Increasing tropical cyclone intensity and potential intensity in the subtropical Atlantic around Bermuda from an ocean heat content perspective 1955-2019

We investigate tropical cyclone (TC) activity and intensity within a 100 km radius of Bermuda between 1955 and 2019. The results show a more easterly genesis over time and significant increasing trends in TC intensity (maximum wind speed (Vmax)) with a decadal Vmax median value increase of 30 kts from 33 to 63 kts (r = 0.94, p = 0.02), together with significant increasing August, September, October sea surface temperature (SST) of 1.1 degrees C (0.17 degrees C per decade) r = 0.4 (p < 0.01) and increasing average ocean temperature between 0.5 degrees C and 0.7 degrees C (0.08 degrees C-0.1 degrees C per decade) r = 0.3(p < 0.01) in the depth range 0-300 m. The strongest correlation is found between TC intensity and ocean temperature averaged through the top 50 m ocean layer ((T-50 m) over bar) r = 0.37 (p < 0.01). We show how TC potential intensity (PI) estimates are closer to actual intensity by using <(T-50 m)over bar> as opposed to SST using the Hydrostation S time-series. We modify the widely used SST PI index by using (T-50 m) over bar to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a (T-50 m) over bar PI ((T-50 m) over bar _PI) index. The average MSLP difference is reduced by 12 mb and proportional (r = 0.74, p < 0.01) to the SST/<(T-50 m)over bar> temperature difference. We also suggest the index could be used over a wider area of the subtropical/tropical Atlantic where there is a shallow mixed layer depth.