Highlight: Marginal warmth for AMO/TNA the next several months may hold hurricanes closer to normal.
Executive Summary: During the mid-to-late 1990’s to 2022, the North Atlantic basin entered a decadal warm cycle. Most of that time the North Atlantic basin has produced warmer-than-normal SST accounting for an uptick in tropical cyclone activity. The Atlantic multi-decadal oscillation (AMO) is a SSTA index covering the entire North Atlantic basin. The tropical North Atlantic (TNA) index covers the main development region (MDR) for hurricanes in-between the Caribbean Sea and northwest Africa Coast. The AMO and TNA are used to identify the strength of North Atlantic basin SST. Currently, AMO and TNA are neutral and not as warm as the warm North Atlantic 25-year climatology. However, using an optimum climate normal (OCN) for the 2016-2021 active tropical cyclone period in the North Atlantic basin, the AMO and TNA are expected to warm for the AUG-NOV 2022 tropical cyclone season and cool to neutral phase late this year and early 2023. Dynamic models are slightly cooler than the OCN forecast. The 2022 AUG-NOV tropical cyclone season encounters marginally warm waters slightly less warm than the 2016-2021 OCN and 25-year climatology. The 2022 number of hurricanes forecast for the North Atlantic basin is 6-10 according to NOAA. The slightly cooler AMO/TNA pattern compared to the OCN and longer-term climatology may hold the 2022 hurricane count closer to 6 (versus 10).
Fig. 1: During 2016-2021 the North Atlantic basin has observed a persistent active tropical cyclone regime.
Discussion: Tropical cyclone activity is forecast above normal in the North Atlantic basin for the 2022 season. If correct, the North Atlantic will observe a unique 7 consecutive years of greater than normal number of tropical cyclones in one season (Fig. 1). A significant contributor to the active period of tropical cyclones is the behavior of North Atlantic SSTA as identified by the Atlantic multi-decadal oscillation (AMO) and tropical North Atlantic (TNA) index. A review of the AMO/TNA behavior during this period to project the 2022-23 forecast is presented.
During 2016-2022, the AMO and TNA monthly index have run approximately parallel (Fig. 2). Except for 2018, the AMO and TNA have been in the warm phase during tropical cyclone season. Based on the AMO/TNA 2016-2022 behavior, an analog forecast of each index for 2022-23 reveals warming to the positive (warm) phase for the AUG-NOV 2022 tropical cyclone season followed by cooling to neutral AMO and eventually neutral TNA by next spring (Fig. 3).
The NMME global SSTA forecast for SEP/OCT/NOV 2022 reveals near normal TNA index which is unusually cool for the late 1990’s to 2022 warm cycle (Fig. 4). The NMME forecast is definitely cooler than the OCN analog. The AMO index is marginally warm and similar to the OCN analog although note that the basin average is the result of large areas of both neutral and warm SSTA plus the cool North Atlantic warm hole (NAWH) west and south of Greenland.
Fig. 2: Monthly AMO and TNA index for 2016-2022 are indicated.
Fig. 3: Based on the 2016-2022 optimum climate normal, a projection of AMO and TNA for 2022-23 is indicated.
Fig. 4: The NMME global SSTA forecast for SEP/OCT/NOV 2022 reveals near normal TNA and slightly above normal AMO.