Geopotential height (GPH) is an extremely effective measure to use when assessing large-scale, long-term weather patterns.
Between the subtropics and the polar regions (mid-latitudes), it reveals the location of major boundaries along which extratropical low pressure systems frequently develop. It also shows where such systems will tend to travel, stall-out and then dissipate. Have a read of section 2 to delve into this further.
In the tropics, GPH strongly affects thunderstorm activity and in turn, the ability of tropical cyclones to develop. Section 3 presents all the juicy details.
Proportional to Air Temperature
To understand the general global pattern of GPH, it helps to know its relationship with air temperature. Thankfully, this is straightforward:
As this diagram explains, GPH reduces as the air becomes colder, and vice-versa.
You’ll likely be familiar with how air temperatures vary between the equator and the poles. GPH varies in tandem with that – generally higher toward the equator and lower toward the poles.
The Master of Mid-Latitude Weather Patterns
Irregularities are a big part of weather; the smallest disturbances can evolve into broad-scale features. All the way up to the boundary with the stratosphere, you can always find areas of relatively low and GPH within the general gradient from equator to pole.
These are commonly referred to lows and highs. These have a steering effect on the air, which in the Northern Hemisphere flows anticlockwise around a low and clockwise around a high (in the Southern Hemisphere, it’s the other way around).
As illustrated below, this flow is far from uniform. This is a consequence of lows having tighter circulations than highs, due to their spin being the same as that of the Earth.
As a result, the amount of ‘room’ for the air flowing between an upper low and high varies. This forces air to converge in some places and diverge in others.
A diagram illustrating, by an example, how upper highs and lows relate to their surface counterparts.
Relatably to us, the air doesn’t like to be squashed, so where it’s being forced to converge from the side, it takes a vertical escape route. Due to the effects of gravity, more goes down than up – often all the way to the surface. This sinking applies more pressure on the surface, so we see high SLP – a surface high.
Where the flow diverges aloft, there’s space being opened up for air to move into from above and below. This is a big deal for surface low pressure systems. A key part of their structure is convergence of air at the surface, which requires an upward escape route for that air.
Together, this all means that surface highs tend to locate beneath or a little poleward of upper highs, while surface lows develop on the periphery of upper troughs. Check out the example below, in which you can see how the contoured surface systems relate to the shaded 500 mb GPH*.
* The shading of this map shows at what height, in decametres, the air pressure falls to 500 mb. It was sourced from the website indicated in the top-right corner. I’ve circled some key areas of upper convergence in black and divergence in white.
While we’re at it, worth noting the offset with height, also shown in the previous diagram. The surface highs and lows are a little to the west of the areas of upper convergence and divergence, respectively.
By taking an average across weeks or even months, GPH can be used to examine broad-scale tendencies to weather patterns. Comparison with long-term averages (usually across 30+ years) allows for easy identification of unusual behaviour. For example, anomalously low GPH in the subtropics indicates low pressure systems tending to travel unusually far equatorward.
The Modulator of Tropical Convection
Those who’ve read section 2 may have noticed that the 500 mb GPH shows high uniformity in the tropics. Here, there are usually only gentle gradients between slightly warmer and cooler regions of upper air.
Even so, there can be large, very important differences between those regions.
You see, warmer, less dense air will inherently tend to rise into colder, denser air above it. The tropics are abundantly warm at the surface, so with no other driving forces, there would be simultaneous rising motion (i.e. vertical convection) right around the globe. As it is, though, the slight variations in GPH are sufficient to greatly enhance vertical convection in some places and suppress it in others.
That’s because of the relationships of both vertical convection and GPH with temperature. Lower GPH corresponds to lower temperature aloft, and the faster temperature drops with height, the more quickly air can rise from the surface.
So, in the tropics, upper lows increase rising motion from the surface and upper highs reduce it.
Now comes the crucial connection: Moisture is also abundant in the tropics and when that’s combined with a lot of rising air, thunderstorms result. Check out the snapshot of infra-red satellite measurements below, taken from this animation of observations. The blue to yellow shading represents tall thunderstorm clouds.
Note the inconsistency across the globe; some areas have huge areas of thunderstorms, while others have few or none. The active zones are beneath relatively low GPH and vice-versa.
Tropical Cyclone Connection
Not only does this greatly affect rainfall distribution, it also determines where tropical cyclones are most likely to develop. You see, most of these grow from ‘seeds’ of long-lasting tropical convection.
When looking to anticipate how active a tropical cyclone season will be, the overall tendency of GPH relative to the long-term average is a very important factor. Similarly, observed and modelled GPH trends can be used to assess how tropical cyclones may respond to climate change.
James Peacock MSc
Head Meteorologist at MetSwift