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Climate-sargassum interactions across scales in the tropical Atlantic [1]

['Robert Marsh', 'School Of Ocean', 'Earth Science', 'University Of Southampton Waterfront Campus', 'National Oceanography Centre', 'European Way', 'Southampton', 'United Kingdom', 'Nikolaos Skliris', 'Emma L. Tompkins']

Date: 2023-07

Changes across the tropical Atlantic associated with large-scale modes of natural variability in atmosphere-ocean processes appear to drive changes in sargassum quantity and drift. Drifting sargassum will also gain or lose biomass in response to local-scale environmental variation encountered along their transport pathways. We review in turn the drivers that act at basin scale and at local scales.

Basin scale

Following the first proliferation of sargassum in the Equatorial Atlantic in 2011, each year has brought variable amounts, distribution, and timing of sargassum blooms, forming the GASB that is associated with surface convergence under prevailing winds [2]. Several studies have explored the physical drivers of basin-scale sargassum proliferation and drift [29,30,34,43–47]. In Fig 3, we summarise a range of drivers that have been considered to explain the GASB.

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TIFF original image Download: Fig 3. Schematic illustrations of various drivers of basin-scale environmental change that may explain recent influxes of sargassum in the Eastern Caribbean and off West Africa. (A) dynamical and riverine drivers, emphasising how sargassum recirculating in the North Equatorial Recirculation Region (NERR) since 2011 forms a belt across the northern tropical Atlantic that follows seasonal migration of the ITCZ [29], potentially subject to waters enriched by nutrient-laden riverine runoff; (B) additional influences from modes of tropical variability, specifically the strong influence of sea surface temperature (SST) anomalies, strengthened trades and associated enhanced upwelling of nutrients off West Africa and along the Equator (green dots), associated with the Atlantic Meridional Mode (in negative phase) and the Atlantic Niño, following [30]. Locations of sargassum beaching events are indicated with ‘x’ symbols. https://doi.org/10.1371/journal.pclm.0000253.g003

With an initial focus on passive drift, high-fidelity ocean model currents and winds were used to obtain trajectories for ensembles of virtual particles released across the tropical Atlantic, in the zone 0–10°N [43]; westward drift through the Caribbean is consistent with satellite evidence for the source of pelagic sargassum being the central tropical and equatorial Atlantic, often referred to as the North Equatorial Recirculation Region (NERR); interannual variability in sargassum drift, and shoreline beaching, may further be related in particular to dynamical changes in the complex North Brazil Current system. Further trajectory experiments confirmed a strong influence on sargassum drift of skin drag and form drag associated with prevailing winds, collectively known as ‘windage’ and typically applied to drifting particles as 1% of the local wind [48]. The variable winds and currents emphasized in Fig 3A were thus a focus of these early studies.

The GASB was further explained in terms of dynamical processes that shift sargassum–more in some years than others–towards the nutrient sources associated with equatorial upwelling and the Amazon outflow. Central to this explanation is a hypothesis for the emergence of sargassum in 2011, and subsequent persistence, that can be specifically attributed to physical mechanisms [29], as highlighted in Fig 3A; it is suggested that the trigger was an exceptionally negative phase of the North Atlantic Oscillation (weak Icelandic low and Azores high pressure centres) in the winter of 2010/11, which drove sargassum from the western subtropics (the Sargasso Sea) to the northeast, as far as Gibraltar. Conveyed southward in the Canary Current as far as the eastern central tropical Atlantic, subsequent spring growth led to the first Caribbean inundation of summer 2011. It is further hypothesized that this sargassum has subsequently recirculated in the NERR, forming the GASB across the northern tropical Atlantic that follows seasonal migration of the Intertropical Convergence Zone (ITCZ), subject to strong zonal flows that seasonally reverse direction.

Acting as a seasonal source, sargassum from the NERR drifts westward in winter/spring to subsequently beach along the windward shores of the Lesser Antilles and throughout the Caribbean. With development of the North Equatorial Counter Current (NECC) in the approximate zone 5-10°N during the second half of the year, the NERR also supplies the sargassum that beaches along West Africa, evident in drifter data [45]. Sargassum may thus be conveyed eastward, joining the Guinea Current to reach West African coastlines from Senegal to Nigeria. In summary, these basin-scale, tropical-extratropical dynamical perspectives have emphasized physical connectivity in understanding sargassum distributions and change.

Complicating hypotheses for the triggers and drivers that have established the GASB is the fact that sargassum distributions, drift and beaching events have been subject to considerable interannual variability since 2011 [30]. An identified southward shift of the ITCZ in years of most excessive sargassum, notably 2015 and 2018 (subsequently also 2021 and 2022), is consistent with the scenario outlined in Fig 3A and developed in Fig 3B. Furthermore, this shift is associated with two leading modes of natural variability in the tropical Atlantic, notably a negative phase of the Atlantic Meridional Mode (AMM) in all four years, and additionally the Atlantic Niño in 2018. The AMM is associated with anomalies in the cross-equatorial meridional gradient of SST in the Atlantic, which tend to peak in boreal spring [49]. A negative phase of the AMM is characterized by negative SST anomalies centred around 10°N and anomalously strong trade winds centred around 5°N, associated with the shifted ITCZ [50]. The Atlantic Niño is also associated with a southward shift of the ITCZ during boreal summer, but with a reduction of (westward) equatorial winds over the east equatorial upwelling zone. This results in strong surface warming along the Equator, with peak warming at around 10°W [50,51].

In addition to physical drivers, evidence is emerging for biogeochemical drivers affecting sargassum proliferation [e.g., 52]. Various physical mechanisms may have raised levels of key macronutrients, of consequence for sargassum growth. A sequence of unusually large Amazon floods since 2011 have been highlighted as a possible source of elevated nutrient levels, along with enhanced nutrient sources to the eastern equatorial Atlantic in some years that may be associated with coastal upwelling off west Africa and Congo floods [53]. Increased nutrient supply has also been attributed to more active hurricane seasons in recent years [53,54]. In this case, an active June-November hurricane season is thought to broadly raise surface nutrient levels through stirring up deep nutrient-rich waters to the surface. Increasing nitrogen availability in the Atlantic Basin is also emerging since the 1980s, in seawater and in sargassum itself [55].

Of consequence for sargassum, a negative phase of the AMM is associated with both increasing trade winds in the equatorial and tropical North Atlantic–enhancing West African and equatorial upwelling–and a southward shift of the ITCZ; in contrast, the Atlantic Niño is only associated with the southward shift of the ITCZ–bringing sargassum closer to equatorial nutrient sources [30], as outlined in Fig 3B. The AMM and Atlantic Niño thus drive extensive interannual variability in both transport and nourishment of sargassum across the tropical Atlantic, compounding a simple explanation of the GASB in relation to anthropogenic climate change.

A recent refinement to our understanding of the drivers that explain variable sargassum strandings is provided through backtracking samples of the three morphotypes collected from east Barbados through 2021–22 [56]. This analysis suggests two district pathways and origins, with S. fluitans III-dominated mats arriving from March to early August via a southerly convoluted pathway from an equatorial source, including the Gulf of Guinea and passage close to the coast of South America–a nutrient-rich pathway. In contrast, significantly higher amounts of S. natans VIII arrived from late-August to March via a northerly and more zonal pathway–a nutrient-poor alternative. The distinct temperature and nutrient conditions along these two broadly defined pathways may explain some of the variability in beached sargassum, including amounts and morphotype composition.

In summary, proliferation of sargassum across the tropical Atlantic was most likely physically triggered, under anomalous Atlantic-wide atmospheric and oceanic conditions around 2011; high sargassum biomass has since been sustained under favourable, although variable, SST patterns and nutrient levels, with sargassum rafts subject to extensive dispersal under prevailing ocean currents and winds.

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[1] Url: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000253

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