Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea

Abstract

Abstract. Understanding the oceanic response to tropical cyclones (TCs) is of importance for studies on climate change. Although the oceanic effects induced by individual TCs have been extensively investigated, studies on the oceanic response to the passage of consecutive TCs are rare. In this work, we assess the upper-oceanic response to the passage of Hurricanes Dorian and Humberto over the western Sargasso Sea in 2019 using satellite remote sensing and modelled data. We found that the combined effects of these slow-moving TCs led to an increased oceanic response during the third and fourth post-storm weeks of Dorian (accounting for both Dorian and Humberto effects) because of the induced mixing and upwelling at this time. Overall, anomalies of sea surface temperature, ocean heat content, and mean temperature from the sea surface to a depth of 100 m were 50 %, 63 %, and 57 % smaller (more negative) in the third–fourth post-storm weeks than in the first–second post-storm weeks of Dorian (accounting only for Dorian effects), respectively. For the biological response, we found that surface chlorophyll a (chl a) concentration anomalies, the mean chl a concentration in the euphotic zone, and the chl a concentration in the deep chlorophyll maximum were 16 %, 4 %, and 16 % higher in the third–fourth post-storm weeks than in the first–second post-storm weeks, respectively. The sea surface cooling and increased biological response induced by these TCs were significantly higher (Mann–Whitney test, p<0.05) compared to climatological records. Our climatological analysis reveals that the strongest TC-induced oceanographic variability in the western Sargasso Sea can be associated with the occurrence of consecutive TCs and long-lasting TC forcing.

Impact of Tropical Cyclones on Pelagic Sargassum

“…On average, the maximum chl-a anomalies occurred 1 day after TCs with a maximum of +0.02 mg m −3 . These increased chl-a concentrations persist for the next 4 days after the passage of the TCs, in good agreement with previous studies (e.g., Avila-Alonso et al, 2021;Babin et al, 2004;Menkes et al, 2016). This is indicative that TCs induce a fertilization of the upper ocean which may also fuel Sargassum growth.…”

“…Because they bring nutrient-rich subsurface waters within the mixed-layer, TCs are known to increase the productivity at the ocean surface. This is revealed by increased concentration of chlorophyll-a in the wake of TCs (Avila-Alonso et al, 2021;Babin et al, 2004;Menkes et al, 2016). Although this effect on the primary productivity appears to be relatively weak in the open ocean, because of a deep nitracline, as suggested in Menkes et al ( 2016), we cannot discard at this stage a possible…”

MESI: A Multiparameter Eddy Significance Index

“…Hurricane Dorian (26 August–9 September 2019) was among the most devastating and powerful Atlantic tropical cyclones in recent memory, with a maximum sustained wind speed of ∼82 mph and minimum central pressure of 910 mb (Ezer, 2020). The system formed in the tropical Atlantic before skirting the Caribbean Islands near Cuba, pausing at the Bahamas, and progressing north along the US East Coast before dissipating off the coast of New England (Avila‐Alonso et al., 2021; Ezer, 2020; Hazelton et al., 2021). While not infamous for its ocean‐eddy interactions such as Rita or Katrina (Jaimes & Shay, 2009), Dorian nonetheless had a significant impact on Gulf Stream volume transport, upper ocean variability along the US East Coast, much of the Sargasso Sea, and other biological productivity in this region (Avila‐Alonso et al., 2021; Ezer, 2020; Hazelton et al., 2021).…”

“…The impact of Dorian along the southern US coast, from Florida up through South Carolina, can be seen in Figure 12, with rainfall rates from Dorian in excess of 50 mm/day. Its strong winds caused massive amounts of Ekman pumping and upwelling that would later lead to large phytoplankton blooms and significantly reduce the flow of the Gulf Stream (Avila‐Alonso et al., 2021; Ezer, 2020). Overlaid with eddy contours, it is clear that only a few strong eddies were present at the peak of the storm, but the data from within these eddies and the total area can still be used to analyze the impact Dorian had on the upper ocean (Figures 12 and 13).…”

The Ocean's labile DOC supply chain

“…Yet, because the models are based on experimentally measured BGE values that simply tally carbon fate, we expect this alternate energy source will not substantially impact model results. Finally, episodic events such as storms and physical mixing can influence primary production, phytoplankton mortality, and microbial community composition (Johnson et al 2010;Garrison and Tang 2014;Avila-Alonso et al 2021;Rii et al 2021;Wang and Zhang 2021). While planktonic communities often return to pre-event status within days or weeks (Rii et al 2021), episodic events may have significant local implications for DOC flux and fate, and these were not captured in the data used here.…”