed over the Mediterranean Sea travelled longer distances.

In this investigation, some aspects of the impact of the Madden-Julian Oscillation (MJO) on the subtropical region of the Northern Hemisphere together with the underlying mechanisms are studied using NCEP/NCAR reanalysis data. The data cover winter months (December to February) from 1974 to 2015. The main method used is that of averaging and analyzing of meteorological parameters associated with convection over the Indian Ocean and the mid-latitude large scale motions in the eight phases of MJO. The indices of MJO provided by the Australia's National Weather, Climate and Water Agency (BMRC) are used to identify the MJO phases. The averaging is carried out over the periods when MJO index is higher than unity and stays in the same phase for at least 5 days. The selected parameters are the “outgoing longwave radiation” (OLR), velocity potential and divergent component of the horizontal wind at 200 hPa level and vertical component of velocity in pressure coordinate denoted by . These parameters have been selected based on their potential to unravel the interaction between tropical and subtropical tropospheric circulations.
The average of OLR in the selected period shows clear movement and amplification of convection cells associated with MJO from the Western Indian Ocean to the east. This confirms that the periods have been selected properly.
The distributions of averaged OLR, divergence at 200 hPa level and at 600 hPa level show that the southwest Asia is significantly affected by MJO. Over the Indian Ocean, convective cells of MJO are strengthened from the phase 1 to phase 4 while anomalous convection at 200 hPa level and the associated downdraft at 600 hPa level in the southwest Asia are manifested. During the phases 3 and 4 of MJO, the convection cells associated with MJO exhibit the strongest anomalies over the east of Indian Ocean. The results thus suggest that the atmospheric circulation pattern provides adverse conditions for cyclogenesis and cyclone development in the southwest Asia and especially over Saudi Arabia and the south of Iran. On the contrary, all anomalous patterns are reversed in the phase 6 to phase 8 in the tropical and subtropical region. In these phases, anomalous convergence at 200 hPa and updraft motion at 600 hPa seen during the phases 3 and 4 in the southwest Asia are replaced by anomalous divergence and updraft motion, respectively. The change is such that the atmosphere circulation provides suitable conditions for cyclogenesis and cyclone development at the downstream end of the Mediterranean storm track.
The current study shows that confluence and diffluence associated with MJO are extended from the Indian Ocean to the Middle East and the east of Mediterranean Sea. The extension is such that the movement of mass from the Indian Ocean to the Middle East at the upper troposphere in the phase 4 results in the formation of a downdraft motion in the east of Mediterranean Sea. The reverse circulation seems to prevail in the phases 7 and 8 of the MJO. Anomalous updraft motion with divergence (convergence) at the upper (lower) troposphere in the east of the Mediterranean Sea are seen when convection is suppressed in the Western Indian Ocean. Another interesting point is that the convergence and divergence in the east of the Mediterranean Sea are dominantly due to variation of wind speed (and not confluence and diffluence), which may be caused by the effects of topography or interaction with mid-latitude flow. Finally, the distribution of OLR confirms the results of the dynamical analysis in the sense that in the Middle East, positive (negative) anomalous values of OLR are seen in the phases 3 and 4 (7 and 8) suggesting less (higher) than normal cloudiness and precipitation. This study shows that confluence and diffluence associated with MJO expand from Indian Ocean to Middle-East and east of Mediterranean Sea. So that mass movement from Indian Ocean to Middle-East in upper troposphere in phase 4 causes to formation a convection center and downdraft motion in the east of Mediterranean Sea. The reverse circulation seems to occur in the phase 7 and 8 of the MJO. Anomalous updraft motion with divergence (convergence) in upper (lower) troposphere in the Eastern Mediterranean Sea are seen when suppression of convection exists in the Western Indian Ocean. Another point is that convergence and divergence in east of Mediterranean Sea is due to the variation of wind speed (not confluence and diffluence) that may becaused by topographic effects or interaction with mid-latitude flow. Distribution of OLR confirms the results of this study so that in the Middle-East, positive (negative) anomalous value of OLR is seen in phase 3 and 4 (7 and 8) which suggests less (more) than normal cloudiness and precipitation.

This research is an attempt to investigate the blocking systems over the Atlantic Ocean and especially over the Mediterranean sea, which leads to surface cyclogenesis in the east of the Mediterranean Sea, and the influence of their effects over the Iranian region. At the first step of study, Mediterranean topography, local Mediterranean winds and cyclogenesis have studied; then structure and types of existing blocking systems on the synoptic weather maps have been introduced with figures.
In the second step of the study, all of the blocking systems that were formed over the Atlantic Ocean were identified from 1989 to 1997, the results of which are shown in the table included in the paper. Of course this part of the work was done with the help of Prof. Colucci from USA and data are given from NCEP.
Atmospheric blocking systems can lead to a stagnation of weather patterns where the patterns remain for several days or even weeks at the same location during the blocking system. This case can lead to flooding, drought, above normal temperatures, below normal temperatures and other weather extremes. Therefore, it is important to recognize a blocking pattern in its initial development. With this awareness, one can forecast up to several days in advance with a high degree of accuracy.