Three Oceanic Conductors Control Iran's Rains

Original: "Spatial Analysis of Seasonal Precipitation over Iran: Co-Variation with Climate Indices"

· Majid Dehghani, Somayeh Salehi, Amir Mosavi, Narjes Nabipour, Shahaboddin Shamshirband, Pedram Ghamisi

arXiv:2001.09757 · 2020-01-15 · CC BY 4.0 · 2 min

Atmospheric and Oceanic Physics Exoplanets

Three climate indices explain where and how much Iran's seasonal rainfall deviates from the norm.

In 2019, northern and southwestern Iran received 50% more rainfall than usual, and rivers overflowed their banks. A year earlier, in 2018, rainfall was 53% below normal, and fields cracked under drought. Such swings are not random; they are responses to three distant oceanic pulses that set the rhythm for Iran's rains.

Three indices: SOI, NAO, and PDO are like three conductors, each with their own score. SOI (the pressure difference between Tahiti and Darwin) leads the Pacific orchestra, NAO (the pressure difference between the Azores and Iceland) leads the Atlantic, and PDO (Pacific sea surface temperature) sets a slow bass rhythm.

The researchers took data from 103 synoptic stations across Iran over 30 years (1987–2016) and compared seasonal precipitation totals with monthly values of these indices from the preceding 12 months. For each station, they identified the month with the strongest correlation, then divided index values into five phases, from strongly negative to strongly positive. For each phase, they calculated the percentage deviation of precipitation from the long-term average.

The most influential index was SOI. In autumn, it correlates significantly with precipitation at 86% of stations, and in winter and spring at over 70%. During winter, El Niño (negative SOI) enhances precipitation across nearly the entire country, while the warm phase of PDO and the cold phase of NAO reduce it.

In the neutral phase, all three indices most often lead to reduced precipitation. Reductions can reach 100%, meaning a season with no rain at all, while increases can exceed 100% of the norm, as in 2019. This sensitivity map allows for early assessment of what to expect in the coming season, given the phase of the indices.

Previously, it was known that climate indices affect precipitation in Iran, but quantitative estimates for all seasons and all three indices together were lacking. Now, for each station and season, one can say by what percentage precipitation will change under a given phase of SOI, NAO, or PDO. These maps can be used in planning dam operations, allocating irrigation water, and assessing drought and flood risks.

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In some parts of Iran, such as Yazd, average annual precipitation is just 51 mm, while on the Caspian coast at Bandar-e Anzali it reaches 1730 mm per year, a 34-fold difference. Yet even in arid regions, an El Niño phase can more than double seasonal precipitation.

\rho = 1 - \frac{6 \sum d_i^2}{n(n^2 - 1)}

A measure of monotonic association between two variables, where d_i is the difference in ranks and n is the number of observations

Concepts

North Atlantic Oscillation Southern Oscillation index El Niño index ENSO spring predictability barrier El Niño interannual variability precipitation monsoon NOAA CPC La Niña precipitation anomaly seasonal cycle Indian Ocean capacitor effect Indian Ocean Dipole natural variability monsoon onset seasonal forcing TAO/TRITON moorings Teleconnection

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Three Oceanic Conductors Control Iran's Rains