عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The role of stratospheric circulations in large-scale and intense anomalies over a large-part of Asia including Iran in Winters of 2007–2008 and 2009–2010 was investigated using NCEP/NCAR reanalysis data available four times a day and in daily and monthly averages for 17 pressure levels as well as for isentropic and sigma levels. The spatial resolution of the data set was 2.5×2.5 in the longitudinal and latitudinal directions which provided adequate resolution to study large-scale dynamical processes.
Anomalies in the wind and temperature fields induced by the interaction of vertically-propagating planetary waves with a stratospheric mean flow first appear in the winter stratosphere and subsequently with downward propagation, they affect the surface climate. The time series of each meteorological quantity contains a set of variability modes. Using Empirical Orthogonal Functions (EOFs), it is possible to extract and determine the contribution of each variability mode in the time series of a given meteorological quantity. The highest variability is contained in the first mode of variability, also called the leading mode. The vertical and horizontal structures of the resulting spatial patterns illustrate the internal variability of each atmospheric layer, telleconnection patterns as well as the interactions of atmospheric layers. Using EOFs, the stratosphere–troposphere interactions in the two above winters were investigated. Comparing the two winters, it was found that the variance of the leading mode of the 10-hPa gepotential height was larger in Winter 2007–2008, indicating a stronger polar vortex than that in Winter 2009-2010. With regard to this situation at various levels, it was shown that the cold winter in the region in Winter 2007–2008 coincided with the existence of a strong polar vortex and minor sudden stratospheric warming (SSW). In Winter 2009–2010, the reverse is true. That is, the warm winter in the region coincided with the existence of a weak polar vortex, early SSW and the displacement of the dipolar pattern of a temperature anomaly to higher latitudes.
As an important tool in understanding the time evolving flows, Eulerian diagnostics were employed to corroborate the results obtained using the statistical method. The results for a temperature anomaly at 850 hPa were consistent with surface observations in which the winters 2007–2008 and 2009-2010 were, respectively, cold and warm over the region. The changes in the temperature pattern in these two winters were believed to be related to the effects of a stratospheric circulation in the surface climate. The SSW events were classified according to the definition provided by the World Meteorological Organization. The SSW events were classified and the stratosphere–troposphere interaction was investigated using Eulerian diagnostics. Consistent with the statistical analysis, the time evolution of Eulerian diagnostics illustrates marked differences in the behavior of a polar vortex in the two winters.
The results obtained using EOFs and Eulerian diagnostics showed that in Winter 2007–2008 the occurrence of a major or minor sudden stratospheric warming was associated with the displacement of a dipolar pattern of temperature to lower (higher) latitudes and thus a prolonged cold anomaly over Iran. The opposite situation was found to prevail in Winter 2009–2010.