جستجوی مقالات مرتبط با کلیدواژه « جریان جتی » در نشریات گروه « زمین شناسی »

The near-surface temperature in the Arctic has increased about four times greater than the global average temperature in recent decades, the feature called rapid Arctic warming or Arctic amplification. Different dynamical pathways have been hypothesized through which Arctic amplification may influence the midlatitude weather. There is also some evidence that extreme weather in midlatitudes of the Northern Hemisphere is linked to rapid Arctic warming. We analyzed the ERA5 data for the period 1965–2021 to investigate the impact of rapid Arctic warming on weather patterns in West Asia. We defined pre-Arctic and post-Arctic amplification periods as 1965–1991 and 1995–2021, respectively. We analyzed wind speeds between 400 and 100 hPa to investigate the response of the upper-tropospheric jet stream in West Asia to rapid Arctic warming. We defined the meridional extent of an isopleth of the geopotential height at 500 hPa (Z500) as the difference between the maximum and minimum latitudes reached by a single Z500 isopleth each day. By averaging these daily values in different seasons during the pre-Arctic and post-Arctic amplification periods, we obtained seasonal averages of the meridional wave extent in these two time periods. In winter, the altitude of the maximum geopotential height in the middle troposphere has increased in the post-Arctic amplification period. Moreover, the latitudinal extension of the maximum amplitude of Rossby waves has decreased. These two changes imply that the speed of the eastward propagation of Rossby waves has intensified in the post-Arctic amplification period.In summer, the subtropical jet stream has wakened and shifted toward the equator in the post-Arctic amplification period. Due to the weakening of the meridional temperature gradient, both the subtropical and subpolar jet streams become weaker and shift poleward in summer. The poleward shift of the subtropical jet stream has also occurred in the post-Arctic amplification period, while the jet has weakened compared to the pre-Arctic amplification period. The analysis of the meridional circulation index (MCI) shows significant positive and negative values in most parts of West Asia in the post-Arctic amplification period, implying that Rossby waves have become wavier and their speed has decreased, associated with which is a higher possibility for the occurrence of extreme weather events in summer. In autumn, in the post-Arctic amplification period, the jet stream has intensified in the eastern parts of West Asia compared to the climatology and the pre-Arctic amplification period. Between 50 and 75 ºE, the subtropical jet stream in West Asia has shifted toward the equator in the post-Arctic amplification period. As the subtropical jet stream separates the warmer tropical air from the colder air of higher latitudes, its equatorward shift in the post-Arctic amplification period implies the development of negative temperature anomalies in most parts of West Asia.

The East Atlantic-West Russia (EA–WR) teleconnection pattern is one of the low-frequency atmospheric phenomena that affects Europe and Asia, especially in the cold season. In this study, the effects of EA–WR on the climate of Southwest Asia are investigated using the NCEP/NCAR reanalysis dataset from 1950 to 2012 for winter months (Dec. to Feb.) and the monthly indices taken from the Climate Prediction Center (CPC). Because of the large zonal extension of the EA–WR teleconnection pattern, all data north of 20°N are taken into account for the analysis. In this paper, the method of composite maps is employed. Considering the critical positive and negative months, the average state of the troposphere is studied for each of the two phases from the synoptic viewpoint. To this end, a month is considered to be a critical positive (negative) month, if the monthly index of EA–WR is higher (lower) than the long-term mean value of EA–WR index plus (minus) its standard deviation. In this way, among 189 winter months during the 63-year period from 1950 to 2012, 26 positive critical months and 29 negative critical months are identified. For the part of analysis based on outgoing longwave radiation for which the data is available from 1974 onwards, there exist 20 positive critical months and 18 negative critical months. The composite map analyses include 500 hPa geopotential height and its anomaly, mean sea level pressure, 300 hPa wind field, 1000-500 hPa thickness, the outgoing longwave radiation and the Eady's parameter for the growth rate of baroclinic eddies: whereis the Coriolis parameter or inertial frequency, is the buoyancy frequency, and is the magnitude of the vertical wind shear. The growth rate is evaluated and compared between the two phases at 800 hPa. In the critical positive months of EA–WR, in 500 hPa geopotential height field, there is a trough from the western part of Russia to the Middle East and a ridge over the eastern part of the North Atlantic. In critical negative months of EA–WR, however, there is a ridge over the western part of Russia, a trough over Europe and a dominantly zonal flow is observed over the Middle East. In the critical positive months, the subtropical jet stream over the southwest of Asia is stronger, and at the same time, the exit region of the polar front is extended to the border between Europe and Asia, which together with the wind field anomaly result in significant cold air advection to the northwest of Iran. Furthermore, the stronger subtropical jet over the Southwest of Asia, the southwest of Iran, and Saudi Arabia in the critical positive months is associated with increased amounts of Eady's parameter for the growth rate and thus baroclinic instability. Overall, results point to a significant effect of EA–WR on the climate of the southwest Asia. Compared with the negative phase, the presence of a mid-tropospheric trough of geopotential height as well as the upper-tropospheric wind anomaly in the form of a stronger subtropical jet stream provide a better ground for the development and organization of synoptic systems and their impact on the climate of Iran in the positive phase. The dynamical effects mentioned are also helped by a more suitable lower-tropospheric moisture transport in the positive phase.

With regard to the adverse effects of air pollution in Tehran on its residents، it has become vital to investigate the meteorological factors that determine conditions favorable for the establishment of critical air-pollution episodes. Among these factors، the largescaledynamical processes acting in the upper troposphere are particularly important، as they can provide a means of predicting the critical episodes using the medium-range weather forecasts. As a step in exploring such factors، this study focused on some aspects of the large-scale upper-tropospheric flow believed to have a significant impacton lowlevel flow. The upper-tropospheric jet stream with its possible split and the Rossby wave breaking as a way of detecting and measuring blocking strength were examined using various diagnostics including the distribution of potential temperature θ on the 2 PVU (a potential vorticity unit is equal to 10-6 m2 s-1Kkg-1) surface corresponding to the position of the dynamical tropopause. The analysis was carried out for a prolonged، acute episode of air pollution in Tehran. For quantification، the previously introduced Split Flow Index (SFI) and the Rossbywave breaking index، referred to as β for brevity were employed to identify the jet split and blocking formation، respectively. The Global Forecast System (GFS) data consisting of geopotential height، temperature and horizontal velocity components were used for the three-month period of 1 Nov 2010 to 31 Jan 2011. The geopotential height at 500 hPa، and the wind speed، relative vorticity and streamfunction fields at 300 hPa were analyzed to determine the synoptic structure associated with the intense air-pollution in Tehran from 22 Nov to 22 Dec 2010 (the month of Azar 1389 in Iranian calendar). The synoptic structure indicates persistent blockings in the Northern Atlantic، and the formation of deep troughs in the southern Europe together with strong ridges in their downstream sides. The high values of the geopotential height field، the negative relative vorticity and low wind speed in the middle and upper troposphere were the dominant features over Iran. To compute the SFI، three regions had to be involved: the subtropical jet (STJ)، the polar front jet (PFJ) and the gap between these two (GAP). The SFI was computed by subtracting the mean relative vorticity of GAP from the sum of mean relative vorticities of the STJ and PFJ regions. The sign and magnitude of the SFI were criteria for the occurrence and strength of a split jet. The results for the SFI were generally sensitive to the longitudinal and temporal interval used in computation of the SFI. By considering broader and longer longitudinal and temporal intervals، respectively، more accurate values for the SFI index could be obtained. With regard to the latter sensitivities، it was shownthat within the positive values of SFI in the large part of Nov. and Dec. 2010 representing non-split flow، there was the period 9 to 14 Dec. 2010 during which the formation of a transient blocking over central Asia led to negative (positive) values of SFI at the beginning (end) of this period. The blocking formation was associated with the reversal of the meridional gradient of θ on the 2 PVU surface. For this reason، the β index was used as a dynamical tool to detect and identify a blocking based on the distribution of θ on potential vorticity surfaces. Results showed positive values of β over Iran for a few days in the middle of Azar (early Dec. 2010)، indicting the the presence of a transient blocking. There was also a second case of positive values of β from 11 to 16 Dec.، which closely followed the negative values of the SFI (split-flow regime) from 9 to 14 Dec. over central Asia.