Mortality rates typically vary smoothly over age and time. Exceptions occur due to events such as wars and epidemics which create ridges in the age-period surface of mortality rates in a particular year or for cohorts born in a particular year. We propose a new practical method for modelling the age-period surface of mortality rates. Our approach uses $L_1$ regularization with bivariate smoothing, and allows for age-varying period and cohort ridges in the otherwise smooth …