Polar amplification of global warming is expected to decrease equator-to-pole temperature gradients, which would tend to slow thermal winds and thus the jet streams. However, increasing tropospheric and decreasing stratospheric temperatures are expected to raise the height of the tropopause, which would tend to elevate and strengthen the jet streams.

Here, we analyzed historical trends in several jet stream parameters in the ERA-40 reanalysis dataset for the period 1959 to 2001. Because the jet streams have a complex three-dimensional structure that varies with time, and because integrated quantities are more stable numerically than simple maxima, we calculated integrated quantities (or centroids) that largely reflect average mass transport and kinetic energy in the jet streams. We defined several regions (northern and southern hemispheres, 100 mb to 400 mb between 10° to 70° north and south, and 100 mb to 400 mb from 10°S to 45°S and from 45°S to 70°S). For each of these volumes and each month, we calculated the average mass transport and kinetic energy, and the mean latitude and altitude of the jet streams.

We found that, on average, the jet streams rose in altitude and moved poleward in both hemispheres. The strongest signal was detected in the southern hemisphere, where the sub-tropical jet moved south, upward, and weakened and the polar jet moved south, upward, and strengthened. These trends may be indicators of ongoing climate change.