Response of the Equatorial Atlantic Cold Tongue to Stratospheric Aerosol Geoengineering

Abstract

By increasing Earth-atmosphere system albedo, Stratospheric Aerosol Geoengineering (SAG) using sulfur dioxide is an artificial potential means, with the goal to mitigate the global warming effects. In this study, we used the simulations from Geoengineering Large Ensemble project realized under the climate change scenario of Representative Concentration Pathway 8.5 (RCP8.5), to investigate the potential impact of SAG on the Sea Surface Temperature (SST) in Equatorial Atlantic Cold Tongue (EACT) and the physical processes driving these changes. Results reveal that in the EACT region, under RCP8.5, SST warms significantly (compared to present‐day climate) with a maximum of 1.7 °C in July, and this increase in SST is mainly due to the local processes related to the weakening of vertical mixing at the base of the mixed layer. This reduction of the vertical mixing is associated to the diminution of the vertical shear from July to April and to the increase of ocean stratification from May to June. However, under SAG, SST decreases significantly throughout the year (compared to present‐day climate) with a maximum cooling of − 0.4 °C in the cold tongue period (May–June). This SST cooling is mainly associated with the non-local processes related to intensification of the westerly equatorial Atlantic wind stress. Finally, results show that the use of SAG to offset all global warming under RCP8.5 results in a slight over compensation of SST in the EACT region.

Impact of Stratospheric Geoengineering on Sea Surface Temperature in the Northern Gulf of Guinea

“…A cold bias of −0.5 °C relative to observations (OISST and ERAI) is also located between 4°W and 4°E from October to December. These biases obtained at the NGG region are similar to those found in the SST reproduced by the HIST model at the equatorial Atlantic cold tongue [17]. It is probably the ocean dynamics and/or the parameterizations used in the model that are responsible for these SST biases.…”

“…Based on recent studies [16,17,54], "control" simulations over 2010-2029 (20 years) were used for current climate, referred to as baseline (BSL), "control", simulations over 2050-2069 (20 years) for climate change without SAG (RCP8.5) and "feedback" simulations with SAG (GLENS) over 2050-2069 (20 years). Additionally in this study, the first three simulation members of the model were used with the monthly data like in recent studies.…”

“…In addition, statistical significance using a two-sided t-test was calculated for changes, and for evaluation of the error estimate of changes in SST, the standard error was calculated. To explain the possible causes of SST changes in the NGG, the mixed-layer heat budget was used like in previous studies [17,29,32]:…”

“…Thus, in addition to the local causes, non-local processes are also investigated by calculating correlations between the zonal wind stress at the western equatorial Atlantic (WEA) and the depth of the thermocline (represented here by the 20 °C isotherm, D20) in the study area on the one hand, and the correlation of the thermocline depth with the SST on the other hand. The wind at the western Atlantic basin is taken from a region called WEA, between 2°S-2°N and 40°W-20°W, as in a recent study [17]. As explained above, upwelling mechanisms east of Cape Palmas (Côte d'Ivoire) are different from those east of Cape Three Points (Ghana), so in this paper we studied these two regions differently (Figure 1), which are defined as in previous studies [29,63]: Sassandra Upwelling in Côte d'Ivoire (SUC, 4°N-6°N, 4°W-7°W) and Takoradi Upwelling in Ghana (TUG, 4°N-6°N, 2°W-1°E).…”

“…These changes in monsoon precipitation are largely related to atmospheric circulation changes. Apart from rainfall, it is shown that SAG decreases Sea Surface Temperature (SST) in the equatorial Atlantic cold tongue and SST changes are predominantly associated with wind modification in the western equatorial Atlantic [17]. However, no study has examined the possible consequences of SAG in the northern Gulf of Guinea (NGG), which is known to have large variations in SST due to the presence of coastal upwelling.…”