The Warm Tropical North Atlantic Index

Discussion: The 50-year climatology for tropical cyclone activity in the North Atlantic Basin is 12.4 tropical storms, 6.4 hurricanes and 2.5 intense hurricanes. The accumulated cyclone energy index is 99.2. During the past 25 years the climatology is somewhat stronger numbers…15.3 tropical storms, 7.6 hurricanes and 3.4 intense hurricanes. ACE index is 125.8. Why are the numbers higher? The tropical North Atlantic (TNA) index identifies sea surface temperature anomalies (SSTA) in the main development region (MDR) for North Atlantic hurricanes located between the far eastern Caribbean Sea to the northwest coast of Africa. Beginning in the middle 1990’s the TNA index during AUG/SEP/OCT (peak activity of TC season) jumped into the warm phase persistently and with amplitude. Only 5 tropical cyclone seasons since 1995 have observed TNA index not in the warm phase. The average TNA index during the past 25 years is nearly +2 St Dev. Last year the TNA index was very warm (+0.58) and one catalyst to the record seasonal activity. The warmest TNA index for AUG/SEP/OCT was in 2010 (+0.83) a year when 12 hurricanes were generated. The most hurricanes on record occurred in 2005 (15) when TNA index was +0.67. At the moment, all global SSTA forecasts for late summer/early autumn 2021 indicate the TNA index in the MDR region will be marginally warm. For this reason the 2021 North Atlantic tropical cyclone season is likely very active (the subtropics will be somewhat warmer than normal) but closer to normal with hurricanes.