The culprit to North Atlantic tropical cyclone development is too much dry air promoted by a strong subtropical high pressure stretched across the North Atlantic basin whereas the 2020-24 climatology produced low pressure in the tropical/subtropical North Atlantic due to presence of tropical cyclones.

During the past week, the Nino SSTA regions have cooled dramatically. The Nino34 SSTA is -0.49C today, very close to the -0.50C La Nina threshold. Off the northwest coast of South America, the Nino12C chilled rapidly to -0.80C. In the tropical Indian Ocean, the western areas have cooled while the southeast tropics have warmed dramatically signaling a moderate-to-strong negative Indian Ocean dipole (-IOD).

The Gulf of Mexico and Caribbean Sea have been exceptionally quiet regarding tropical cyclone activity through late August despite moderately warmer than normal ocean surface temperatures. The primary reason for the lack of tropical cyclone activity is the dry mid-troposphere. The dryness in this region is compensation for a wet mid-troposphere across the eastern tropical Pacific and eastern half of the North Atlantic tropics.

Other than Erin, the 2025 season has been quiet so far. However, during the 2016-24 active period, after September 1 is typically quite active featuring 7 hurricanes and 3 intense hurricanes. As September 2025 approaches, the North Atlantic basin is warmer than normal but arguably the least warm since 2015. Previously, 2018 produced least warm North Atlantic basin SSTA during September when peak tropical cyclone activity occurs. However, in 2018 a weak La Nina was forming and in 2025, signs of a weak La Nina are ahead. Therefore, despite the less warm North Atlantic, the La Nina tendency should produce a favorable atmospheric environment for tropical cyclone development during SEP/OCT/NOV 2025.