Response of atmospheric quasi‐stationary waves to La Niña conditions in Northern Hemisphere winter

Large‐scale atmospheric quasi‐stationary waves (QSWs) are strongly linked to synoptic variability and extreme events such as blocking and heatwaves. It is therefore essential to gain a deeper insight into the drivers of QSW variability. Previous research highlighted the El Niño Southern Oscillation as one of the main drivers for global QSW variability with La Niña conditions of particular interest because of their link to anomalous strong QSW amplitudes. This connection between La Niña and QSW activity in the Northern Hemisphere is analysed in this study using a set of aquaplanet experiments with a slab ocean, mimicking La Niña conditions of varying strengths. The experiments demonstrate La Niña conditions are consistent with a weaker zonal mean Hadley cell and a corresponding northward shift in the midlatitude wave guide associated with strong meridional gradients in absolute vorticity. The QSW region generally follows this shift in the wave guide with an overall increase in QSW amplitudes along a slightly weaker mean vorticity gradient. For weak to moderate modifications of the ocean heat flux convergence, there is an approximately linear relationship with the associated QSW response, though for stronger slab heat fluxes the overall response becomes highly nonlinear. It is therefore concluded that the ENSO state may have significant consequences for QSW activity across the Northern Hemisphere. Anomalous QSW amplitudes at 300 hPa for strong La Niña phase. Shading in (a) shows anomalous QSW amplitudes (EXP075‐CTRL). Grey contour lines show QSW climatology, spaced every 0.5 m s−1$$ {}^{-1} $$ starting with 4 m s−1$$ {}^{-1} $$. (b) shows the zonal average of the anomalous QSW amplitudes between 225∘$$ 22{5}^{\circ } $$W and 45∘$$ 4{5}^{\circ } $$W. Errorbars show the standard error of the mean (standard deviation of the annual means, divided by the square root of the numbers of years, reduced by one).