Is Upper Air Pattern Associated NAWH Inhibiting TC Season?

Climate Impact Company Tropical Feature

Issued: Monday September 8, 2025

Highlight: Is the upper air pattern associated with a strong North Atlantic warm hole contributing to the inactive North Atlantic tropics?

Fig. 1-2: The North Atlantic warm hole and the northern hemisphere ECM ENS 15-day 500 MB anomaly forecast.

Discussion: During the climatological peak of tropical cyclone season, the North Atlantic warm hole (NAWH) is vividly present located due south of Greenland in the North Atlantic basin (Fig. 1). NAWH is a large stratified upper ocean cool pool associated with the springtime Greenland icesheet melting and surrounded by the warming of the North Atlantic basin common during the past 3 decades. There is a tendency for the atmosphere to cool across and/or downwind the NAWH which is observed during summer of 2025 and remains in the 15-day outlook according to ECM ENS (Fig. 2). The NAWH trough has pushed the North Atlantic subtropical ridge farther south compared to recent years enabling attendant dry air to work into the subtropics and northern tropics helping to prevent tropical waves from organizing into tropical cyclones. The recent evolution of an Interior East U.S. upper trough has increased westerly shear across the western North Atlantic basin further inhibiting tropical development. The NAWH phenomenon in September was last observed in 2009, 2014, 2015, 2016, and 2018. The 2009 and 2015 analog years featured moderate-to-strong El Nino, not present in 2025. ENSO was closer to neutral in 2014, 2016, and 2018. During the later 3 analog years, tropical cyclone activity after September 8^th ranged from 4 to 8 tropical storms, 3-5 hurricanes, and 2-3 major hurricanes. During the 3 analog years, the middle 3^rd of September was active. Operational models indicate potential for one tropical cyclone through the next 15 days. Consequently, based on the operational forecast, the lower end of the 4-8 tropical storms, 3-5 hurricanes, and 2-3 major hurricanes observed during the analog years fits for 2025. Based on the lower end of the range in tropical cyclone activity after September 8^th during the NAWH analog years, the 2025 seasonal tropical cyclone total is likely 11 tropical storms, 4 hurricanes, and 3 major hurricanes which are considerably lower than most operational seasonal North Atlantic basin tropical cyclone forecasts. The analog years (2014, 2016, 2018) each produced several significant tropical cyclones during October.