Tier II: Volcanic response — Pinatubo (1991–1993)

## Description
Evaluate whether models reproduce a realistic volcanic signature during the post-Pinatubo period.

## Specification
- Assess from historical simulation (no separate volcanic experiment)
- Ensemble-mean anomalies of surface SW↓ and near-surface temperature for 1991–1993
- Require correct **co-variation** of radiative and thermal anomalies (not in isolation — guards against overfitting)
- Compare against BSRN-derived and observational estimates

## Required variables
`rsds`, `tas`

## Required simulations
historical

## References
Protocol §Tier II — Volcanic response