The relationship between the stratospheric conditions and evolution, and the surface weather, not only has revolutionized our understanding of the functions of different atmospheric layers and their dynamics, but has also brought about potential implications for weather and climate predictions in almost any region of the planet. Obtaining a better understanding of the field, particularly in Southwest Asia, is the motivation for this study. The, NCEP/NCAR reanalysis data including minimum and average daily temperature, geopotential height, the precipitation rate, pressure and relative humidity at the surface or various atmospheric levels are used. Due to the fact that between 1948 and 1957, observations of the upper atmosphere were less frequent and were made at synoptic hours different from today’s main synoptic times, the reanalysis data are less reliable (Kistler et al., 2001); hence this period has been omitted in the present study; consequently only the data from 1958 to 2011 were used. The study region consists of an area between 25-45°N and longitudes 35-65°E, which includes Iran and extends westward to the Mediterranean Sea. The grid points are 2.5 degrees apart in both longitude and latitude. In a procedure similar to that of Thompson et al. (2002), the mean daily surface temperature and the frequency of cold events are compared in a 60-day interval, following weak and strong vortex conditions. A cold event is defined as a day in which the minimum temperature falls more than one standard deviation beneath the January to March (JFM) climatological mean. The stratospheric polar vortex is called weak or strong when the absolute value of the daily geopotential height anomaly at 10 hPa, averaged from 60- 90°N, is more than twice the JFM climatological standard deviation. Other variables such as relative humidity at 850 hPa, surface pressure, the precipitation rate and temperature anomaly are also compared during these intervals. For each day, in evaluating the temperature anomaly and its standard deviation, use is made of the climatological mean value for the month that the day is in it. Values of a winter severity index (Thompson et al., 2002) are calculated and compared for every winter day using 54 years of temperature data in the two cases of weak and strong polar vortex. The index is proportional to the standardized squared minimum daily temperature and is nonzero if and only if the latter temperature is below a specified threshold value. The winter severity index is averaged over all grid points within our domain of study. A randomization test is used to estimate the significance of the differences observed in the number of cold events after the weak and strong vortex conditions.
It is shown in weak vortex events, daily mean surface temperatures decrease compared to the strong vortex conditions in most parts of the under study region, however cold events become less frequent. Exceptions are two distinct locations in the east and northwest part of the country which appear to have higher daily mean temperatures following the weak vortex conditions. Also, the entire region shows a decrease in surface pressure relative to the strong events for which it can reach up to 30 hPa in some parts including a region located in the southwest of Iran. Weak vortex events are followed by higher relative humidity conditions in most parts of the region— the enhancement reaches six percent at certain areas— which may be linked to the aforementioned drop in surface pressure. The difference in precipitation rate following weak and strong vortices varies from one part of the region to another, showing both increases and decreases, with a maximum absolute value of 12.5 millimeters per month. A positive temperature anomaly differenceis identified in almost the entire region, in agreement with the expectations the frequency and magnitude of positive and negative temperature anomalies. The performed randomization test reveals a confidence level of 92 to 99 percent for the observed differences between the frequency of cold events after the weak and strong vortex conditions depending on the severity of events. The winter severity index also confirms previous findings regarding the frequency of cold events following weak and strong vortices, demonstrating a higher value in almost every winter day for the case of a strong vortex compared to its climatological mean value and the weak vortex conditions.
Although the strength of the stratospheric polar vortex has a less dramatic impact on the study region compared to regions like northern parts of Asia and North America, the current study reveals patterns of weather and climate variability related to the polar vortex conditions in the region which can have important implications for long-term forecasting purposes.