Impact of stratospheric conditions on the tropospheric state

Contact: Hauke Schmidt, Elisa Manzini, Felix Bunzel, Andreas Miller

The most obvious correlation between the stratosphere and the troposphere is depicted by the Northern Annular Mode (NAM) index, which reflects the dominant form of dynamic variability in the Northern hemisphere. This index is commonly defined as the leading empirical orthogonal function (EOF) of geopotential height. Near the surface the NAM pattern is also called the Arctic Oscillation (AO), which is highly correlated to a key climate index for weather and climate in Europe and Eurasia, the index of the North Atlantic Oscillation (NAO).
The positive phase of the NAO is associated with a relatively strong and cold polar vortex in the stratosphere, while the propagation of stationary waves originating from the troposphere is reduced. Westerlies over the North Atlantic are enhanced and the cold polar air is locked in the Arctic keeping the surface temperature anomalies in Europe and Eurasia positive. During the negative phase of the NAO, on the other hand, the stratospheric polar vortex is weaker and warmer than in the positive phase. Propagation of stationary waves from the troposphere to the stratosphere is enhanced, westerlies over the North Atlantic are suppressed and cold polar air enters the mid-latitudes.