Ocean currents show global intensification of weak tropical cyclones.

Theory¹ and numerical modelling² suggest that tropical cyclones (TCs) will strengthen with rising ocean temperatures. Even though models have reached broad agreement on projected TC intensification^3-5, observed trends in TC intensity remain inconclusive and under active debate^6-10 in all ocean basins except the North Atlantic, where aircraft reconnaissance data greatly reduce uncertainties¹¹. The conventional satellite-based estimates are not accurate enough to ascertain the trend in TC intensity^6,11, suffering from contamination by heavy rain, clouds, breaking waves and spray¹². Here we show that weak TCs (that is, tropical storms to category-1 TCs based on the Saffir-Simpson scale) have intensified in all ocean basins during the period 1991-2020, based on huge amounts of highly accurate ocean current data derived from surface drifters. These drifters have submerged 'holy sock' drogues at 15 m depth to reduce biases induced by processes at the air-sea interface and thereby accurately measure near-surface currents, even under the most destructive TCs. The ocean current speeds show a robust upward trend of ~4.0 cm s^-1 per decade globally, corresponding to a positive trend of 1.8 m s^-1 per decade in the TC intensity. Our analysis further indicates that globally TCs have strengthened across the entirety of the intensity distribution. These results serve as a historical baseline that is crucial for assessing model physics, simulations and projections given the failure of state-of-the-art climate models in fully replicating these trends¹³.