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 () r = 0.37 (p < 0.01). We show how TC potential intensity (PI) estimates are closer to actual intensity by using as opposed to SST using the Hydrostation S time-series. We modify the widely used SST PI index by using to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a PI (_PI) index. The average MSLP difference is reduced by 12 mb and proportional (r = 0.74, p < 0.01) to the SST/ 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.
Highlights
In September 2019 Hurricane Humberto was the latest category 3 hurricane to impact Bermuda, with 100 knot winds and causing power outage to over 80% of the island, and there have been several other intense hurricanes impacting the island in the recent past: Fabian (2003), Gonzalo (2014), Nicole (2016) and Paulette (2020).Observed Atlantic hurricane frequency has been found to correlate with the variability of sea surface temperatures (SSTs) on seasonal (Hallam et al 2019) to multidecadal timescales, as measured by the Atlantic multi-decadal variability (AMV) index (Goldenberg et al 2001), which is the North Atlantic area-averaged (0–60◦ N, 0–80◦ W) sea surface temperature anomaly (SSTA)
In this study we have reviewed tropical cyclone (TC) in the Bermuda area from 1955 to 2019 and highlighted the more easterly genesis of tracks in the period 2000–2019 compared to 1980–1999, in line with the all Atlantic trend, and suggest, from an ocean perspective, that this is related to the increase in ASO SST and T50m in the eastern Atlantic off the west coast of Africa (10–20◦ N, 10–20◦ W)
In addition to the more easterly track genesis we have observed a statistically significant increase (r = 0.94, p = 0.02) in TC intensity in the Bermuda area with an increase in the Vmax median value of 30 kts (4.9 kts per decade) from 33 to 63 kts (1955–2019) and an increased trend observed for the period 1980–2019 of 7.7 kts which is in line with the increase of 8 kts per decade (1982–2009) observed over the wider Atlantic region by (Kossin et al 2007, 2013)
Summary
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 (T50m) r = 0.37 (p < 0.01). The average MSLP difference is reduced by 12 mb and proportional (r = 0.74, p < 0.01) to the SST/T50m temperature difference.
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