The troposphere, which this post concerns, can be divided into two subsections: an upper layer, known as the Free Atmosphere, and a lower layer, known as the Planetary Boundary Layer. The Boundary Layer usually runs up from the surface to about 1,000 m above it (sometimes a bit more, sometimes a bit less), but basically it's a relatively thin layer in which the air movements and temperatures are influenced not only by major weather patterns but also by localized effects relating to the interaction of the air with the planet's surface. Such effects include frictional drag as winds cross land areas, eddies, veering and lifting due to hills and headlands, and convection initiated directly by heat radiation from sun-warmed ground. Low-level air currents, such as the cool sea breezes that push inland from coasts on warm summer days, likewise aid and abet convection and thereby thunderstorm formation as they undercut and lift warmed air masses along zones of convergence -- where different air currents come together. These factors are all low-level forcing mechanisms that set air currents in motion or perturb existing currents.
Above the Boundary Layer, winds are directed by two factors: the gradients that exist between centres of high and low pressure (anticyclones and cyclones respectively) -- air will always flow from a high-pressure zone to a low-pressure zone -- and the modifying factor known as the Coriolis Effect, which is the force exerted by the Earth's rotation. In the Northern Hemisphere, it causes air masses to be deflected to the right of their trajectory, and this effect is strongest at the poles and weakest at the Equator. In the Northern Hemisphere, the effect is to make the winds around a high-pressure centre circulate in a clockwise manner and those around a low-pressure centre circulate in an anti-clockwise manner: on a larger scale, the Coriolis Effect helps to maintain the prevailing west-to-east airflow.
Although the weather charts seen on TV forecasts show only what is happening close to the surface, the forecasts themselves are made with much reference to goings-on in the upper troposphere. In upper-air meteorology, pressure patterns are as important as they are down here at the surface. Atmospheric pressure is simply an expression of the force applied by a column of air upon a fixed point of known area and is measured in pascals (Pa). Meteorologists use the hectopascal (hPa) because the numbers are the same whether expressed in hectopascals or the older unit, millibars.
The greater the altitude, the lower the atmospheric pressure because there's less air above. In meteorology, above-surface observations are made remotely with satellites and directly by weather balloons carrying measuring instruments. The results of the balloon ascents, called soundings, are plotted on charts at different pressure levels, some typical examples of which are as follows:
Other properties of the upper air, such as temperature, are important, too. For example, storm formation in an unstable lower troposphere is markedly encouraged if cold, dry air is present aloft, which makes the rising warm, moist air much more buoyant, increasing the instability. Storm forecasters will look at soundings for indications that cold, upper air is either already present or is upwind and can be expected to be transported into the forecast area. The process by which air (with its intrinsic physical properties such as temperature or moisture content) is transported horizontally is known as advection, an important term that will appear elsewhere in this post.
In addition to the Longwaves, there are similar, but much smaller ridges and troughs, known as Shortwaves. The chart above also shows how, locally, the jet stream can split in two around a so-called cut-off upper high or low, reuniting again downstream. Longwaves, shortwaves and cut-off highs and lows all have a strong bearing on the weather to be expected at ground level.
In highly zonal conditions, weather systems move along rather quickly, giving rise to changeable weather. However, in highly meridional conditions, the Longwaves can slow down in their eastwards progression to the point of stalling, to form what are known as blocks. When a block forms, whatever weather type an area is experiencing will tend to persist. During some winters, for example, a blocking ridge forms in the mid-Atlantic, with high pressure extending from the Azores all the way up towards Greenland. Provided the block is far enough west, it can induce a cold northerly-to-easterly airflow over NW Europe, a synoptic pattern that brings cold weather and, in recent winters, heavy snowfalls.
Positive vorticity in the upper troposphere encourages air at lower levels to ascend en masse. Rising air encourages deepening of low-pressure systems, assists convective storm development and so can lead to severe weather such as heavy precipitation and flooding. As an upper trough moves in, air with positive vorticity is advected ahead of its axis in the process known as positive vorticity advection, usually abbreviated to PVA. Thus, to identify areas of PVA when forecasting, look on the upper-air charts for approaching upper Longwave or Shortwave troughs: PVA will be at its most intense just ahead of the trough and that is where the mass ascent of air will most likely occur.
Fast jet streaks with winds as high as 200 knots pull in air upstream (to their west) at what is called an Entrance Region and throw it out down stream (to their east) at what is called an Exit Region. These are further subdivided, as in the diagram above, into Left (to the north) and Right (to the south). Because the behaviour of air currents is determined by the interaction of the Coriolis effect and the pressure gradient, the Right-Entrance and Left-Exit regions of jet streaks are areas where winds aloft diverge, allowing air below to rise. This in turn further encourages storm development. In Right-Exit and Left-Entrance regions, the opposite occurs, with upper-level winds converging leading to air sinking and inhibiting storm formation. The reason, in terms of storm development, it is divergence as opposed to convergence that is important at height (the opposite being the case at low levels) is that converging air at height cannot go upwards because of the effective ceiling provided by the tropopause. There is only one vertical direction in which the air can freely go -- downwards.
"Hard to say if it's linear or otherwise -- not enough years of data yet, and it's not clear if models are able to capture the behavior realistically. Some recent papers suggest they don't simulate blocking patterns well, for example, which are key for extreme weather. We have looked at a 4xCO2 run of the NCAR GCM, however, which suggests that (like the real atmosphere) the 500-hPa zonal winds will weaken substantially in all seasons (not just fall, which is the strongest signal in the real world), and also that the flow will become more meridional, that is, the ratio of north-south winds relative to the total flow will increase. I think the tendencies we're seeing in the real world will continue to increase. As we lose all the summer ice, the response in the fall may plateau somewhat (although Arctic Amplification will continue via the other factors), but as ice in the other seasons declines, we should see the response become stronger all year long."