Jet streams and pressure · Observatoire des phénomènes naturels

Jet streams are rivers of westerly wind circling the planet at about 10 km. The eddy-driven jet, which reaches down to the ground, steers Atlantic depressions toward Europe: its latitude decides whether winter is mild and wet in the north or in the south. This page follows, month by month since 1979, the latitude and strength of the jets (ERA5 reanalysis, wind at 850 and 250 hPa) and the two pressure centres of action that go with them, the Icelandic Low and the Azores High, whose difference makes the North Atlantic Oscillation (NAO).

ERA5 monthly means have not been retrieved from the Climate Data Store yet. No data imported yet. Run: php bin/console --no-debug app:import era5-circulation

Method and sources

Data. ERA5 reanalysis (Hersbach et al., 2020), monthly means since 1979 regridded to 1° by the Climate Data Store: zonal and meridional wind at 250 hPa (about 10 km) and 850 hPa (about 1.5 km), mean sea-level pressure. Recent months come from ERA5T, published about five days after the end of the month and subject to revision.

Jets. For the eddy-driven jet, the method of Woollings et al. (2010) is adapted to monthly means: 850 hPa zonal wind averaged over a sector (North Atlantic 60° W–0°, 15–75° N; the whole Southern Hemisphere, 20–75° S), latitude and value of the maximum refined by a parabola between grid rows. Upper-level jets are the maximum of the 250 hPa zonal wind averaged over the North Atlantic (20–70° N) and the North Pacific (140° E–140° W, 15–70° N). The axis of the month on the maps is the latitude of the strongest 250 hPa wind at each longitude.

Pressure. Icelandic Low: lowest pressure between 50 and 75° N, 60° W–0°; Azores High: highest between 20 and 50° N. Station-like NAO (Hurrell, 1995) at the grid points nearest Ponta Delgada and Reykjavík. Normals and departures over 1991–2020, least-squares trends with a standard error corrected for autocorrelation.

Reading these data

A monthly mean smooths a jet that meanders every day: the monthly latitude is not the position on a given day, and a month with a split jet gives a weaker maximum at an intermediate latitude. Before 1979 ERA5 has fewer upper-air observations: the page starts in 1979. Centres of action are sought in fixed boxes: an Azores High shifted toward Europe or a split low are poorly described. Trends over 45 years remain short against decade-to-decade variability.

SOURCES OF THIS PAGE

Hersbach et al. (2020): Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., et al. (2020). The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146(730), 1999–2049. · doi:10.1002/qj.3803
Licence : Published literature; cited
Woollings et al. (2010): Woollings, T., Hannachi, A., & Hoskins, B. (2010). Variability of the North Atlantic eddy-driven jet stream. Quarterly Journal of the Royal Meteorological Society, 136(649), 856–868. · doi:10.1002/qj.625
Licence : Published literature; cited
Hurrell (1995): Hurrell, J. W. (1995). Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269(5224), 676–679. · doi:10.1126/science.269.5224.676
Licence : Published literature; cited
Barnes et al. (2013): Barnes, E. A., & Polvani, L. (2013). Response of the midlatitude jets, and of their variability, to increased greenhouse gases in the CMIP5 models. Journal of Climate, 26(18), 7117–7135. · doi:10.1175/JCLI-D-12-00536.1
Licence : Published literature; cited
Thompson et al. (2002): Thompson, D. W. J., & Solomon, S. (2002). Interpretation of recent Southern Hemisphere climate change. Science, 296(5569), 895–899. · doi:10.1126/science.1069270
Licence : Published literature; cited
Woollings et al. (2018): Woollings, T., Barnes, E., Hoskins, B., Kwon, Y.-O., Lee, R. W., Li, C., Madonna, E., McGraw, M., Parker, T., Rodrigues, R., Spensberger, C., & Williams, K. (2018). Daily to decadal modulation of jet variability. Journal of Climate, 31(4), 1297–1314. · doi:10.1175/JCLI-D-17-0286.1
Licence : Published literature; cited

Jet streams and pressure Jet latitude and strength, Icelandic Low, Azores High (ERA5, since 1979)