Residual delay maps unveil global patterns of atmospheric nonlinearity andproduce improved local forecasts

We use residual-delay maps of observational field date for barometric press ure to demonstrate the structure of latitudinal gradients in nonlinearity i n the atmosphere. Nonlinearity is weak and largely lacking in tropical and subtropical sites and increases rapidly into the temperate regions where th e time series also appear lo be much noisier. The degree of nonlinearity cl osely follows the meridional variation of midlatitude storm track frequency . We extract the specific functional form of this nonlinearity, a V shape i n the lagged residuals that appears to be a basic feature of midlatitude sy noptic weather systems associated with frontal passages. We present evidenc e that this form arises from the relative time scales of high-pressure vers us low-pressure events. Finally, we show that this nonlinear feature is wea ker in a well regarded numerical forecast model (European Centre for Medium -Range Forecasts) because small-scale temporal and spatial variation is smo othed out in the grided inputs. This is significant, in that it allows us t o demonstrate how application of statistical corrections based on the resid ual-delay map may provide marked increases in local forecast accuracy, espe cially for severe weather systems.