The Influence of Large‐Scale Spatial Warming on Jet Stream Extreme Waviness on an Aquaplanet.

The effect of modified equator‐to‐pole temperature gradients on the jet stream by low‐level polar warming and upper‐level tropical warming is not fully understood. We perform aquaplanet simulations to quantify the impact of different sea surface temperature distributions on jet stream strength, large wave amplitudes and extreme waviness. The responses to warming in the waviness metrics Sinuosity Index and Local Wave Activity are sensitive to the latitude range over which they are calculated. Therefore, we use a latitude range that accurately represents the position of the jet. The uniform warming scenario strengthens the jet and reduces large wave amplitudes. Reductions in meridional temperature gradients lead to weakened mid‐latitudinal jet strength and show significant decreases in large wave amplitudes and jet stream waviness. These findings contradict the mechanism that weakened jet streams increase wave amplitudes and extreme jet stream waviness. We conclude that weakened jet streams do not necessarily become wavier. Plain Language Summary: This research letter considers how different patterns of atmospheric warming, like low‐level warming at the poles and at high altitude in the tropics, impact the jet stream, which is a strong "river" of high‐altitude wind. We use numerical model simulations to mimic different scenarios of warming that maintain or reduce the temperature gradient between equator and poles. We find that when an Earth‐like planet completely covered by water warms in specific ways, it strengthens the jet stream when it is warmed uniformly or weakens the jet stream when only mid‐ and high latitudes are warmed. For the simulations with a reduced mid‐latitudinal temperature gradient, the size of largest jet stream waves reduces and makes the extreme waviness episodes less wavy. This research letter conclude that weakened jet streams do not necessarily become wavier, which opposes the popular paradigm that weakened jet streams become wavier due to warming in polar regions. Key Points: On an aquaplanet, weakened jet streams do not become wavier with reduced temperature gradients due to warming in mid‐ and high latitudesIn reduced temperature gradient simulations, the magnitude of large wave amplitudes and jet stream extreme waviness decrease robustlyThe Sinuosity Index and the Local Wave Activity waviness metrics are sensitive to the latitude range over which they are calculated [ABSTRACT FROM AUTHOR]