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Meeting report: Autumn retreat

29-31 October 2018, Bekkjarvik, Norway
By Stina Olandersson and Peter Siew
Link to schedule

A group of scientists from the large-scale dynamics group had an autumn retreat in Bekkjarvik, Norway 29-31 October. Apart from the folks in Bergen, some guests were invited to participate, including David Battisti (University of Washington), Justin Wettstein (Oregon State University), Tim Woollings (University of Oxford) and William Roberts (Northumbria university). The aim of this workshop was to explore how the weather and climate interact in midlatitudes, and their combined role in setting the midlatitude circulation of the Northern Hemisphere. Continue reading →

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.

Atmospheric jet streams

Atmospheric jet streams are belts of strong westerly (west-to-east) winds circling the Earth. Associated with the jet streams are the storm tracks, preferred regions for the growth and propagation of midlatitude weather systems.

Northern Hemisphere jet streams during the winter season (December to February). Blue shading shows the upper level jet (250 hPa; contour intervals of 15 m/s). The arrows show the lower tropospheric jet (at 850 hPa; 5 m/s reference arrow in red).

Pictured above are the Northern Hemisphere jet streams during the winter season (December to February). Three main jets are observed: the North Atlantic jet, the African jet, and the North Pacific jet.

The shading shows the jet in the upper troposphere, about 10 km up from the surface. The westerly winds are strongest at these levels. The contour interval is 15 m/s, which means the North Pacific jet has an average wintertime speed of over 60 m/s (that’s more than 200 km/h!). The North Atlantic jet is about half that strength. Another difference is that the North Pacific jet is rather zonal (oriented mainly east-west), while the North Atlantic jet has a pronounced southwest-northeast tilt. Such differences are due to the placement of continents, large mountain chains such as the Rockies and Tibetan Plateau, and warm ocean currents on Earth.

The arrows show the jet in the lower troposphere, about 1.5 km above the surface of the Earth. Winds at this level generally match up with winds in the upper troposphere over the ocean basins, but there are some differences over land. Lower tropospheric winds over northern Eurasia are relatively strong compared to upper tropospheric winds. Conversely, the African jet does not exist at these lower levels.

Northern Hemisphere jet streams during the summer season (June to August). Blue shading shows the upper level jet (250 hPa; contour intervals of 15 m/s). The arrows show the lower tropospheric jet (at 850 hPa; 5 m/s reference arrow in red).

In summertime, winds are much weaker owing to weaker temperature gradients between the equator and pole. The main jet streams are still evident, but slower and shifted north relative to the winter jets. The strong lower tropospheric winds in the Indian Ocean are a signature of the monsoon.

Data used to create the plots shown here are from ECMWF’s ERA-Interim reanalysis for the period 1979 to 2015.

Meeting report: Oxford

16-18 March 2016, University of Oxford
By Erica Madonna

P1010131
On the roof of AOPP (J. LaCasce)

The Norwegian jetSTREAM (Atmospheric jet variability: linking STRucture, Evolution And Mechanisms) crew met for a three days’ workshop in Oxford March 16-18. The aim of the meeting was to exchange knowledge and ideas with people working on the SummerTIME (Summer: Testing Influences and Mechanisms for Europe) project, led by Tim Woollings and based at the University of Oxford, whose motivating scientific questions are closely related to jetSTREAM.
Continue reading →

Meeting report: SPARC Workshop on Storm Tracks

24-28 August 2015, Grindelwald, Switzerland
By Laura Ciasto and Paul Hezel

Lunch break discussions under a not so terrible view (C. Li)

Lunch break discussions under a not so terrible view (C. Li)

In week 35, a group of jetSTREAM scientists converged in Grindelwald, Switzerland for the SPARC Workshop on Storm Tracks (SPARC: Stratosphere-troposphere processes and their role in climate, a program of the WCRP (World Climate Research Program)). Co-organized by Camille Li, the workshop brought together a wide range of experts in the field of storm track and jet variability. The workshop goal was to shed light on questions such as “What controls the position, strength, and variability of a storm track?” The project team was well represented in this endeavour with presentations on how jets and storm tracks are influenced by internal atmospheric processes (Thomas Spengler, Clemens Spensberger, Justin Wettstein) as well as external forcing (Laura Ciasto, Paul Hezel, Joe LaCasce, Tim Woollings). See the Presentations page for details. Continue reading →

Kickoff meeting

3-4 November 2014, GFI, Bergen

Bergen
Brainstorming over dinner

Monday (GFI Ø321)
12:15 Lunch at GFI
13:00
  – Camille: Introduction, objectives, research components
  – Tim: North Atlantic jet variability
  – Justin: Task B – observed jet variability and 3D teleconnection patterns
  – Thomas (+ Stefan K in spirit): Task A – recipes for jet variability from idealized modelling experiments
  – Wil: examples of interactive visualization techniques for exploring complex objects/datasets
19:00 Dinner at 1877

Tuesday (GFI Ø321)
9:00
  – Clemens: jet axis view of jet variability
  – Joe: storm track – SST relationships
12:00 Lunch at Hanne på Høyden
13:30
  – Laura: remote vs local storm track – SST relationships
  – a brief word on tools and planned experiments: Clemens – Bedymo, Paul – CAM5 experiments, Stefan S (TBC)
  – Camille: synergy and wrap-up
16:00