The Indonesian Throughflow circulation under solar geoengineering

The Indonesia Throughflow (ITF) is the only low-latitude channel between the Pacific and Indian oceans, and its variability has important effects on global climate and biogeochemical cycles. Climate models consistently predict a decline in ITF transport under global warming, but it has not yet been examined under solar geoengineering scenarios. We use standard parameterized methods for estimating the ITF - the Amended Island Rule and buoyancy forcing - to investigate the ITF under the SSP2-4.5 and SSP5-8.5 greenhouse gas scenarios and the geoengineering experiments G6solar and G6sulfur, which reduce net global mean radiative forcing from SSP5-8.5 levels to SSP2-4.5 levels using solar dimming and sulfate aerosol injection strategies, respectively. Six-model ensemble-mean projections for 2080-2100 show reductions of 19ĝ€¯% under the G6solar scenario and 28ĝ€¯% under the G6sulfur scenario relative to the historical (1980-2014) ITF, which should be compared with reductions of 23ĝ€¯% and 27ĝ€¯% under SSP2-4.5 and SSP5-8.5. Despite standard deviations amounting to 5ĝ€¯%-8ĝ€¯% for each scenario, all scenarios are significantly different from each other (p<0.05) when the whole 2020-2100 simulation period is considered. Thus, significant weakening of the ITF occurs under all scenarios, but G6solar more closely approximates SSP2-4.5 than G6sulfur does. In contrast with the other three scenarios, which show only reductions in forcing due to ocean upwelling, the G6sulfur experiment shows a large reduction in ocean surface wind stress forcing accounting for 47ĝ€¯% (38ĝ€¯%-65ĝ€¯% across the model range) of the decline in windĝ€¯+ĝ€¯upwelling-driven ITF transport. There are also reductions in deep-sea upwelling in extratropical western boundary currents.