The relationship between intraseasonal precipitation of Iran and combined effects of MJO and NAO

Because of low precipitation and its severe fluctuations in Iran, understanding the dynamics of large scale climate modes and probability of annual and intraseasonal precipitation variation is essential for water management. This study investigates the characteristics of the combined effects of Madden-Julian Oscillation (MJO) and North Atlantic Oscillation (NAO) on precipitation over Iran. Daily precipitation and atmospheric data (relative humidity and vertical velocity) were analysed over Iran during wet season (October to May) for the period 1961 to 2018. The results indicated that: 1) Distinct difference can be observed in spatial distribution of the probability of daily precipitation above upper tercile for MJO phases, phase 1 and 2 wetter while 4 and 5 are drier. The relative humidity is higher in phases (1-3 and 7-8) and lower in phases (4-6). The vertical velocity shows upward (downword) motion in phases 1-2 and 7-8 (3-6). 2) Response of rainy season precipitation over Iran to MJO is more affected by the large-scale atmospheric variation associated with negative NAO as compared to positive NAO. In the negative NAO, the MJO increase (decrease) the probability of upper tercile precipitation 1.2 (0.7) times in phases 2-3 (4-6) and significant tests show a significantly large response for west and North west of Iran. In contrast of positive NAO, the relative humidity and vertical velocity is more affected by negative NAO state. The more (less) humidity and upward (downward) motions increase (decrease) precipitation in phases 2-3 (4-6).