A nonlinear projection of the tropical Pacific sea surface temperature anomalies (SSTA) onto the Northern Hemisphere winter 500-mbar geopotential height (Z500) by a neural network reveals asymmetric atmospheric patterns associated with El Ni¿o and La Ni¿a. While the linear response of Z500 to tropical Pacific SSTA is largely confined over North Pacific and North America, the nonlinear response, mainly a quadratic response to the SSTA, reveals that regardless of the sign of the SSTA, positive Z500 anomalies are found over the central-eastern North America, North Atlantic and western Europe, and negative Z500 anomalies over the west coast of North America, Scandinavia and eastern Europe-consistent with the positive Pacific-North America (PNA) teleconnection pattern and the positive North Atlantic Oscillation (NAO) pattern being excited.