Spin Correlations and Magnetic Field Effects in the Weakly Anisotropic Square-Lattice AntiferromagnetSr2CuO2Cl2

${}^{35}\mathrm{Cl}$ nuclear spin-lattice relaxation rate (NSLR) and magnetic susceptibility $\left(\chi \right)$ measurements are reported for ${\mathrm{Sr}}_2\mathrm{Cu}{\mathrm{O}}_2{\mathrm{Cl}}_2$ single crystals as functions of temperature and of magnetic field intensity and orientation. From the anisotropy of the ${}^{35}\mathrm{Cl}$ NSLR and $\chi$ we obtained evidence for a crossover of the ${\mathrm{Cu}}^{2+}$ spin correlations from Heisenberg to $\mathrm{XY}$-like at $T\approx 290$ K. For $\mathbf{H}\Vert c$ the 3D antiferromagnetic transition is at $T_N=257\mathrm{}$ K and is field independent while for $\mathbf{H}\Vert \mathrm{ab}$ $T_N$ increases to 265 K at $H=8.2\mathrm{}$ T. The field-dependent $T_N$ for $\mathbf{H}\Vert \mathrm{ab}$ is explained by a field-induced Ising-like anisotropy in the $\mathrm{ab}$ plane.