Observing the spread of Agulhas Leakage into the Western South Atlantic by tracking mode waters within ocean rings

Abstract

The Agulhas rings transport warm and salty waters that feed the surface limb of the Atlantic Meridional Overturning Circulation. Some studies have focused on the conveying capacity of ocean eddies, and recently, the role of the Agulhas rings in advecting water masses and organisms has been explored. Here we show evidence that the Agulhas rings are responsible for the advection of mode waters from the Cape Basin to the western side of the Atlantic. We analyzed more than 3,200 temperature profiles and 2,400 salinity profiles from historical databases collocated with 52 long-lived Agulhas rings tracked from 1993 through 2016. An automated algorithm was used to identify thermostads in the profiles acquired within the rings. The data revealed mode water layers trapped inside 88% of the rings. The joint distribution of temperature and salinity indicated two types of mode waters in the range 16.2 ± 0.6°C, 35.6 ± 0.1 (Type I) and 12.9 ± 0.7°C, 35.2 ± 0.1 (Type II). The majority (67%) of the rings carrying mode waters had both types detected inside. Moreover, considering only those rings sampled west of the Mid-Atlantic Ridge, we found that 45% of them advected mode waters to the western basin. Therefore, our results demonstrate that, despite the long journey, interaction with the bottom topography and other vortices, ocean-atmosphere exchanges, and decay, the Agulhas rings are responsible for spreading mode waters initially available at the Cape Basin throughout the South Atlantic, contributing to a positive anomaly in temperature and salinity along the eddy corridor joining the Cape Basin to the Brazil Basin.

Possible transport pathway of diazotrophic Trichodesmium by Agulhas Leakage from the Indian into the Atlantic Ocean

“…Agulhas Leakage was detected by sea surface anomalies in the sBUS (Fig. 2 ) and tracks of Agulhas eddies and leakages into the investigated area and further into the western South Atlantic are published by Raith et al 48 , Guerra et al 49 and Wei and Wang 50 . Water with an Agulhas signature was even found at the western side of the Atlantic 49 .…”

“…2 ) and tracks of Agulhas eddies and leakages into the investigated area and further into the western South Atlantic are published by Raith et al 48 , Guerra et al 49 and Wei and Wang 50 . Water with an Agulhas signature was even found at the western side of the Atlantic 49 . We assume that these eddies taking the Trichodesmium colonies northwards to the offshore regions of the Benguela Current.…”

Subsurface warming in the tropical Atlantic for the last 3 deglaciations: Insights from organic molecular proxies

“…Therefore, these results indicate that the signature of Agulhas Leakage waters was observed during past glacial terminations along their pathway from the Indian Ocean to the western boundary of the South Atlantic through the Cape Basin. In addition, Agulhas Leakage waters were reported to reach the western South Atlantic on modern times (Castellanos et al, 2017;Guerra et al, 2022;Lübbecke et al, 2015) and over past deglaciations (Ballalai et al, 2019;Santos et al, 2014). Altogether, these observations suggest that the upper ocean warming in the western tropical Atlantic during the last three glacial terminations is due, at least partly, to the spreading of Agulhas Leakage waters in the South Atlantic.…”

“…The increase outflow of Agulhas waters during glacial terminations has also been suggested to explain the increase in upper ocean temperature and salinity reconstructed in the western South Atlantic during TII (Ballalai et al, 2019) and the eastern South Atlantic during TI and TII (Kasper et al, 2014;Scussolini et al, 2015). Agulhas warm and salty mode waters are spreading into the South Atlantic mainly within the thermocline (Guerra et al, 2022), and constitute the main source of heat and salt to the upper limb layers of this basin (Castellanos et al, 2017). The abovementioned records reported a stronger temperature and salinity increase at upper (Kasper et al, 2014) and permanent thermocline (Ballalai et al, 2019;Scussolini et al, 2015) levels compared to the surface.…”

“…Existing records in the eastern (GeoB9528-3, Lopes dos and western (SO164-03-4, M78/1-235-1, VM12-107 Reißig et al, 2019;Schmidt et al, 2012) tropical North Atlantic exhibiting a pronounced surface and subsurface warming over glacial terminations indicates the propagation of South Atlantic waters, although interhemispheric upper ocean transport is reduced (Laurian and Drijfhout, 2011) (Figure 8). These South Atlantic waters spreaded meridionally and zonally within the tropical Atlantic, through mode waters at thermocline and intermediate levels (Guerra et al, 2022;Laurian and Drijfhout, 2011), explaining the large accumulation of warm waters up to 20°N (Schmidt et al, 2012;Weldeab et al, 2006). Due to air-sea interactions, the atmospheric and oceanic reorganizations greatly impact the heat transport in the tropical thermocline (Laurian and Drijfhout, 2011;Xie et al, 1999).…”