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 100km radius of Bermuda between 1955 and 2019. The results show a more easterly genesis over time and significant increasing trends in tropical cyclone intensity (maximum wind speed (Vmax)) with a decadal Vmax median value increase of 30kts from 33 to 63kts (r=0.94, p=0.02), together with significant increasing August, September, October (ASO) sea surface temperature (SST) of 1.1°C (0.17°C per decade) r= 0.4 (p<0.01) and increasing average ocean temperature between 0.5–0.7°C (0.08-0.1°C per decade) r=0.3(p<0.01) in the depth range 0-300m. The strongest correlation is found between TC intensity and ocean temperature averaged through the top 50m ocean layer (T50m ) r=0.37 (p<0.01). We show how tropical cyclone potential intensity estimates are closer to actual intensity by using T50m as opposed to SST using the Bermuda Atlantic Timeseries Hydrostation S dataset. We modify the widely used sea surface temperature potential intensity index by using T50m to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a T50m potential intensity (T50m_PI) index. The average MSLP difference is reduced by 12mb and proportional (r=0.74, p<0.01) to the SST/(T50m ) 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.