Tropical Pacific climate variability under solar geoengineering: impacts on ENSO extremes

Abstract

Abstract. Many modelling studies suggest that the El Niño–Southern Oscillation (ENSO), in interaction with the tropical Pacific background climate, will change with rising atmospheric greenhouse gas concentrations. Solar geoengineering (reducing the solar flux from outer space) has been proposed as a means to counteract anthropogenic climate change. However, the effectiveness of solar geoengineering concerning a variety of aspects of Earth's climate is uncertain. Robust results are particularly challenging to obtain for ENSO because existing geoengineering simulations are too short (typically ∼ 50 years) to detect statistically significant changes in the highly variable tropical Pacific background climate. We here present results from a 1000-year-long solar-geoengineering simulation, G1, carried out with the coupled atmosphere–ocean general circulation model HadCM3L. In agreement with previous studies, reducing the solar irradiance (4 %) to offset global mean surface warming in the model more than compensates the warming in the tropical Pacific that develops in the 4 × CO2 scenario. We see an overcooling of 0.3 ∘C and a 0.23 mm d−1 (5 %) reduction in mean rainfall over the tropical Pacific relative to preindustrial conditions in the G1 simulation, owing to the different latitudinal distributions of the shortwave (solar) and longwave (CO2) forcings. The location of the Intertropical Convergence Zone (ITCZ) in the tropical Pacific, which moved 7.5∘ southwards under 4 × CO2, is restored to its preindustrial position. However, other aspects of the tropical Pacific mean climate are not reset as effectively. Relative to preindustrial conditions, in G1 the time-averaged zonal wind stress, zonal sea surface temperature (SST) gradient, and meridional SST gradient are each statistically significantly reduced by around 10 %, and the Pacific Walker Circulation (PWC) is consistently weakened, resulting in conditions conducive to increased frequency of El Niño events. The overall amplitude of ENSO strengthens by 9 %–10 % in G1, but there is a 65 % reduction in the asymmetry between cold and warm events: cold events intensify more than warm events. Notably, the frequency of extreme El Niño and La Niña events increases by ca. 60 % and 30 %, respectively, while the total number of El Niño events increases by around 10 %. All of these changes are statistically significant at either 95 or 99 % confidence level. Somewhat paradoxically, while the number of total and extreme events increases, the extreme El Niño events become weaker relative to the preindustrial state, while the extreme La Niña events become even stronger. That is, such extreme El Niño events in G1 become less intense than under preindustrial conditions but also more frequent. In contrast, extreme La Niña events become stronger in G1, which is in agreement with the general overcooling of the tropical Pacific in G1 relative to preindustrial conditions.

Robust winter warming over Eurasia under stratospheric sulfate geoengineering – the role of stratospheric dynamics

“…As shown in Simpson et al (2019), there are some enhancements in tropical East Pacific sea surface temperatures and reductions in the tropical West Pacific in GEO8.5, which are only partially explained by stratospheric heating. Changes in ENSO variability have also been reported in a solar dimming experiment (Malik et al, 2020) and following volcanic eruptions (Khodri et al, 2017). The Pacific climate response in GEO8.5 and under sulfate geoengineering in general merits future study.…”

Indices of Extremes: Geographic patterns of change in extremes and associated vegetation impacts under climate intervention

“…Furthermore, many of the projected changes in ETCCDI in GLENS EC are not significantly different from the simulated current climate (BASE), indicating that SAI could offset the worst effects projected by RCP8.5 EC. However, we also note that SAI is preferentially more effective for daytime temperatures than nighttime due to the reduction in incoming solar irradiation, resulting in warmer minimum temperatures and cooler maximum temperatures (Curry et al, 2014;Malik et al, 2020). In addition to the winter warming over Europe, North America and Asia (Banerjee et al, 2021), our results indicate asymmetric increases in warm nights (TNx, TR) compared to cooler days in the summer (TXx, SU).…”

“…Changes in the frequency and strength of El Niño and La Niña events as a result of SAI are difficult to confirm (Gabriel and Robock, 2015). However, GLENS EC projected equatorward shifts in westerlies and storm tracks (Karami et al, 2020), together with increasing precipitation over Australia and weakening of the African and Indian Monsoons (Da Allada et al, 2020;Bhowmick et al, 2021) support a weakened ENSO signal compared to present day (Malik et al, 2020). In keeping with these results, we find that projected changes in the annual total precipitation in GLENS arise from a change in the number of days with precipitation, not only the intensity on those days.…”

“…It is widely acknowledged that the Earth system responses to SAI will vary temporally and spatially (Cheng et al, 2019;Simpson et al, 2019) as well as in response to the injection location (MacMartin et al, 2018;Kravitz et al, 2019). Responses in the hydrological cycle are complicated by intra-to inter-annual changes in the location of the Inter Tropical Convergence Zone (Haywood et al, 2013) and conflicting signals in atmospheric-oceanic teleconnections such as ENSO (Gabriel and Robock, 2015;Malik et al, 2020), in addition to energetic constraints (Allen and Ingram, 2002;Ingram, 2016). Furthermore, the spatial and temporal distribution of extreme precipitation has become more skewed in response to temperature increases, such that the most extreme events may not have as linear a correlative relationship with Clausius-Clapeyron as previously assumed (Guerreiro et al, 2018;Pendergrass and Knutti, 2018;Allan et al, 2020).…”

Impacts of three types of solar geoengineering on the North Atlantic Meridional Overturning Circulation

“…To date, little research on the oceanic response at high northern latitudes under SRM has been published (Malik et al, 2020;Muri et al, 2018;Smyth et al, 2017). Some research has been done on AMOC under sunshade geoengineering (Hong et al, 2017) and under SAI (Muri et al, 2018;Moore et al, 2019;Tilmes et al, 2020).…”