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Nat. Hazards Earth Syst. Sci., 21, 837–859, 2021 https://doi.org/10.5194/nhess-21-837-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea Dailé Avila-Alonso1,2, Jan M. Baetens2, Rolando Cardenas1, and Bernard De Baets2 1Laboratory of Planetary Science, Department of Physics, Universidad Central “Marta Abreu” de Las Villas, 54830, Santa Clara, Villa Clara, Cuba 2KERMIT, Department of Data Analysis and Mathematical Modelling, Faculty of Bioscience Engineering, Ghent University, 9000 Ghent, Belgium Correspondence: Dailé Avila-Alonso ([email protected]) Received: 7 September 2020 – Discussion started: 27 October 2020 Revised: 18 December 2020 – Accepted: 13 January 2021 – Published: 2 March 2021 Abstract. Understanding the oceanic response to tropical cy- 1 Introduction clones (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 re- Hurricanes and typhoons (or more generally, tropical cy- sponse to the passage of consecutive TCs are rare. In this clones (TCs)) are among the most destructive natural phe- work, we assess the upper-oceanic response to the passage of nomena on Earth, leading to great social and economic losses Hurricanes Dorian and Humberto over the western Sargasso (Welker and Faust, 2013; Lenzen et al., 2019), as well as eco- Sea in 2019 using satellite remote sensing and modelled data. logical perturbations of both marine and terrestrial ecosys- We found that the combined effects of these slow-moving tems (Fiedler et al., 2013; de Beurs et al., 2019; Lin et al., TCs led to an increased oceanic response during the third 2020). Given the devastating effects of TCs, the question of and fourth post-storm weeks of Dorian (accounting for both how they will be affected by climate change has received Dorian and Humberto effects) because of the induced mix- considerable scientific attention (Henderson-Sellers et al., ing and upwelling at this time. Overall, anomalies of sea sur- 1998; Knutson et al., 2010; Walsh et al., 2016). Modelling face temperature, ocean heat content, and mean temperature studies project either a decrease in TC frequency accompa- from the sea surface to a depth of 100 m were 50 %, 63 %, nied by an increased frequency of the strongest storms or and 57 % smaller (more negative) in the third–fourth post- an increase in the number of TCs in general, depending on storm weeks than in the first–second post-storm weeks of the spatial resolution of the models (Knutson et al., 2010; Dorian (accounting only for Dorian effects), respectively. For Camargo and Wing, 2016; Walsh et al., 2016; Zhang et al., the biological response, we found that surface chlorophyll a 2017; Bacmeister et al., 2018; Bhatia et al., 2018). In any (chl a) concentration anomalies, the mean chl a concentra- case, an assessment of the impact of climate change on fu- tion in the euphotic zone, and the chl a concentration in the ture TC activity needs to be based on whether or not the past deep chlorophyll maximum were 16 %, 4 %, and 16 % higher and present changes in climate have had a detectable effect in the third–fourth post-storm weeks than in the first–second on TCs to date (Walsh et al., 2016). post-storm weeks, respectively. The sea surface cooling and The ocean is the main source of energy for TC intensifica- increased biological response induced by these TCs were sig- tion; hence, changes in oceanic environments considerably nificantly higher (Mann–Whitney test, p < 0:05) compared affect TC activity (Knutson et al., 2010; Huang et al., 2015; to climatological records. Our climatological analysis reveals Sun et al., 2017; Trepanier, 2020). Sea surface temperature that the strongest TC-induced oceanographic variability in (SST) and ocean heat content (OHC) have risen significantly the western Sargasso Sea can be associated with the occur- over the past several decades in regions of TC formation rence of consecutive TCs and long-lasting TC forcing. (Santer et al., 2006; Defforge and Merlis, 2017; Trenberth et al., 2018; Cheng et al., 2019; Zanna et al., 2019; Chih and Published by Copernicus Publications on behalf of the European Geosciences Union. 838 D. Avila-Alonso et al.: Oceanic response to the Hurricanes Dorian and Humberto (2019) Wu, 2020). Accordingly, TC lifetime maximum intensity sig- storms on upper-ocean oceanographic conditions. Although nificantly increased during 1981–2016 for both the Northern there have been extensive studies investigating the oceanic Hemisphere and Southern Hemisphere (Song et al., 2018). response described above to the passage of individual TCs, More specifically, both the frequency and intensity of TCs in the effects induced by consecutive TCs have been much less the North Atlantic basin increased over the past few decades documented (Wu and Li, 2018; Ning et al., 2019). More (Deo et al., 2011; Walsh et al., 2016), while a significant in- specifically, extensive and long-lasting SST cooling and increase in TC intensification rates in the period 1982–2009 tense post-storm phytoplankton blooms after the passage of has been documented with a positive contribution from an- consecutive TCs have been documented in the northwestern thropogenic forcing (Bhatia et al., 2019). Pacific Ocean (e.g. Wu and Li, 2018; Ning et al., 2019; Wang The most recent Atlantic hurricane seasons have shown et al., 2020). However, to the best of our knowledge, there are well above normal activity (Trenberth et al., 2018; Bang no previous studies assessing the biological response to con- et al., 2019). For instance, the 2017 hurricane season was secutive TCs in the western Sargasso Sea. extremely active with 17 named storms (1981–2010 median Given that the greatest ocean warming is projected to oc- is 12.0), 10 hurricanes (median is 6.5), and six major hurri- cur by the end of the century (Cheng et al., 2019), we may canes (Klotzbach et al., 2018, median is 2.0). It will be re- anticipate a further increase in TC intensity and/or frequency. membered for the unprecedented devastation caused by the The assessment of the oceanic response to TCs has been a hot major Hurricanes Harvey, Irma, and Maria, breaking many topic given its importance for studies on climate change, eco- historical records (Todd et al., 2018; Trenberth et al., 2018; logical variability, and environmental protection (Fu et al., Bang et al., 2019). This high activity was associated with un- 2014). More specifically, insights into the phytoplankton re- usually high SST in the eastern Atlantic region, where many sponse to severe weather events are essential in order to as- storms developed, together with record-breaking OHC val- certain the capacity of the oceans to absorb carbon diox- ues favouring TC intensification (Lim et al., 2018). In ad- ide through photosynthesis (Davis and Yan, 2004). Hence, dition, the 2019 hurricane season was relevant because of several studies have assessed the oceanic response to re- the high number of named storms (18 storms) and the devel- cent major hurricanes in the North Atlantic basin (e.g. Tren- opment of long-lasting Hurricane Dorian, which broke the berth et al., 2018; Miller et al., 2019b; Avila-Alonso et al., record for the strongest Atlantic hurricane outside the trop- 2020; Hernández et al., 2020) and others in very active hur- ics (Klotzbach et al., 2019; Ezer, 2020, > 23:5◦ N). About ricane seasons such as in 2005 (e.g. Oey et al., 2006, 2007; 2 weeks after the passage of Dorian across the western Sar- Shi and Wang, 2007; Gierach and Subrahmanyam, 2008). In gasso Sea, TC Humberto moved across this area. The interac- this work, we assess the upper-oceanic responses induced by tion of two storms closely related in time and space provides Hurricanes Dorian and Humberto in the western Sargasso an in situ experiment for studying oceanic response (Bara- Sea in 2019. This gives insights into the implications of a nowski et al., 2014). simultaneous increase in both the frequency and intensity of Over oceans, TC-induced wind forcing mixes the surface TCs in the North Atlantic basin. layer, deepens the mixed layer, and uplifts the thermocline, leading to a decreased upper-ocean temperature and heat potential (Price, 1981; Shay and Elsberry, 1987; Trenberth 2 Materials and methods et al., 2018). Vertical mixing and upwelling also lead to an 2.1 Study area increased abundance of surface phytoplankton due to entrainment of nutrient-rich waters from the nitracline to the ocean The Sargasso Sea is the part of the North Atlantic Ocean surface and/or entrainment of phytoplankton from the deep (known as the North Atlantic gyre) that is bounded by the chlorophyll maximum (DCM) (Babin et al., 2004; Walker surrounding clockwise-rotating system of major currents, i.e. et al., 2005; Gierach and Subrahmanyam, 2008; Shropshire the North Atlantic Current in the north, the Gulf Stream in the et al., 2016). The nutrient influx stimulates phytoplankton west, the North Atlantic Equatorial Current in the south, and growth and can lead to phytoplankton blooms lasting sev- the Canary Current in the east (Deacon, 1942; Laffoley et al., eral days after the TC passage in the oligotrophic oceanic 2011). Hence, the Sargasso Sea essentially lies between the waters (Babin et al., 2004; Hanshaw et al., 2008; Shropshire parallels 20–35◦ N and the meridians 30–70◦ W(Augustyn et al., 2016). Moreover, rainfall associated with these ex- et al., 2013). We considered the western Sargasso Sea as our treme meteorological phenomena modulates surface cooling study area (shown in Fig.1), since it was affected by Dorian and phytoplankton blooms since rainfall freshens the near- and Humberto.

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