نوع مقاله : مقاله پژوهشی
نویسندگان
1 دانشجوی دکتری، گروه مهندسی آب، دانشکده کشاورزی، دانشگاه شهرکرد، شهرکرد، ایران
2 گروه مهندسی آب/ دانشکده کشاورزی/ دانشگاه شهرکرد/ شهرکرد/ ایران
3 گروه مهندسی آب، دانشکده کشاورزی، دانشگاه شهرکرد، شهرکرد، ایران
چکیده
کلیدواژه‌ها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Introduction:
There is some well-known large-scale pattern in earth atmosphere which could affect the faraway locations. Studying these patterns that influence regional climate led to identify the factors affecting the weather and climate and help to better prediction of atmospheric phenomena such as wet weather and drought.
This research examines the impact of the El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) on the frequency and intensity of Atmospheric Rivers (ARs), which affect the precipitation of Iran. The Atmospheric Rivers (ARs), the phenomena which were identified in 1992 first time, are long and narrow filamentary path ways that transport about 90% of the water vapor in the atmosphere to the Earth's middle latitudes and transport the moist air of faraway sources around the Earth. They play a significant role in the world's weather and regional hydrology. Former studies covered the effect of large-scale phenomena on precipitation and temperature, or detecting the frequency, path, and strength of ARs, individually. Here, the relation between various phases of the NAO and ENSO with the frequency and strength of ARs are examined.
Methods:
The six hours ERA5 reanalysis data, available from 1940, were used from November to May between 1979 to 2020, for computing Integrated water Vapor Transport (IVT) to recognize Atmospheric Rivers (ARs). The study area is covered from 20◦W to 80◦E longitude and 10◦S to 50◦N latitude. The first condition to identify the ARs is recognizing an area where the IVT is more than a certain value of IVT, which is noted as the threshold value. In this study, the IVT threshold was calculated by the 90th percentile of all IVT value computed during the period under study on each point of a 101*61 grids. The geometrical characteristics such as the length of more than 2000 km, the ratio of length to the width of more than 2, and the angle of the main axis of more than 10 to the orbits which presented in Ried et al. (2020), applied to identify ARs’ regions which affect Iran. The IVT average of ARs' regions on a limited area around Iran (40◦E to 64◦E longitude and 15◦N to 40◦N latitude) used to calculate the intensity of each AR. In the following, the frequency of ARs, the Odd Ratio (OR), Non-occurrence Ratio (NR), and Occurrence Ratio (OR) are examined in different phases of the ENSO and NAO (neutral and critical phases; negative and positive phases) by a 2*2 cross tables. Also, the intensity of ARs analyzed for different phases of surveyed phenomenon. The monthly ENSO index (SOI) is classified in critical positive (negative) mode when its monthly index is more (less) than 7 (-7). In the same way, the +1 (-1) value are used for critical positive (negative) phases monthly NAO index. Neutral phases of ENSO and NAO are defined when monthly index is between the critical value.
Results:
The results showed the annual frequency of ARs affecting Iran varies between 8 to 25. The monthly frequency range is between zero and 6. December and February of 1980 received maximum event (6) of ARs during the studied period. Here, the results showed that the frequency percentage of ARs is 55.81 and 67.11 in the neutral phase of the SO and NAO indexes, respectively. In addition, the frequency of this phenomenon is about 26 percent in the negative phase of ENSO which is more than positive phases. For the positive phase of NAO, the frequency of atmospheric rivers is much higher (4 times) than the occurrence in the negative phase of NAO. The former studies on the effect of SOI and NAO indexes on the precipitation of Iran showed the negative phase of SOI (or El Nino) and the positive phase of NAO have occurred more in wet years. The high occurrence ratio of ARs in the negative phase (positive) of ENSO (NAO) indicates the results confirmed the former study on the impact of large-scale atmospheric patterns like ENSO and NAO. The intensity of ARs indicates the measure of water vapor transferred by this phenomenon, and this study shows the monthly average intensity of atmospheric rivers is higher in the negative phases of ENSO and positive phases of NAO. The monthly average of ARs’ intensity in the negative phase of SOI is 263.46 kg/ms-1 which is more than the neutral and positive phases. In comparison, the monthly average of ARs’ intensity in the positive phase of NAO is 263.04 kg/ms-1 which means that the ARs occurred in negative and neutral phases of NAO experience lower intensity and transfer less moisture to Iran.
کلیدواژه‌ها [English]
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