A new ocean state after abrupt cooling events

Like volcanic eruptions, nuclear war would transport light-blocking aerosols to the upper atmosphere, resulting in global cooling. We simulate climate impacts of nuclear war scenarios in CESM-WACCM spanning 5-150 Tg of soot delivery into the upper atmosphere, representing a range of regional and superpower nuclear exchanges. These scenarios result in -11 to -115 W m -2 shortwave radiation and -0.5 to -6.4°C sea surface temperature (SST) anomalies. For comparison, the Samalas eruption (1257 CE) is estimated to have caused a -32.8 ± 9.6 W m -2 radiation anomaly (Sigl et al. 2015), and a -1 to -2°C SST anomaly (Chickamoto et al. 2016), comparable to our 16 Tg India-Pakistan nuclear war case (-31.1 W m -2 , -1.4°C). In this study, we investigate the impact of these global cooling events on ocean physics and biogeochemistry, including meridional overturning, sea ice, and phytoplankton community dynamics. Global surface cooling drives intensified ocean vertical mixing, which is deeper, longer and expanded further into the subtropical gyres than normal winter mixing. This deeper mixing, combined with reduced solar radiation, inflicts light limitation on phytoplankton populations and curtails global ocean productivity, with larger impacts at high latitudes. High latitude surface buoyancy forcing temporarily increases deep water production and strengthens meridional overturning magnitude in both hemispheres, proportional to the magnitude of the cooling event. The result is a new physical and biogeochemical ocean state stabilizing roughly a decade after the war and continuing for decades further. In this new state, the pycnocline, thermocline and nutriclines all have shoaled, deep water masses have been ventilated; extreme cooling has expanded and thickened Arctic sea ice, as in the Little Ice Age after the Samalas eruption. The anomalies in these variables are proportional to the magnitude of the cooling event.