  The Brown-York stress tensor provides a means for defining quasilocal
gravitational charges in subregions bounded by a timelike hypersurface. We
consider the generalization of this stress tensor to null hypersurfaces. Such a
stress tensor can be derived from the on-shell subregion action of general
relativity associated with a Dirichlet variational principle, which fixes an
induced Carroll structure on the null boundary. The formula for the mixed-index
tensor $T{}^i{}_j$ takes a remarkably simple form that is manifestly
independent of the choice of auxiliary null vector at the null surface, and we
compare this expression to previous proposals for null Brown-York stress
tensors. The stress tensor we obtain satisfies a covariant conservation
equation with respect to any connection induced from a rigging vector at the
hypersurface, as a result of the null constraint equations. For transformations
that act covariantly on the boundary structures, the Brown-York charges
coincide with canonical charges constructed from a version of the Wald-Zoupas
procedure. For anomalous transformations, the charges differ by an intrinsic
functional of the boundary geometry, which we explicity verify for a set of
symmetries associated with finite null hypersurfaces. Applications of the null
Brown-York stress tensor to symmetries of asymptotically flat spacetimes and
celestial holography are discussed.
