بررسي ماهيت، ساختار و وردايي زماني گردش بزرگ مقياس جو تابستانه بر روي جنوب غرب آسيا

Investigating the Nature, Structure and Temporal Variations of Summertime Atmospheric Circulation over Southwest Asia

در اين پژوهش ساختار گردش بزرگ مقياس جو بر روي جنوب غرب آسيا در طول دوره گرم سال مورد توجه قرار گرفت. ابتدا ويژگي‌هاي بنيادي و ماهيت گردش بزرگ مقياس جو بررسي شد و سپس وردايي زماني آن براي يك دوره 61 ساله (2008-1948) مورد تجزيه و تحليل قرار گرفت. يافته‌هاي تحقيق بيان گر آن است با آغاز گردش موسمي در جنوب و جنوب شرق آسيا جريان مداري غرب سو و مداومي در وردسپهر فوقاني شكل مي‌گيرد كه محل اصلي همگرايي و نزول آن چاهه گرمايي جنوب غرب آسيا و شرق مديترانه است. تداوم نزول بزرگ مقياس هواي منشأ يافته از منطقه موسمي با ايجاد گردش واچرخندي گسترده، جهش شمال سوي جت جنب حاره و معكوس شدن شيب نصف النهاري دما را در پي خواهد داشت. شرايط ياد شده، ساختار گردش بزرگ مقياس جو را در امتداد نصف النهاري تغيير داده و پيدايش يك گردش هادلي معكوس و شروع تابستان را بر روي جنوب غرب آسيا موجب مي‌گردد. در بررسي ماهيت پرفشارهاي تابستانه مشخص گرديد كه پرفشارهاي جنب حاره‌اي تابستانه بر روي جنوب غرب آسيا در تمامي ترازهاي جو قابل مشاهده بوده و از جهت منشأ اصلي تشكيل شان، يا ناشي از نزول هوا با منشأ موسمي، يا فرارفت قائم گرما از سطح مناطق كوهستاني مرتفع و يا تركيبي از اين دو مي‌باشند. نتايج تحقيق همچنين بيان­گر آن است كه از ميان سه شاخص فيزيكي، جت و شاخص ديناميكي كه در اين تحقيق ارائه شده و مورد استفاده قرار گرفته اند، شاخص فيزيكي به نحو مطلوب تري خصوصيات گردش تابستانه را نشان مي‌دهد. بر اساس اين شاخص، به طور متوسط فصل تابستان در جنوب غرب آسيا در روز 7 جون آغاز و پس از 113 روز در 28 سپتامبر به پايان مي‌رسد. بررسي وردايي زماني گردش جو بيان گر كاهش قابل ملاحظه طول فصل تابستان در طي دهه‌هاي اخير بر روي منطقه مي‌باشد. بر اساس نتايج، اگرچه زمان آغاز تابستان بر روي جنوب غرب آسيا در روند درازمدت خود تنها تغييرات بسيار ناچيزي را تجربه نموده، اما خط روند يك كاهش 8 تا 9روزه را در طي دوره مطالعه براي طول فصل تابستان نشان مي‌دهد. يافته ها بيان گر آن است كه كوتاه شدن طول فصل تابستان از 117روز به 109روز بر روي جنوب غرب آسيا ناشي از پايان زودرس فصل تابستان در دهه‌هاي اخير مي باشد. يافته ها همچنين نشان دهنده آن است كه در طول دوره 61 ساله، گردش بزرگ مقياس تابستانه در هر دو منطقه جنوب غرب آسيا و منطقه موسمي روند كاهشي مشابهي را در شدت خود تجربه نموده است. بررسي مؤلفه‌هاي مقياس همديد نشان دهنده آن است كه در وردسپهر مياني و زيرين از يك سو در شبه جزيره عربستان مركز پرفشار و كم فشار عربستان و همچنين باد شمال، به طور محسوسي تقويت گرديده‌اند و از سوي ديگر واچرخند تركمنستان، كم فشار پاكستان و باد سيستان در شمال شرق و شرق فلات ايران كاهشي را در شدت فعاليت خود تجربه نموده‌اند. ميدان واگرايي افقي بر روي غرب ايران نيز از وجود يك روند افزايشي در شدت گردش در مركز پرفشار ايران و در مركز همگرايي زاگرس حكايت دارد.

Introduction In this paper, the authors have studied the largescale atmospheric circulation over southwest Asia during a boreal summer. First, substantial characteristics and nature of the largescale atmospheric circulation are investigated. Then, temporal variability of the circulation was examined during a 61years period (19482008). The findings indicate that the onset of the Asian monsoon is associated with the formation of a permanent westward flow in the upper troposphere. The corresponding maximum convergence and subsidence locations of the flow are the energy sinks over the southwest Asia and east of the Mediterranean Sea, respectively. Maintenance of the largescale atmospheric subsidence creates a transverse meridional gradient in the temperature field therefore, the meridional largescale circulation is changed and this in return creates a transverse Hadley circulation which forms over southwest Asia. Investigating nature of the summertime anticyclones over southwest Asia reveals that the anticyclones are dominant in all tropospheric levels. They are originally formed either as a result of the descending air which is basically originated from the monsoon region, or by accumulated heat of the surface of high plateaus in the midtroposphere due to intensive vertical advection of heat over southwest Asia, or a combination of both. The result also reveals that among the physical, dynamical, and Jet indices, the physical index more favorably shows characteristics of the summertime circulation. Based on the physical index, it’s found that the averaged summer season timing starts on June 7 and ends on September 28 over southwest Asia. Investigations on the temporal variability of the atmospheric circulation, indicates a considerable decreasing of summer length in the recent decades. On this basis, although the starting time of the summer shows very small changes in its longterm trend, the trend line shows an eight to nine days decreasing in the season’s length during the study period. The result revealed that the decreasing of summer length from 117 days to 109 days over southwest Asia is mainly due to the early ending of summertime in recent decades. It is also found that both the southwest Asia and the Asian monsoon regions have experienced similar decreasing trend in their summer largescale circulation strength during the study period. Examining the synoptic scale atmospheric components showed a considerable strengthening of the Arabian high and low and the Shomal wind over the Arabian Peninsula. On the other hand, the Turkmenistan anticyclone, the Pakistan low and the Sistan wind in the east and northeast of Iran plateau experienced a slight weakening in their activity. Also, Examination of the horizontal divergence field shows an increasing trend in the intensity of anticyclonic circulation on Iran and on the Zagros convergence zone in recent years.   Material and methods The NCEP/NCAR daily reanalysis dataset (Kalnayet al. 1996) in a 61years period (19482008) were used to study the summer atmospheric circulation. In this research, variables of the Geopotential Height (gpm), Temperature, and Outgoing Long wave Radiation (OLR), zonal and meridonal wind components the dataset were used to calculate horizontal divergence field, divergent wind, relative vorticity and diabatic heating. We first examined the sinks and sources of energy in regional scale. Then, zonal and meridional largescale circulations as well as their vertical structure were analyzed for AprilOctober months in the period over southwest Asia to derive prevailing circulation patterns. Three regional indices including physical index, Jet Index, and dynamical index were designated to determine the starting/ending time and the length of summer seasons as well as to study the intensity of the atmospheric circulation over southwest Asia. The indices provided the possibility of assessing different aspects of the summer circulation. We also used the gpm, relative vorticity, horizontal divergent fields and diabatic heating charts to investigate structure of anticyclones on Iran and Arab countries in midtroposphere and the Turkmenistan anticyclone, the Pakistan low and the Zagros convergence zone in lower troposphere. Finally, the temporal variations of the summer circulation components were studied using the most suitable variable for each component.   Results and discussion The results show that the monsoon circulation is the main factor controlling the largescale summer circulation over southwest Asia. However, the surface heat forcing has the main role in the formation and maintenance of atmospheric circulation in the lower and middle troposphere. Investigation of the synoptic scale circulations, show that the Zagros chain is effective on formation of anticyclones over Iran. In fact, the vertical advection of heat induced by the elevated heat source in western Iran has a key role on the formation of a thermallydriven anticyclonic circulation in midtroposphere. On the other hand, formation of the Turkmenistan anticyclone and the Arabian anticyclone has different mechanisms. Formation of the Turkmenistan anticyclone in the lower troposphere is mainly due to the convergence and subsidence of the air with monsoon origin in southwest Asia at the levels below 600 hPa. While the Arabian anticyclone is strengthened by the descending air originated from the monsoon system as well as thermal forcing from surface and vertical advection of heat from the high lands in the western Arabian Peninsula. Application of circulation indices in this research led to recognition of the main characteristics of summertime atmospheric circulations including the starting/ending time, length, and intensity of the boreal summer over the southwest Asia. Examination of the temporal variability of the summertime atmospheric circulation over southwest Asia, show that the summer duration has been reduced considerably in recent years. Early ending of the summer can be physically related to the early ending of the transverse gradient in meridional temperature field during the recent 23 decades in the study region. The documents also indicated that shortening of the summer and weakening of the atmospheric circulation over southwest Asia has a relationship with shortening of the monsoon season and also weakening of the monsoon circulation over south and Southeast Asia. However, the correlation between the monsoon circulation and largescale atmospheric circulation over southwest Asia shows a decreasing trend. The latter can be an indicator of increasing regionalscale factors role on the southwest Asian climate. The overall result of this study indicates that from 1980 onward, the interannual variation in the intensity of the activity of atmospheric circulation components has been considerably decreased compared to the time before 1980.