Previous studies revealed that the interannual variation of precipitation in the northwest of Iran (NWI) was controlled mainly by the Mediterranean cyclones and local mechanisms. Drawing upon the ...literature, the aim of this study is twofold: examining precipitation variability resulting from each of the abovementioned mechanism and investigating variation in precipitation regime in the NWI during 1961–2010. To provide a better understanding of the features of precipitation regime in the NWI, in the present study, I put the spotlight on the changes in precipitation patterns occurring over five decades in a context experiencing climate change. Accordingly, the annual, semiannual, and triple cycle of precipitation data from 1961 to 2010, which were obtained from 250 stations and by which a gridded dataset of monthly precipitation for the period above was created, were analyzed using harmonic analysis. The results showed that the regional average of precipitation during five successive decades decreased from 385.8 mm in the first decade to 297.3 mm in the last decade. The differences in the precipitation values during the five decades under study can be due to the slight positive trends in precipitation in summer and considerable negative trends in other seasons. Moreover, global warming and changes in the path of storm tracks can be considered as the possible reasons behind the observed changes in the precipitation in the NWI.
Climate change can be traced in any given region by searching for changes in climatic elements' characteristics. Extreme precipitation, for example, is a characteristic of precipitation whose ...changes, especially in recent years, can be attributed to global warming and climate change. This study aimed to investigate possible changes in extreme precipitation in Iran by analysing the frequency of extreme rainy days duration. To this end, gridded precipitation datasets from the hydrological years 1971–1972 to 2015–2016 were used, and days associated with extreme precipitation were extracted by using thresholds of 90th, 95th, and 99th percentiles (as heavy, very heavy, and extremely heavy precipitation, respectively). Latitude and longitude showed the most significant correlation with precipitation characteristics in the spatial distribution during the study period. In addition, heavy (HP), very heavy, and extremely heavy precipitation experienced durations of 1–8, 1–7, and 1–4 days, respectively. At all extremes, the 1‐day duration covered more areas and had more repetition, while the longer duration had the lowest tempo–spatial repetition. According to the results of the present study, the one‐day duration of HP contributed more significantly to the annual precipitation. The proportion of this type of precipitation has been more significant in the southern and central regions of the country. Furthermore, it was revealed that the frequency of durations of short‐term HP increased during the study period, whereas the duration of long‐term HP declined.
A rainy day time series with three types of heavy precipitation in Iran was extracted based on the thresholds of percentiles criteria, and its frequency changes were investigated. The heavy, very heavy, and extremely heavy precipitation occurred in successions of 1–8, 1–7, and 1–4 days, respectively. The one‐day heavy precipitation duration had the highest share of annual rainfall. In some parts of the country, the short‐term duration of extreme precipitation increased, while the long‐term duration decreased or showed stationary behavior.
The intensity of precipitation, particularly high extreme precipitation (HEP), has always been responsible for numerous natural hazards, such as floods and landslides. Therefore, it is crucial to ...investigate the behavior change of precipitation intensity as a consequence of climate change. This research aims to investigate precipitation intensity, its decadal changes, and precipitation duration for all types of extreme precipitation in Iran. To this end, gridded data of precipitation from the hydrological year 1971–1972 through 2015–2016 was utilized, and the days of heavy (HP), very heavy (VHP), and extremely heavy (EHP) precipitation were extracted using the 90th, 95th, and 99th percentile thresholds, respectively. The investigation results indicated that the maximum intensity of the extreme precipitation types occurred mostly in the Zagros Mountains, the Iranian Coast of the Caspian Sea (ICCS), and parts of the coasts of the Oman Sea and the Persian Gulf. The correlation coefficient between precipitation intensity and spatial features revealed a strong and inverse relationship between precipitation intensity and geographic coordinates (i.e., longitude and latitude). Our findings revealed that the durations of rainy days for HP, VHP, and EHP were 1–8, 1–7, and 1–4 days, respectively. Furthermore, the most intense precipitation occurred in short-term durations (e.g., 1-day duration). The findings also exposed an increasing trend in precipitation intensity of short-term durations in mountainous regions. Moreover, the intensity of HP and VHP increased, while EHP decreased in large areas of the country. Decadal analysis revealed that the maximum intensity of all types of extreme precipitation occurred mainly in the second and third decades. Additionally, the heaviest precipitation events took place in the second decade, which coincided with the dominance of the positive North Atlantic Oscillation (NAO) phase.
Temperature is one of the most important elements of climate, which undergoes temporal and spatial changes under the influence of local and external factors. These changes can cover micro and ...macroclimate areas as well as short to long-term periods. Therefore, in the present study, the spatial patterns of temperature change in Iran over the past half-century are discussed. To this end, average annual temperature of 383 synoptic stations in Iran during 50 years (1961–2010) were collected from Iran Meteorological Organization. Linear regression and local Moran’s I were used to analyze the trend of changes and the spatial patterns respectively. The results of intra-decade changes in temperature showed that the average temperature of Iran has increased significantly in recent decades, and have experienced less significant daily fluctuations at the same time. Results obtained from analysis of intra-decade temperature change trend showed that temperature has dramatically increased as a result of rises in mean temperature levels in recent decades, such that in the fourth (1991 to 2000) and fifth decade (2001 to 2010) the temperature variation trend has increased by 0.17 and 0.1 respectively, while the temperature of Iran in the first and third decade has declined by −0.03. The results obtained from analysis of spatial pattern of intra-decade temperature variation showed that spatial temperature pattern in Iran is mostly influenced by local factors such as latitudes variations. Generally, with increasing latitude in Iran, the temperature decreases. The temperature in the mountainous areas (negative spatial autocorrelation), southern parts, and areas around Persian Gulf, follow a high spatial pattern (positive spatial autocorrelation) due to lower latitudes and special condition of greenhouse gas emission. However, in recent decades, the positive spatial pattern zones have increased significantly as a result of upward temperature trends.
The tropopause serves a critical role in shaping global and regional weather and climate dynamics. Changes in tropopause characteristics can significantly impact other atmospheric components, thereby ...influencing Earth’s climate systems. In the long run, variations in tropopause features can lead to shifts in the thermal, dynamic, and chemical properties of the tropospheric layer. This study aims to investigate the descriptive attributes of tropopause pressure levels (TPLs) during different months, as well as the temporal and spatial trends in TPL across the Northern Hemisphere spanning from 1979 to 2022. Utilizing ERA5 temperature data for the 700 to 50 hPa range, the tropopause was identified using the lapse rate of tropopause (LRT), and its changes were analyzed employing the linear regression model with the least squares error approach. The results indicated that the spatial pattern of TPLs changed across various latitudes varies seasonally. Generally, the changes in TPLs did not exhibit a linear relationship with latitude, and in most observed months, the highest and lowest TPLs did not correspond to the lowest and highest latitudes, respectively. Examination of the trend in TPLs revealed that in numerous significant areas across different seasons, the trends were statistically insignificant. Where significant, the trends predominantly indicated negative changes (decreases), suggesting a reduction in pressure and potentially an increase in tropopause altitude in these regions, possibly reflecting the influence of global warming.
In any given region, climate change can be manifested in the form of various characteristics of climatic elements. In the present study, the frequency of different duration of rainy days was examined ...to investigate the precipitation variations as a sign of climate change. To this end, gridded precipitation data were used from 1971 through 2016, and days with more than 1 mm of precipitation were considered rainy days. According to the rainy days’ frequency, it was revealed that during the study period, 1 to 36 days of duration occurred in the country. The 1-day duration had the highest frequencies and covered the vastest areas, while 36-day duration had the lowest frequencies and covered the minor areas. Accordingly, the 1-day duration played the most significant role in annual precipitation. The proportion of these rainfall events in some country’s dry and semi-dry parts was more than 80%. The findings also revealed an increased frequency of short-term, especially 2-day duration, in large parts of the country and a decrease in the long-term duration. The results showed that latitude and longitude had the most significant impact on the frequency distribution of the duration of rainy days. Latitude had a direct effect (excluding the 1-day duration of annual precipitation), and longitude had an inverse effect (except the 1-day duration of annual precipitation) on the precipitation duration.
The investigation of the height variations of the westerly in the mid-latitudes of the Northern Hemisphere is crucial for monitoring changes in the intricate atmospheric system. This study examined ...the hypothesis of changes in the thickness of the westerly domain by analyzing the height of the 500 hPa pressure level. To this end, monthly data from the European Centre for Medium-Range Weather Forecasts (ECMWF) spanning from 1959 to 2021 was utilized to investigate three components: geopotential height (GH), zonal, and meridional wind components. The dataset underwent statistical tests, including trend analysis using Least Squares Error linear regression (LSE-LR) and shift (jump) analysis using the Standard Normal Homogeneity Test (SNHT). The results revealed a significant increasing trend in the height of the troposphere and the westerlies in the mid-latitudes of the Northern Hemisphere, accompanied by significant fluctuations, indicating an increase in the height of the 500 hPa pressure level and a decrease in the thickness of the westerly domain. The extent of these trends was observed to be greater in cold months, particularly in January and February, while the changes were minimal in November. Notably, the spatial distribution of upward jumps, indicative of change, was highest in January, July, and August in the eastern and southeastern regions of Asia. Furthermore, the convergence section of the westerlies trough exhibited a substantial number of jumps, particularly in March, spanning Central America and southern parts of North America. Regarding decadal coincidence onset, the highest number of jumps occurred in February, encompassing the western Pacific Ocean, western Atlantic Ocean, Eastern Europe, and western Asia. The end of these jumps coincided in February, and the spatial proximity of the end times was closer compared to the onset times in February, March, April, July, and August.
In the present study, the precipitation regime (PR) was investigated as a feature of the structure of Iran’s climate and its relationship with spatial-topographic variables and its spatial pattern. ...Spatial patterns of the indices of the PR used in the present study exhibited significant agreement in illustrating the diverse PRs of Iran. The diversity of spatial factors provided the spatial diversity of precipitation characteristics, including PR. Harmonic analysis (HA) results confirmed the country’s diverse PRs, which were attributed to different mechanisms affecting Iran’s precipitation patterns throughout the year. Using multiple methods in the present study, four PRs were identified. According to the methods used, the increase in precipitation in each location was associated with a reduction in the month-to-month difference in precipitation and an enhancement in the uniform distribution of precipitation. All indices indicated that the northern regions of Iran, including the Iranian coast of the Caspian Sea (ICCS), exhibited a uniform PR. The northwestern (NWI) and southeastern (SEI) regions displayed a relatively uniform regime, while the remaining parts of the country experienced a seasonal regime characterized by a longer dry season. Furthermore, there was a notable correlation between the PR and latitude, particularly from approximately 35 degrees northward. Moreover, there was a notable correlation between the PR and latitude, particularly from approximately 35 degrees northward. As for the southern regions of the country, beyond the latitude of 35 degrees, the spatial distribution of PRs was influenced not only by latitude but also by other variables such as longitude, altitude, and the shape and length of the Persian Gulf coast adjacent to the eastern deserts.
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