1 استادیار، گروه مهندسی آب، دانشگاه شهرکرد، شهرکرد، ایران
2 دانشیار، گروه فیزیک فضا، موسسه ژئوفیزیک دانشگاه تهران، ایران
عنوان مقاله [English]
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.