Cumulative Solar UV Irradiance as a Driver of Southern Ocean Temperature Variability and Antarctic Sea Ice Regime Shift

The Southern Ocean has absorbed 45-62% of global upper-ocean heat gain since 2005 and experienced an unprecedented Antarctic sea ice regime shift beginning in 2016.Standard solar-climate analyses correlate instantaneous total solar irradiance (TSI) with climate variables and find negligible effects.Here we propose an alternative framing: the ocean acts as a cumulative thermal integrator of spectrally variable solar forcing, with ultraviolet irradiance (200-400 nm) as the high-variance component driving departures from equilibrium.Using NRLSSI2 spectral solar irradiance (1874-2024), Kaplan extended SST, Argo float temperatures , and NSIDC sea ice extent , we construct a running cumulative sum of monthly UV departures from the 150-year mean and test its correlation with Southern Ocean (>40°S) sea surface temperature (SST) at multiple lags.During ascending phases of solar cycles (minimum to maximum), cumulative UV correlates with Southern Ocean SST at r [?] 0.45 (p < 0.01 after effective-degrees-of-freedom correction), peaking at 24-48 month lag.This signal survives Benjamini-Hochberg false discovery rate correction across 11 tested lags, and is robust to partial correlation controls for the Pacific Decadal Oscillation, Southern Annular Mode, and Quasi-Biennial Oscillation.A spectral fingerprint analysis reveals sign reversal at the 300 nm ozone absorption edge, consistent with distinct top-down (stratospheric) and bottom-up (direct ocean absorption) pathways.Independent validation using SATIRE-S solar irradiance reproduces the correlation profiles.Using TSI instead of cumulative UV yields fundamentally different lag structures: TSI-SST peaks at lag 0 (instantaneous response) versus 30-48 months for cumulative UV (reservoir effect), with mediation through the Southern Ocean accounting for 35% of the TSI-ENSO signal versus 364% for cumulative UV (indicating a suppressor structure).Critical slowing down indicators in the sea ice record show statistically significant trends (rolling variance Kendall τ = 0.384,p < 0.001; AR(1) τ = 0.512, p < 0.001) preceding the 2016 collapse, consistent with a system approaching a tipping point.The ENSO connection via Southern Ocean mediation is structurally interesting but marginal by superposed epoch analysis (Theiler phase-randomisation p = 0.89).We propose that cumulative solar UV loading is an underweighted contributor to Southern Ocean temperature variability that interacts with, rather than replaces, greenhouse gas forcing.