| The atmospheric northward energy transport plays a crucial role for the Arctic
climate; the transport brings to the Arctic an amount of energy comparable to that
provided directly by the sun. During recent decades warming of the Arctic surface
air has been more than twice as large as the warming averaged over the Northern
Hemisphere. This is known as Arctic amplification. Climate models predict that Arctic
amplification will continue during the 21st century. The models also show that the
atmospheric energy transport to the Arctic will remain almost unchanged or will even
decrease in the future. This has led to the conclusion that atmospheric energy transport
does not contribute but rather opposes Arctic amplification. Here we show that the
atmospheric energy transport will indeed contribute to Arctic amplification even
while decreasing. A split of the transport into latent and dry-static components
reveals that a change of the latent transport compared to a change of the dry-static
has a much larger effect on the Arctic climate. This is because the latent transport
brings not only energy, but also water vapour into the Arctic. This water vapour
enhances the local greenhouse effect, both in itself and through the formation of
clouds. An increase of the latent transport at the Arctic boundary therefore causes
Arctic warming, both directly due to latent heat release, and indirectly due to an
enhancement of the local greenhouse effect. Climate models tend to agree that the latent
energy transport will increase on the expense of the dry-static transport in future
simulations. Our results imply that the Arctic cooling caused by the reduction of the
dry-static transport is more than compensated for by the warming induced by the latent
transport. |