Discussion: Yesterday NOAA reported the April 2018 tropical North Atlantic (TNA) index at -0.29 which is 2nd coolest in the mid-to-late 1990’s to present long-term warm cycle of the Atlantic multi-decadal oscillation (AMO) regime. The cool tropics combined with anomalous warmth across the middle latitudes of the North Atlantic (Fig. 1) lead to a neutral AMO for April (+0.064).
The AMO and especially TNA regimes are important contributors to the North Atlantic tropical cyclone season outlooks provided by NOAA/NHC, Tropical Meteorology Project at Colorado State University, Tropical Storm Risk U.K., Climate Impact Company and several private companies. Seasonal forecasts by these providers will be updated just prior or during the June 1 onset of the 2018 season.
The cooler sea surface temperature anomalies (SSTA) across the tropical North Atlantic also measured in the main development region (MDR) for tropical cyclones as represented by the TNA index indicates a suppressing influence on seasonal activity if the cooler than normal waters prevail into the core of the tropical cyclone season.
The European model SSTA forecast for AUG/SEP/OCT 2018 not only indicates cooler than normal waters in the equatorial/tropical North Atlantic but an evolving El Nino in the equatorial East Pacific (Fig. 2). Meanwhile the NOAA (CFS V2) model is similar in theme, lack of anomalous warming in the North Atlantic tropics with some slight El Nino warming in the equatorial East Pacific (Fig. 3).
Fig. 1: The weekly North Atlantic sea surface temperature anomaly analysis provided by NOAA/NHC indicates the North Atlantic tropics are cooler than normal.
Fig. 2: ECMWF AUG/SEP/OCT 2018 global SSTA forecast indicates an evolving El Nino and cool waters in the North Atlantic tropics (suppressing 2018 tropical cyclone activity in the North Atlantic).
Fig. 3: NCEP CFS V2 AUG/SEP/OCT 2018 global SSTA forecast indicates an evolving a weak El Nino-like warming of the equatorial East Pacific and near normal temperature waters in the North Atlantic tropics keeping 2018 seasonal activity of tropical cyclones in the North Atlantic near normal.