Category Archives: Science

Jet variability related to jet strength

Strong jets tend to be steady, while weaker jets tend to fluctuate more in their north-south positioning. This relationship is found to be remarkably general — it applies to midlatitude jet streams in observations, in climate models, in idealized numerical models, and over a range of time scales from daily to decadal.

Observational jet analysis using the method of Spensberger et al. (2017) applied to the 2-PVU surface in ERA-Interim in winter (1979–2014). Each panel shows the frequency of identified jets from 6-hourly data partitioned according to the speed of the jet (m/s). Blue lines indicate the positions of the climatological jet axes. Adapted from Figure 4 of Woollings et al. 2018.

The figure above shows wintertime jet streams at the tropopause, the surface separating the troposphere from the stratosphere. The shading indicates how often a jet is detected, and each panel groups these jet frequencies according to the speed of the detected jets (the speeds in m/s are indicated at the top left of each panel). Weaker jets are seen to occur over a wide range of latitudes. As the jet speed increases, the possible jet locations contract around the climatological jet axes, shown in blue.

An underlying barotropic mechanism is proposed to explain this behavior, related to the change in refractive properties of a jet as it strengthens, and the subsequent effect on the distribution of Rossby wave breaking. Read more about this work at the links below.

Spensberger, C., T. Spengler and C. Li, Upper tropospheric jet axis detection and application to the boreal winter 2013/14, Mon. Wea. Rev., 145, 2363–2374, 2017. link

Woollings, T., E.A. Barnes, B. Hoskins, Y.-O. Kwon, R. Lee, C. Li, E. Madonna, M. McGraw, T. Parker, R. Rodrigues, C. Spensberger and K. Williams, Daily to decadal modulation of jet variability, J. Climate, 31, 1297-1314, 2018. link

Split jet stream brings cold, clear winter days to Scandinavia

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.

Visualizing the atmospheric circulation

Water transport in the atmosphere. Shown are the column atmospheric water content in white, evaporation in pink, and precipitation in blue. Data from ECMWF ERA-Interim reanalysis (September to November 2014). The running date is indicated at the bottom right. Credit: Mats Bentsen, Uni Research Climate.

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.