Records of the past climates show a wide range of values of the equator-to-pole temperature gradient, with an apparent universal relationship between the temperature gradient and the globalmean temperature: relative to a reference climate, if the global-mean temperature is higher (lower), the greatest warming (cooling) occurs at the polar regions. This phenomenon is known as polar amplification. Understanding this equator-to-pole temperature gradient is fundamental to climate and general circulation, yet there is no established theory from a perspective of the general circulation. Here, a general circulation-based theory for polar amplification is presented. Recognizing the fact that most of the available potential energy (APE) in the atmosphere is untapped, this theory invokes that La-Niña-like tropical heating can help tap APE and warm the Arctic by exciting poleward and upward propagating Rossby waves.