Shift of extratropical storm tracks

The climate in mid-latitudes as well as in higher latitudes is strongly depending on tracks and intensity of mid-latitude cyclones. The stimulus for their formation are meridional temperature gradients in the free troposphere. Their intensity depends in addition on the amount of humidity in the atmosphere. The statistics of cyclones on global scale has revealed that the North Atlantic region experiences winds as intense in winter as within the circumpolar Southern Ocean storm track, where strong winds prevail nearly throughout the entire year.

Will the storm tracks change or have they already changed?

Evaluations of long time series of pressure measurements at several stations, which are more reliable for pressure gradient estimates and thus geostrophic wind vectors than direct wind vector measurements, have shown that there is no general intensification of gale force winds. However, there is an indication of a northward shift of cyclone tracks in the Atlantic. Model results for climate change scenarios in the 21st century point to a clear northward shift jointly with an intensification mainly in the southern part of the storm track, i.e. for Great Britain and Northern Central Europe. The plausible explanation is: Despite the northward shift, a consequence of reduced meridional gradients, intensification occurs due to enhanced latent heat release especially in the southern portions of the cyclones as the air is warmer and contains more humidity.

Figure 2.5

The global conveyor belt in the world ocean

Figure 2.5

The global conveyor belt in the world ocean

1850 1000 1950 Year 2000 2050 2100

Figure 2.6: Weakening of the thermohaline overturning circulation as a function of climate change scenarios (MPI, 2006b).

1850 1000 1950 Year 2000 2050 2100

Figure 2.6: Weakening of the thermohaline overturning circulation as a function of climate change scenarios (MPI, 2006b).

Figure 2.7: Changes in the number of cyclones per month in the northern North Atlantic for the winter period (DJF) in the climate change scenario A1B (Bengtsson, 2006)

Figure 2.8:

As figure 2.7 but for storm intensity

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