Analysis and clustering of Changes on the Atmospheric Middle-Level in Recent Decades

Investigating the behavior of atmospheric circulation during different periods can broaden our knowledge of large-scale changes in the climate. Based on recent Intergovernmental Panel on Climate Change assessments, the increase in greenhouse gases and other human activities will lead to the warming of the troposphere, atmosphere temperature drop, thickening of tropopause, less atmospheric circulation activity in the tropics, migration of atmospheric disturbance from poles toward middle latitudes, increasing of precipitation in tropical areas, etc. Since the middle-level atmosphere has the greatest effect on the occurrence of atmospheric phenomena in the surface of the earth, many studies have investigated the temporal and spatial changes of this level across the world. Most of these studies confirm the changes in the geopotential height anomalies at 500 hPa in some areas and identify them useful in forecasting temperature and precipitation variables, as well as phenomena such as flood and frost.

This study explored changes at 500 hPa using daily data of the US National Center for Atmospheric Research for the period from 1972 to 2015. Using cluster analysis, the behavioral patterns at 500 hPa were analyzed for a 44-year period, and the Mann-Kendall graphical test was used to identify the trend of change and its significance.

The clustering results show two main long-term patterns in middle-level atmosphere. In the first group with more days, the domination of the Mediterranean high landing over a large part of the study area and the its penetration toward Iran is noteworthy, whereas in the second group with far less days, there is the dominance of the sub-tropical high with one of its bulges over a vast area of Iran, which leads to sustainability and hot air in the warm days of the year. The analysis of the trend of changes in the middle-level atmosphere height using the Mann-Kendall method indicates significant changes in the study area during the period; with the significant increase being recorded for the last decade (after 2000) during the annual period and in the summer months of July and August. These results also showed no significant trends in the autumn, especially in October and November, either individually or collectively.

One of the results of climate change is the change in atmospheric patterns such as sea level pressure and the height of different layers of the atmosphere. The results show that the middle-level atmosphere height, which is a key components in atmospheric disturbances and precipitation, or lack thereof, especially in regions such as Iran, has experienced increase in various periods and throughout the statistical period under study. This height increase can lead to a decrease in rainfall and an increase in the duration and severity of droughts in these areas. On the other hand, occurrence of extreme temperature and precipitation anomalies in these areas is another result of this height increase. A suitable solution for dealing with its adverse effects is the optimal management of water resources.