Contribution of Increased Agulhas Leakage to Tropical Atlantic Warming

Abstract

The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here, model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948-2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation, it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300 m of the tropical Atlantic Ocean within two decades and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in part originate from the Agulhas, leading to higher SSTs in the tropics.

Indian Ocean sources of Agulhas leakage

“…Agulhas leakage is commonly defined as the portion of the Agulhas Current, for example, at 32°S that leaks into the Atlantic across the GoodHope section within 3–5 years, and is intuitive measured in models using a Lagrangian framework [ Durgadoo et al ., ; Biastoch et al ., ; Lübbecke et al ., ]. In the present model, employing this definition and over the period 1963–2008, the mean value of Agulhas leakage is 14.1 Sv with an annual standard deviation of 2.2 Sv.…”

“…In order to further address this question, sensitivity model experiments could be designed similar to Lübbecke et al . [] who assessed a warming signal transported across the South Atlantic from an increase in transport of Agulhas leakage.…”

“…[] and Lübbecke et al . [] for Agulhas water spreading in the Atlantic), and along‐track changes in tracer properties.…”

The Impact of Agulhas Leakage on the Central Water Masses in the Benguela Upwelling System From A High‐Resolution Ocean Simulation

“…Previous studies based on model simulations analyze the impact of the Agulhas leakage on the western boundary systems of the tropical Atlantic Ocean (Castellanos et al, ) and on the North Atlantic (Rühs et al, ), its correlation with the Atlantic multidecadal Oscillation (Biastoch et al, ), its contribution to tropical Atlantic warming (Lübbecke et al, ), the source water of the leakage (Durgadoo et al, ), and the contribution of Agulhas water to the Benguela Current (Veitch & Penven, ). We focus on the amount of Benguela central water masses originating from the Agulhas Current and the Brazil‐Malvinas Confluence Zone, the pathways of the water masses into the upwelling region, and their age.…”

Submesoscale Impacts on Mesoscale Agulhas Dynamics

“…The turbulent upper ocean inflow of Indian Ocean waters into the Atlantic, the “Agulhas leakage,” strongly controls the South Atlantic heat and salt budgets and thus its stratification (De Ruijter et al, ; Weijer et al, ). A part of this water reaches the South Atlantic western boundary and is transported into the equatorial current system where it is an important driver of decadal sea surface temperature (SST) variability (Lübbecke et al, ; Rühs et al, ). In respect to the far‐field influence, the Atlantic multidecadal oscillation is found to correlate with the strength of the Agulhas leakage with a lag of 15 years (Biastoch et al, ).…”