Atlantic tropical cyclones downscaled from climate reanalyses show increasing activity over past 150 years

Historical records of Atlantic hurricane activity, extending back to 1851, show increasing activity over time, but much or all of this trend has been attributed to lack of observations in the early portion of the record. Here we use a tropical cyclone downscaling model driven by three global climate analyses that are based mostly on sea surface temperature and surface pressure data. The results support earlier statistically-based inferences that storms were undercounted in the 19th century, but in contrast to earlier work, show increasing tropical cyclone activity through the period, interrupted by a prominent hurricane drought in the 1970s and 80 s that we attribute to anthropogenic aerosols. In agreement with earlier work, we show that most of the variability of North Atlantic tropical cyclone activity over the last century was directly related to regional rather than global climate change. Most metrics of tropical cyclones downscaled over all the tropics show weak and/or insignificant trends over the last century, illustrating the special nature of North Atlantic tropical cyclone climatology.

The Combined QBO and ENSO Influence on Tropical Cyclone Activity over the North Atlantic Ocean

The Quasi-Biennal Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO) largely modulate the zonal wind in the tropics. Previous studies showed that QBO phases produce changes in deep convection through an increase/decrease in the tropopause height over the tropics and subtropics. This study investigates the combined effects of QBO and ENSO on tropical cyclone activity by modulating tropopause height. We found that tropopause height increases over the Gulf of Mexico, the Caribbean region, and the Western North Atlantic Ocean during La Niña + QBOW, allowing deeper tropical convection to develop over those regions. As a consequence, TC activity over those regions is not only increased in number but also enhanced in intensity. Conversely, during El Niño + QBOE, most deep tropical convection is inhibited over those same regions due to the decrease in tropopause height over the subtropics. We conclude that QBO effects on TCs and deep convection should be studied in combination with ENSO. Additional comparative studies using long record data at high vertical resolution are needed to fully understand to what extent QBO interacts with ENSO in the lower tropical stratosphere and upper tropical troposphere.

Extreme Atlantic hurricane seasons made more likely by ocean warming

Tropical cyclones are among the most damaging and fatal extreme weather events. An increase in Atlantic tropical cyclone activity has been observed, but attribution to global warming remains challenging due to large inter-annual variability and modelling challenges. Here we show that the increase in Atlantic tropical cyclone activity since the 1980s can be robustly ascribed to changes in atmospheric circulation as well as sea surface temperature (SST) increase. Using a novel weather pattern based statistical model, we find that the forced warming trend in Atlantic SSTs over the 1982–2018 period increased the probability of extremely active tropical cyclone seasons by 14 %. Seasonal atmospheric circulation remains the dominant factor explaining both inter-annual variability and the observed increase. Our weather pattern-based statistical decomposition helps to understand the role of atmospheric variability for the Atlantic tropical cyclone activity and provides a new perspective on the role of ocean warming.