Tropically forced tapping of potential energy by planetary scale waves and the subsequent Arctic warming

One of the challenging tasks in climate science is to understand the equator-to-pole temperature gradient. This is directly relevant for understanding Arctic amplification which refers to the phenomenon that the greatest warming (cooling) occurs over the Arctic region when the average temperature in the Northern Hemisphere is warmer (cooler) than some reference climatic state. During warm climates, Arctic amplification implies a reduction in the meridional equator-to-pole temperature gradient and zonal available potential energy (ZAPE). However, according to well-established theories on baroclinic instability and the flux-gradient relationship, a reduction in ZAPE should also be accompanied by a reduction in poleward eddy heat flux. Therefore, the question arises as to how a warm climate maintains a warm Arctic, particularly during polar night. We show, using reanalysis data, the existence of a poleward heat transport process that is independent of the flux-gradient relationship and baroclinic instability. Rather, the process arises from a forced tapping of atmospheric ZAPE by planetary-scale waves that are triggered by enhanced tropical convection over the Pacific warm pool region. During polar night, when the current warming is most rapid, this forced tapping of ZAPE produces more warming than the eddy fluxes that presumably arise from baroclinic instability. In the current climate, there is a large reservoir of ZAPE left untapped. Therefore, being unconstrained by the flux-gradient relationship, this forced tapping process can potentially account for a wide range of equator-to-pole temperature gradients.