Expected Impact of Global Warming on Middle Latitude Circulation
Meteorologists and climate scientists studying the impact of global warming predict that most of the warming will take place at higher latitudes while the temperatures in the equatorial regions would change the least. This will result in a weaker equator-to-pole temperature gradient than exists today.
The strength of the polar jet stream is related to the height gradient found in middle latitudes, with higher 500 mb heights to the south and lower middle latitude heights to the north, and the slope of the isobaric surfaces in the middle latitudes. If the temperatures in the equatorial regions stay the same, while temperatures in the higher latitudes increase, this height gradient and isobaric slopes will get weaker, and so will the average strength of the polar jet stream.
Fig. 1: Schematic cross-section of the Troposphere at the time of the winter solstice in the Northern Hemisphere. This is taken from Lab 10.
A. Low Index Flow Types
There is an interesting twist, pun intended, to all of this. In a pattern of lower zonal wind speeds, the meridionality (the tendency for larger wavelength, larger amplitude disturbances in the middle and upper troposphere) is expected to increase. Such waves are slower moving, but such waves also have an interesting pattern in the height contour field.
Fig. 2: A typical 500 mb map during a so-called "Low Index" (low zonal index).
Fig. 2 shows a forecast 500 mb height pattern typical of a highly meridional flow. Such patterns can occur during the core of the winter due to the evolution of temperature advection patterns, and are termed "Low Zonal Index" patterns. But they become more typical during times of relaxed pole-equator temperature differences. This type of pattern is thus expected to become more frequent during the 21st century.
These longer wave troughs and ridges are very slow moving and can become stationary. When their positions and patterns are averaged out, they are really symptomatic of the Rossby (Long) Wave pattern. When ridges become stationary in certain ranges of longitudes long term droughts will occur and vice versa. This is consistent with the foreshadowings of higher variability of rainfall (a greater occurrence of high and low extremes) in the middle latitudes.
B. Short Waves Impossible to Anticipate
Even though the background jet stream flow is projected to have longer waves in quasi-anchored positions, this does not mean that shorter wavelength disturbances will not move through them. There will still be temperature gradients and temperature advection associated with wave cyclones. However, these short wave disturbances are impossible predict with the larger scale linked atmosphere-ocean models. Even in today's climate, such waves react to local temperature gradients (such as occluded lows approaching the West Coast during the winter) that in themselves are ephemeral producing a non-linear system (meaning, the same disturbance approaching a different temperature gradient will interact with that gradient and have a different ulitmate development).
All one can say is that in low index circulation types, the fixed position of the jet stream will allow certain longitudes to have more frequent precipitation associated with whatever short wave disturbances exist in the flow. And less frequent precipitation in areas in which a ridge is at a fixed longitude. In the case of the former occuring along the West Coast, atmospheric rivers (AR) carrying higher water vapor (due to the increased sea-surface temperature) would contribute to much higher AR-related rainfall.