Nonlinear association between Northern Hemisphere winter geopotential height and the stratospheric Quasi-Biennial Oscillation

The equatorial stratospheric quasi-biennial oscillation (QBO) signal is nonlinearly projected onto the 500 hPa geopotential height (Z500) in the winter extra-tropical Northern Hemisphere by a neural network (NN) model to reveal the association between QBO and extra-tropical tropospheric height variations. While the linear response of Z500 to QBO is weak, the NN method demonstrates a stronger, predominantly cubic, nonlinear response of Z500 to QBO in both the westerly phase and the easterly phase of QBO, which is further affirmed by fitting linear polynomial regression models with cubic terms. Nevertheless, the NN result is superior as it captures more detailed structures than the linear regression by polynomials. Z500 responds asymmetrically in the two phases of QBO, it is shown that Z500 resembles an Arctic Oscillation (AO) mode circulation and a positive Pacific-North America (PNA) pattern in the westerly phase of QBO. The predominance of cubic nonlinearity is unusual as previous studies on the nonlinear projections of the El Nino-Southern Oscillation, the AO and the Madden-Julian Oscillation onto the extra-tropical Northern Hemisphere winter climate anomalies have all revealed predominantly quadratic nonlinearity.