Recent tropical expansion: Natural variability or forced response?

Grise, K. M., Davis, S. M., Simpson, I. R., Waugh, D. W., Fu, Q., Allen, R. J., … Staten, P. W. (2019). Recent tropical expansion: Natural variability or forced response? Journal Of Climate, 32, 1551-1571. doi:10.1175/JCLI-D-18-0444.1