Rossby Waves and concurrent weather extremes
The jet-stream and its interaction with cyclones and anticyclones can lead to pronounced co-variability of weather in far-away regions. The effect is most pronounced when the circulation features weak zonal flow and exhibits amplified planetary waves visible as large North-South meanders in the jet. This enables cold air to be carried far south while warm air reaches northern regions, leading to unusual local temperatures across the entire hemisphere. Such patterns can last for several days or weeks and turn unusual weather into persistent extremes. Recent examples include the summer 2018 where several record breaking and persistent heat and rainfall extremes occurred near simultaneously and the record-breaking heatwaves of summer 2019.
Here we show that some wavenumbers have a preferred phase position and thus constitute recurrent hemispheric wide circulation regimes. The atmospheric organization associated with such patterns can lead to co-occurring heatwaves and floods across the northern hemisphere. We quantify to what extent those patterns increase the likelihood of simultaneous heat extremes across affected regions and will discuss drivers and mechanisms that could lead to wave amplification.