The El Niño Southern Oscillation and Solar Geoengineering

Abstract:

The El Niño Southern Oscillation (ENSO) is a primary source of interannual natural variability in the Earth’s climate system. Here, we address whether the overall character of ENSO is likely to evolve differently over time under a regime of solar geoengineering than it would under unimpeded global warming. Analysis of output from the GeoMIP G1 (instantaneous quadrupling of carbon dioxide with a concurrent fully offsetting solar constant reduction) and G4 (continuous 5 Tg SO₂ per year stratospheric injections for 50 years coupled with RCP4.5) experiments reveals no clear signal that ENSO amplitude, frequency, spectrum, or seasonality will be different under the respective climate regimes. Solar heating is an important control of Western Pacific sea surface temperatures. In the eastern Pacific, horizontal advection of water makes a large contribution to the surface mixed layer heating budget. Application of very strong negative radiative forcing would generate an initial La Niña response due to this dynamical delay in the eastern Pacific. However, we find that the initial negative forcing applied in G1 or G4 is not strong enough to detect this fast response. The same dynamical delay implies a slow El Niño-like response after 10-20 years. This response is also undetectable in G1 and G4. While state of the art global climate models have demonstrated the ability to produce plausible ENSO cycles, analysis of process-based variables, which quantify the physical mechanisms underlying ENSO, reveals that different models generate similar ENSO variability results for different physical reasons. If the physical processes underlying ENSO become more well-resolved in models, subsequent studies should be performed to take another look at the ENSO response to geoengineering.