Item

Abstract

The surface temperature at high latitudes — especially in the Arctic — has warmed faster than the global mean temperature since the 1990s. This process, called Polar amplification, is expected to continue this century. Concurrent with Polar amplification, upper tropospheric Tropical warming is projected in the 21st century. Low-level Polar and upper-level Tropical warming influence the equator-to-pole temperature gradient in opposite ways and hence the mid-latitude jet stream. However, the effect of modified equator-to-pole temperature gradients is not fully understood. Earlier studies argued that low-level Polar warming causes a weaker and wavier jet stream. Here, we study the influence of Polar warming on the jet stream speed, position and its waviness by increased SSTs. We performed four idealised aquaplanet simulations with the Open Integrated Forecast System. First, we compared the jet stream intensity and position of the control simulation with the warmed simulations. Second, we studied the influence on jet stream waviness quantified by a modified Sinuosity Index: we adjusted the original metric to better capture the jet on an aquaplanet. Despite strong Polar and uniform warming, the mean of the Sinuosity Index distributions of the warmed simulations changed negligibly. However, the most extreme waviness events decreased with Polar warming and increased with uniform warming. Our results contradict findings in other studies on the consequences of low-level Polar warming and its influence on jet stream waviness. We conclude that a weaker jet stream does not have to become wavier with reduced temperature gradient and discuss the implications.