Influence of enhanced variability with zonal wavenumber 1 on Arctic Oscillation in late winter to early spring in El Niño conditions

Abstract

We investigated the relationship between the atmospheric variability in El Niño conditions and Arctic Oscillation (AO) during the period from late autumn to early spring, focusing on the vertical linkage between the troposphere and the stratosphere, based on a composite analysis. The results of the composite analysis indicate that the vertical linkage is the clearest in late winter to early spring, particularly in the three-month of February - April (FMA). In FMA, the upper tropospheric patterns and upward propagation of planetary waves with zonal wavenumber 1 are enhanced and contribute to a negative phase of the stratospheric Northern Annular Mode (NAM) in El Niño conditions. The results also indicate that the stratospheric negative potential vorticity (PV) anomalies associated with the negative phase of NAM induce a lowered tropopause, vertical compression of the tropospheric column, positive surface pressure anomalies in the polar region and hence the negative phase of the AO. This vertical linkage and the impact of sea level pressure near the pole are consistent with a quantitative estimation based on geostrophic and hydrostatic adjustment associated with the stratospheric PV anomalies.