Linking Northern Hemisphere extreme cold weather events to upper atmospheric circulation

Previous studies have indicated that increased probability of extreme events in many regions of mid-latitude is linked to amplified waviness and slow upper-atmosphere circulation. However, this linkage appears to be dependent on region, and details regarding the waviness or slowness required to promote extreme events locally remain unclear. The objective of this study is to examine the upper atmospheric circulation and the linkage between the occurrence of cold extremes in different regions of the Northern Hemisphere in winter. We examine this link using the fifth-generation ECMWF reanalysis data (ERA5).

The upper atmospheric waviness—both in the vertical and the meridional direction—is computed based on geopotential height at 300 hPa. At each latitude, the vertical waviness is estimated as the circumglobal amplitude of the first five zonal wave numbers based on a Fourier decomposition. For the meridional waviness, the amplitude of each specific ridge and trough is defined as the latitudinal deviation of the isoheight taken as the zonal geopotential height mean over the region of the ridges and troughs. The speed of planetary wave zonal propagation is computed through the utilization of a top-ridge and bottom-trough tracking algorithm.

In most of the studied regions, there is a significant slowdown in the upstream ridge and downstream trough during a cold spell, confirming earlier results. For the North American cold spells, this pattern is mainly observed at high latitudes, particularly between 60° and 75° N. During cold spells over the British Isles, Europe, and Nordic countries, the speed of the ridges and troughs decreases at mid-latitude yet continues moving eastward. For cold spells over Central Asia, the ridges and troughs become significantly slower at high latitudes (60°N–80°N) but faster at lower latitudes (35°N–45°N). Contrary to our expectations, the circumglobal vertical amplitude over mid-latitudes for most regions’ cold spells exhibits less waviness. However, each local meridional wave amplitude associated with upstream ridges and downstream troughs in the vicinity of the cold spell’s location becomes significantly larger. Hence, the waviness and slow upper-atmosphere circulation associated with each region's cold extremes occur more locally than globally. Our results also indicate that amplified local meridional wave amplitude always precedes cold spells, but ridges and troughs become slower—depending on the locations of the cold spells—before or during cold spells.