Responses of Midlatitude Blocking Activity and Wave Amplitude to Reduced Meridional Temperature Gradient: Arctic Amplification versus Arctic Oscillation

Atmospheric blocks and large amplitude waves strongly affect the surface weather and can lead to extreme weather events such as heat waves, cold spells, droughts, and episodes of heavy precipitation. Therefore, a question of significant scientific and societal importance is how blocking activity and waviness of the midlatitudes jets respond to the reduced midlatitude-to-pole temperature gradient caused by Arctic amplification. However, a lack of long enough observations/simulations, disagreements on blocking and waviness indices, deficiencies of climate models, and an incomplete dynamical understanding of blocks and amplified waves have seriously hindered the efforts in answering this question.

The meridional temperature gradient also decreases in the negative phase of Arctic Oscillation, which is an internal mode of variability. Observations and models show that in the negative phase of Arctic Oscillation, the midlatitudes jet shifts equatorward and blocking activity and wave amplitude increase. It has been suggested that blocks and waviness might respond in the same way to reduced temperature gradient that is externally forced by Arctic Amplification.

Using an idealized dry GCM, the dry dynamical core with Held-Suarez forcing, we show that blocking activity responds differently to reduced temperature gradient caused by an Arctic Amplification-like forcing, and in the negative phase of Arctic Oscillation. With Arctic Amplification-like forcings, the jets and eddy fluxes weaken and shift equatorward. Blocking activity decreases and its distribution shifts equatorward as well (as discussed in Hassanzadeh et al. 2014 GRL). In the negative phase of Arctic Oscillation (in the control run and also with Arctic-Amplification-like forcings), the jets shift equatorward, eddy fluxes weaken, blocking distribution shifts poleward, and the blocking activity increases, which is consistent with observation. Therefore, the changes in the magnitude of blocking activity and blocking latitudinal distribution in response to Arctic Amplification and Arctic Oscillation are opposite. Identifying the underlying dynamics of these responses, exploring changes in wave amplitude, and the robustness of these results in more complex GCMs are in progress.