The Climate Dynamics of Category 3 Tropical Cyclone Idai
Discussion: A large mass of somewhat warmer than normal ocean surface temperatures in the western tropical Indian Ocean was present during the evolution of Category 3 Tropical Cyclone Idai earlier this month. There is a tendency for subtropical ridging across or pole ward of large areas of warmer than normal sea surface temperature anomalies (SSTA) in the tropics and subtropics. In this case a relatively small but strong upper ridge was present southwest of Madagascar preventing much movement of Idai early in its life-cycle while over land (northern Mozambique). Once Idai drifted offshore to take full advantage of the warmer than normal surface water steady intensification to a category 3 tropical cyclone took place. Between a strong upper trough over South Africa and the upper ridge southwest of Madagascar Tropical Cyclone Idai was guided westward making a second landfall with a 14+ foot storm surge and 25+ in. of rain over a large area as this system weakened slowly drifting west and inland Mozambique to Zimbabwe. This system was similar to many in recent years stalling and causing long-lasting damage due to the presence of blocking subtropical ridging associated with large areas of warmer than normal waters in the tropics and subtropics and not related to the El Nino southern oscillation. In the U.S. Harvey and Florence are recent examples.