Drought and wetness periods in Iran under the influences of subtropical sea surface temperature anomalies and large scale atmospheric circulation

Abstract

Abstract In this study, Drought and wetness periods in Iran under the influences of sea surface temperature (SST) anomalies and large scale atmospheric circulation, were studied using the SPI from 1979 to 2020. To illustrate changes in atmospheric circulation patterns, sinuosity, humidity, jet stream and u, v were analyzed. Severe droughts in autumn (OND) are associated with negative SST anomalies in the North Atlantic, the Caspian Sea, Western Mediterranean and Oman Sea that affect the drought through decreasing atmospheric humidity and strengthening high pressure. Wetness are also associated with positive SST anomalies in the Mediterranean, North Atlantic and Black Sea. These conditions are associated to the positive SST anomaly over the region (-80° W to 70° E, 10° N to 60° N) in winter (JFM).In some cases, the drought occurred simultaneously with the positive temperature anomaly in the Mediterranean and Persian Gulf. Importantly, a large proportion of the wet periods during JFM were associated with positive SST anomalies in the North Atlantic, the Oman Sea, and the western Mediterranean. The increase in SST occurred simultaneously with the increase in precipitation in October and November from 1979 to 2010, while decrease in JFM precipitation was observed. Precipitation anomaly is associated with the position of jet stream centers. Wetness occur when the jet stream is located over Southern Mediterranean and the Arabian Peninsula. The pattern revealed that the occurrence of severe droughts is related to jet stream and it’s the retreated toward the west that leads to the reduction of westerlies waves.