By Dr. Kristopher Karnauskas, University of Colorado Boulder — The Conversation US — 18 May 2021
As summer in the Northern Hemisphere approaches, forecasters begin watching every bout of rainy weather between the Gulf of Mexico and Africa. The seeds of tropical storms are small weather disturbances that develop in the tropics — some beginning as clusters of thunderstorms over Africa, or as clouds near the Cape Verde Islands off Africa’s west coast. The Atlantic hurricane season officially starts 1 June, and the National Hurricane Center will be issuing its first seasonal forecast. To understand how bad a given season will be, forecasters watch three key indicators.
First, the African monsoon: more storm “seeds” emerge from the African coastline in high-monsoon years, creating more potential tropical systems. Second, ocean temperatures: tropical storms thrive where surface ocean temperatures reach 80°F (26.7°C) or warmer, and a deep layer of warm water provides more sustained fuel for major hurricanes. Third, ENSO conditions — El Niño or La Niña: El Niño tends to increase wind shear over the Atlantic, suppressing tropical storms, while La Niña favours more storms by reducing shear. Conditions are near neutral right now, and forecasters are watching closely for a possible late-blooming La Niña. The 2020 season produced 30 named storms — a record; a quieter 2014 saw only eight.