نوع مقاله : مقاله تحقیقی (پژوهشی)
1 پژوهشگر پسادکتری، گروه فیزیک فضا، موسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
2 گروه فیزیک فضا، موسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
3 پژوهشگر، پژوهشکده تغییر اقلیم و گرمایش زمین، دانشگاه تحصیلات تکمیلی علوم پایه زنجان، زنجان، ایران
عنوان مقاله [English]
We detect the downward planetary wave reflection from the stratosphere back to the troposphere in the Northern hemisphere extended winter season (November-March), using ECMWF (ERA-Interim) reanalysis data (1979-2014). In the previous studies, the wave activity is defined as departure from a long-term time mean. However, we demonstrate some of the shortcomings of the above-mentioned definition. For instance, negative values of the heat flux at the lower stratosphere does not necessarily show the downward wave propagation, but a lower upward wave propagation. Perlwitz-Harnik index of reflection is used to categorize the stratospheric wind regimes into the two distinct states: reflective and non-reflective. Negative and positive values of this index indicate a reflective and non-reflective (either absorptive or propagative) stratospheric states, respectively. Our results show that the negative values of this index during early winter (November-December) suggests that the downward wave coupling is less likely to happen in early winter and most of the direct downward wave coupling occurs during mid-winter and early spring (January-March), which is in agreement with previous studies. Furthermore, 10 stratospheric winter states (out of 34) are reflective, while the remaining states (24 winters) are non-reflective winters. Winters whose their stratosphere experiences a major Sudden Stratospheric Warming (SSW) event is identified to detect the absorptive states of the stratosphere. Our analysis suggests that only in the %33 of the winters with reflective stratosphere a SSW can occur. In the %62 and %38 of the winters with non-reflective stratosphere, a SSW event can occur or Rossby waves can propagate upward freely to the upper stratosphere, respectively. Analysis of the heat flux at the lower stratosphere (50 hPa) using two different definitions (wave as a deviation from the time mean and as a deviation from the zonal mean) provides some useful information. If the waves are defined as a deviation from the zonal mean (), during the reflective years, this quantity is negative (indicating a downward reflection of wave activity from the stratosphere to the troposphere). Similarly, during non-reflective years, this quantity has positive values (suggesting either upward wave propagation or absorption by the mean flow). While the above-mentioned definition is in harmony with our expectation, the definition of the wave as a deviation from the long-term time mean () results in an oscillation curve around zero line, without any useful information either about the upward wave propagation or a downward wave reflection. In order to understand the role of mean flow in influencing the upward wave propagation, we calculate the Rossby wave refractive index (or vertical wavenumber alternatively). Our analysis show some of the problematic features (for instance, a very noisy structure) of this index in understanding the differences of reflective and non-reflective stratospheric states. This is most probably due to the overlapping of very large or very small values of the refractive index which cancel each other and results in a noisy structure. To overcome this problem, we use a modified diagnostic tool (compared to the refractive index), probability of favorable propagation condition for Rossby waves. This index has some clear advantages over the traditional refractive index. Analysis of this index shows that during the non-reflective stratospheric states, Rossby waves are more likely to propagate upward (with higher values of the probability), in comparison to the reflective stratospheric regimes which is a superior demonstration of the influences of the basic mean flow on the upward wave propagation over the traditional refractive index.