Ocean-atmosphere circulation controls on integrated meteorological and agricultural drought over Iran

Drought may affect more people than any other climate extremes worldwide, especially in (semi)arid climates such as Iran. In this paper, the empirical Multivariate Standardized Drought Index (MSDI) that incorporates the meteorological and agricultural drought conditions is first computed. The wavelet analysis is then used to identify teleconnections between ocean-atmosphere oscillations from Atlantic (AMO, AO, and NAO) and Pacific (ENSO, PDO, and PNA) oceans and Iran's drought at a highly dense network of regularly gridded points (2485) during 1979-2016. It is found that among the droughts of 1980, 1985-86, 1990-91, 1998-99, 2000, 2007-08, and 2010-11, both the 2007-08 and 2000 are the broadest and strongest across the country. Overall, those droughts that occurred in the 2000-2016 period (average MSDI = -1.14) were, on the whole, stronger than those happened over the latest 20 years of the 20th century (average MSDI = -0.77), reflecting the strengthening of the influence that ENSO has on the Middle East climate in the most recent decades. According to the wavelet coherence results, unlike the Atlantic-based oscillations that almost have an anti-correlation with droughts, there is a positive correlation between Pacific-based oscillations and Iran's drought. It is also identified that the severe historical droughts across the country were usually associated with a strong La Nina coupled with cold PDO and positive NAO phases in the latest 30 years. Overall, droughts in Iran tend to be substantially controlled by multiple interacting of climate indices since three-coupled indices have stronger coherence with drought than either two-coupled indices or single ones. A combination of the three indices of ENSO, PDO, and NAO has the strongest coherence with Iran's drought compared to all the other three- and two-coupled, and all single indices, indicating that integrated impact of climate indices can better capture hydroclimate teleconnections. These multiple interacting impacts from the different large-scale climate circulation patterns mark a significant step towards an improved prediction of severe droughts across Iran and can also be used for other parts of the world.

Automated summary

Drought in Iran is influenced by ocean-atmosphere oscillations from the Atlantic and Pacific oceans. Droughts from 2000-2016 were more severe than those in the last 20 years of the 20th century, and severe historical droughts in Iran were linked to strong La Niña events paired with cold PDO and positive NAO phases.