Potential Impacts of Anthropogenic Forcing on the Frequency of Tropical Depressions in the North Indian Ocean in 2018

Fourteen tropical depressions formed in the North Indian Ocean during 2018-the most active season since 1986 and the second most active season since 1980. Among the 14 tropical depressions during 2018, seven developed into cyclonic storms, with five intensifying into severe cyclonic storms-three of which became very severe cyclonic storms. The sea surface temperature anomaly associated with El Nino appears to have played a minor role in shaping this extreme event (i.e., the 14 tropical depressions in the North Indian Ocean). Using large ensemble experiments performed by the Community Earth System Model developed by the National Center for Atmospheric Research, we detected an important role potentially played by anthropogenic forcing in increasing the risk of the 14 tropical depressions in the North Indian Ocean that were observed in the active 2018 season. Moreover, the projection experiments suggest a rising frequency of tropical depressions in the second half of the 21st century.