3D characterisation of the thermohaline structure in the southwestern tropical Atlantic derived from functional data analysis of in situ profiles

Abstract

The dynamic of the thermohaline structure of the upper ocean, which depends on ocean-atmosphere interactions, drives most near surface oceanic processes, including the control of gases and heat fluxes, and nutrient availability in the photic layer. The thermohaline structure of the southwestern tropical Atlantic (SWTA), a key region for diagnosing variation of the Atlantic Meridional Overturning Circulation, has prime impact on global climate. Characterising the thermohaline structure is typically based on the application of classical statistical methods on vertical profiles. Such approach has important limitations since classical methods do not explicitly contemplate the vertical nature of the profiles. Functional Data Analysis (FDA) is a new alternative to solve such drawbacks. Here, we apply an FDA approach to characterise the 3D canonical thermohaline structure of the SWTA in austral spring and fall. Our results reveal a clear spatial pattern with the presence of three areas with significantly different thermohaline structure. Area 1, mostly located along the continental slope, reflects the western boundary current system, with low static stability and high frequency of occurrence of barrier layer (BL). Conversely, Area 2, located along the Fernando de Noronha chain, presents strong static stability with a well-marked thermocline. This area, under the influence of the eastern Atlantic, is characterised by a low BL frequency, which is seasonally modulated by the latitudinal oscillation of the Intertropical Convergence Zone, controlling the regime of precipitation. In turn, Area 3 behaves as a transition zone between A1 and A2 with the presence of the water core of maximum salinity in subsurface, and therefore presence of strong-moderate BL. Beyond this study, FDA approach emerges as a powerful way to describe, characterise, classify and compare ocean patterns and Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.processes. It can be applied to in situ data but could also be used to deeply and comprehensively explore ocean model output. Highlights► The thermohaline structure drives most near surface oceanic processes. ► A functional data analysis approach is used to characterise in 3D the thermohaline structure of the southwestern tropical Atlantic in spring and fall. ► We reveal a clear spatial pattern with the presence of three areas with significantly different thermohaline structure. ► The picture provided can serve as a reference for diagnosing future variation in the Atlantic Meridional Overturning Circulation.

Sightings trends and behaviour of manta rays in Fernando de Noronha Archipelago, Brazil

“…Predominant winds are southeasterly with an average velocity of 6.6 m/s, with increased wind speeds during July and August (Tchamabi et al 2017;Cristiano et al 2020). The mean sea temperature at FNA is 26°C (Ávila et al 2018;Figueiredo et al 2020) and mean salinity is around 36‰ (Assunção et al 2020;Santana et al 2020). The region is influenced by the central branch of the South Equatorial Current, which flows westward, and the Equatorial Undercurrent, flowing in the opposite direction (Stramma 1991).…”

“…The region is influenced by the central branch of the South Equatorial Current, which flows westward, and the Equatorial Undercurrent, flowing in the opposite direction (Stramma 1991). The currents interact with the local topography and may induce upwellings events, with the potential to enrich the surface layers of the normally oligotrophic oceanic environment (Wingfield et al 2011;Tchamabi et al 2017;Assunção et al 2020;Leitner et al 2020). Such events likely increase primary productivity around FNA and provide a suitable oceanic habitat for filter-feeding species, like manta rays.…”

Surface Circulation and Vertical Structure of Upper Ocean Variability Around Fernando de Noronha Archipelago and Rocas Atoll During Spring 2015 and Fall 2017

“…The tropical Atlantic presents a relatively strong static stability with a well-marked thermocline, which is seasonally modulated by the meridional displacement of the Intertropical Convergence Zone (ITCZ), controlling the regime of precipitation and trade winds (Araujo et al, 2011;Nogueira Neto et al, 2018;Assunção et al, 2020). In such regions, vertical mixing, and upwelling are usually restricted to local mechanisms such as divergence of currents, winds and interactions between ocean currents and topography.…”

“…To provide a detailed picture of the currentisland interactions, here we use current, hydrographic, and satellite data collected during the Acoustic along the BRAzilian COaSt (ABRACOS) cruises in Austral spring 2015 and fall 2017 (Bertrand et al, 2015;. These two periods were found to be representative of canonical spring and fall conditions in the area (Assunção et al, 2020;Dossa et al, 2021). More specifically, we describe the upper-ocean circulation around FN and RA and highlight some mesoscale features observed on currents, thermohaline structure and primary productivity.…”

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“…In the southwestern tropical Atlantic, negative ocean-atmosphere heat fluxes and stronger winds are observed in spring, leading to a lower SST and deeper mixed-layer. In contrast, in fall, positive buoyancy due to positive heat fluxes and the relaxation of southeast trade winds lead to a higher SST and shallower mixedlayer (Araujo et al, 2011;Servain et al, 2014;Nogueira Neto et al, 2018;Assunção et al, 2020). Similarly, in situ ABRACOS measurements showed higher SST values in fall 2017 (28.8 • C) than in spring 2015 (26.7 • C).…”