Beyond everyday weather, Earth's atmosphere and oceans produce powerful recurring phenomena. The El Niño–Southern Oscillation (ENSO) is a coupled ocean-atmosphere cycle that alternates roughly every 2–7 years between three phases. In the El Niño phase, warmer-than-normal sea surface temperatures develop in the central and eastern tropical Pacific, often bringing wetter conditions to the southern United States and South America and drier conditions to Australia and Southeast Asia. The Southern Oscillation Index (SOI) measures the air pressure difference between Tahiti and Darwin, Australia; negative SOI values indicate El Niño, while positive values indicate La Niña, the opposite phase with cooler-than-normal tropical Pacific waters. La Niña also tends to reduce wind shear over the Atlantic, increasing both the number and intensity of Atlantic hurricanes.
Hurricanes are large rotating storm systems with sustained winds of at least 119 km/h (74 mph) that form over warm tropical ocean water of at least 26.5°C. The Saffir-Simpson Hurricane Wind Scale ranks them from Category 1 (119–153 km/h) to Category 5 (≥252 km/h) based on maximum sustained wind speed. At the center of a hurricane lies the eye, a calm and clear region surrounded by the eyewall, which contains the storm's most intense winds and rainfall. Although high winds cause severe damage, the deadliest aspect of a hurricane is often storm surge — the abnormal rise in sea level caused by winds pushing ocean water ashore.
Tornadoes form from severe thunderstorms when wind shear creates a rotating updraft called a mesocyclone; this rotation can extend to the ground as a visible funnel. The Enhanced Fujita (EF) Scale rates tornadoes from EF0 (light damage) to EF5 (incredible destruction). Tornado Alley, in the central United States, experiences frequent tornadoes because warm, moist Gulf air collides with cold, dry Arctic air over the Great Plains. A derecho, by contrast, is a widespread, long-lived straight-line windstorm associated with fast-moving bands of severe thunderstorms. Underpinning all of this weather is the thermohaline circulation, a global ocean conveyor belt driven by differences in temperature and salinity that redistributes heat around the planet. Climate change could disrupt it: freshwater from melting ice sheets may dilute salty North Atlantic water, weakening the sinking motion that drives the circulation and potentially cooling Europe.