A new study by jetSTREAM postdoctoral researcher Erica Madonna finds that a weather pattern known as European-Scandinavian blocking tends to occur when the North Atlantic jet stream is in a “split” configuration.

Schematic of four typical configurations of the North Atlantic jet stream (black arrow) and their associated pressure patterns (colour shading). Red indicates anticylonic pressure anomalies (also known as highs or ridges), where winds circulate clockwise; these are a signature of blocking events.

Previous studies identified connections between other types of blocking and the North Atlantic jet: blocking over Greenland with southern jets, blocking/ridge anomalies off the Iberian peninsula with northern jets, and unblocked flow with central jets. However, European-Scandinavian blocking remained somewhat unclear. The cluster analysis used in this study was able to pick out an additional split jet structure, which is slightly more complicated, and tie it to blocking over the continent.

Read more about the work here or download the research article:
Madonna, E., C. Li, C. Grams and T. Woollings, The link between eddy-driven jet variability and weather regimes in the North Atlantic-European sector, QJRMS, 143, 2960-2972, 2017.

This movie is a fantastic visualization of the atmospheric flow as it evolved over the autumn of 2014, revealing many fundamental features of the general circulation.

In the midlatitudes, sweeping weather systems travel eastwards, wrapping tendril-like fronts around their centres. The weather systems follow the jet streams, belts of strong winds blowing west to east, and are concentrated in regions known as “storm tracks” in the North Pacific, North Atlantic, and South Pacific Oceans. These systems bring precipitation, which becomes snow as autumn progresses into winter, blanketing the Eurasian continent by late October and North America by late November. Around the Great Lakes, “lake effect snow” can be seen when weather systems bring cold air masses across the warm lake surfaces from west to east, depositing snow on the eastern shores (watch this region around 1 November and 16 November, for example).

In the tropics, small-scale convective systems travel westwards at a rather modest speed. From mid-September until mid-October, a number of typhoons (tight, circular structures spinning counter-clockwise) can be seen racing westwards across the tropical Pacific. Most of them veer northwards towards Japan as they approach the Asian coast.