climate Article Synoptic–Dynamic Patterns Affecting Iran’s Autumn Precipitation during ENSO Phase Transitions Faranak Bahrami 1 , Abbas Ranjbar Saadatabadi 1 , Nir Y. Krakauer 2, * , Tayyebeh Mesbahzadeh 3 and Farshad Soleimani Sardoo 4   Citation: Bahrami, F.; Saadatabadi, A.R.; Krakauer, N.Y.; Mesbahzadeh, T.; Sardoo, F.S. Synoptic–Dynamic Patterns Affecting Iran’s Autumn Precipitation during ENSO Phase Transitions. Climate 2021, 9, 106. https://doi.org/10.3390/cli9070106 Academic Editors: Salvatore Magazù and Maria Teresa Caccamo Received: 8 May 2021 Accepted: 23 June 2021 Published: 28 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Atmospheric Science and Meteorological Research, Tehran 16385-14977, Iran; bahrami.faranak@yahoo.com (F.B.); a-ranjbar@irimo.ir (A.R.S.) 2 Department of Civil Engineering, The City College of New York, New York, NY 10031, USA 3 Department of Reclamation of Arid and Mountain Regions, University of Tehran, Tehran 31587-77871, Iran; tmesbah@ut.ac.ir 4 Department of Ecological engineering, Faculty of Natural Resources, University of Jiroft, Kerman 78671-61167, Iran; farshad.soleimani@ut.ac.ir * Correspondence: nkrakauer@ccny.cuny.edu Abstract: We compared the effect on autumn (October, November, December) precipitation over Iran during two types of El Niño–Southern Oscillation (ENSO) phase transitions from the perspective of anomalies in wave activity flux and sea level pressure along the Atlantic–Mediterranean storm track, as well as precipitation. We used Oceanic Niño Index (ONI) to identify the transition phases of ENSO (El Niño to La Niña and also La Niña to El Niño, referred to as type 1 and type 2, respectively). Climate data during the period of 1950 to 2019 used in this study is derived from NCEP-NCAR reanalysis. In order to investigate the intensity and direction of Rossby wave trains in different ENSO transitions, we used the wave activity flux parameter, and to evaluate the statistical significance of values, we calculated Student’s t-test. The impact of the Atlantic storm track on the Mediterranean storm track was shown to be greater in type 2 transitions. Further, the existence of a stronger wave source region in the Mediterranean region during type 2 transitions was established. Results also showed the weakening of the Iceland low and Azores high pressure in type 1 transitions and the reinforcement of both in type 2, with the differences being significant at up to a 99% confidence level. Pressure values over Iran were at or below normal in type 1 years and below normal in type 2. Finally, the composite analysis of precipitation anomaly revealed that during ENSO type 1 transitions, most regions of Iran experienced low precipitation, while in type 2, the precipitation was more than average, statistically significant at 75% confidence level or higher over the northern half of the country. Keywords: wave activity flux; Mediterranean storm track; Atlantic storm track; sea level pressure; precipitation anomaly; ENSO 1. Introduction Iran’s climate is relatively arid, marked by low rainfall and high evapotranspiration capacity. Precipitation varies greatly between different regions, but averages about 250 mm per year for the country. Most precipitation occurs from October to March [1]. The low amount of precipitation and high interannual variability over most of Iran lead to frequent droughts, with destructive effects in sectors such as agriculture, environment, and water resources. One factor that affects the climate of this area is El Niño–Southern Oscillation (ENSO), defined in terms of temperature and pressure patterns over the Equa- torial Pacific Ocean. ENSO phase impacts precipitation, temperature, and risk of drought over Iran [2–9]. Nazemosadat and Cordery [7] demonstrated ENSO effects on autumn and winter rainfall, which accounts for about 80% of Iran’s water resources. In autumn, a strong Climate 2021, 9, 106. https://doi.org/10.3390/cli9070106 https://www.mdpi.com/journal/climate