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

Abstract

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 °C (0.17 °C per decade) r = 0.4 ( p < 0.01) and increasing average ocean temperature between 0.5 °C and 0.7 °C (0.08 °C–0.1 °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 ( $\overline {{T_{{\text{50}}\,{\text{m}}}}} $ ) r = 0.37 ( p < 0.01). We show how TC potential intensity (PI) estimates are closer to actual intensity by using $\overline {{T_{50\,{\text{m}}}}} $ as opposed to SST using the Hydrostation S time-series. We modify the widely used SST PI index by using $\overline {{T_{{\text{50}}\,{\text{m}}}}} $ to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a $\overline {{T_{{\text{50}}\,{\text{m}}}}} { }$ PI ( $\overline {{T_{{\text{50}}\,{\text{m}}}}} $ _PI) index. The average MSLP difference is reduced by 12 mb and proportional ( r = 0.74, p < 0.01) to the SST/ $\overline {{T_{50\,{\text{m}}}}} $ 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.

Keywords

• tropical cyclone
• ocean heat content
• subtropical Atlantic
• Bermuda
• potential intensity
• Hydrostation S time-series