Environmental Research Letters (Jan 2021)
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.
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