Introduction:
There are some well-known large-scale patterns in the earth atmosphere which could affect the faraways locations. Studying these patterns that influence regional climate has led to identify the factors affecting the weather and climate and helps improve 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 first identified in 1992 time, are long and narrow filamentary pathways 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. Previous studies covered the effect of large-scale phenomena on precipitation and temperature, or the detection of the frequency, path, and strength of ARs, individually. Here, the relationship between various phases of the NAO and ENSO and the frequency and strength of ARs are examined.
Materials and Methods:
The six-hour ERA5 reanalysis data, available from 1940, were used from November to May between 1979 and 2020, for computing Integrated water Vapor Transport (IVT) to identify Atmospheric Rivers (ARs). The study area 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 exceed a certain value of IVT, which is noted as the threshold value. In this study, the IVT threshold was calculated as the 90th percentile of all IVT value computed during the period under study at each point of a 101*61 grid. The geometrical characteristics such as the length of more than 2000 km, the length-to-width ratio of more than 2, and the angle of the main axis of more than 10 relative to the orbits, were applied to identify ARs’ regions that affect Iran. The IVT average of ARs' regions over a limited area around Iran (40^◦E to 64^◦E longitude and 15^◦N to 40^◦N latitude) was used to calculate the intensity of each AR. Afterward, the frequency of ARs, the Odd Ratio (OR), Non-occurrence Ratio (NR), and Occurrence Ratio (OR) were 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 phenomena. The monthly ENSO index (SOI) were classified in critical positive (negative) mode when its monthly index was more (less) than 7 (-7). In the same way, the +1 (-1) value were used for critical positive (negative) phases monthly NAO index. Neutral phases of ENSO and NAO were defined when monthly index was between the critical value.

Result and Discussion:
The results showed that the annual frequency of ARs affecting Iran varies between 8 and 25. The monthly frequency range is between zero and 6. December and February 1980 received the maximum number of the events (6) of ARs during the studied period. Regarding the results, the frequency percentage of ARs was 55.81 and 67.11 in the neutral phase of the SO and NAO indexes, respectively. In addition, the frequency of this phenomenon was about 26 percent in the negative phase of ENSO which is more than in positive phases. For the positive phase of NAO, the frequency of atmospheric rivers was much higher (4 times) than the occurrence in the negative phase of NAO. The previous 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 occurred more in wet years. The high occurrence ratio of ARs in the negative phase (positive) of ENSO (NAO) revealed that the results confirmed the former study on the impact of large-scale atmospheric patterns like ENSO and NAO. The intensity of ARs indicated the measure of water vapor transferred by this phenomenon, and this study showed that the monthly average intensity of atmospheric rivers was higher in the negative phases of ENSO and positive phases of NAO. The monthly average of ARs’ intensity in the negative phase of SOI was 263.46 kg/ms^-1 which was more than that in the neutral and positive phases. In comparison, the monthly average intensity of AR in the positive phase of NAO was 263.04 kg/ms^-1 which means that the ARs occurred in negative and neutral phases of NAO experienced lower intensity and transferred less moisture to Iran.