Gp Capt (Dr) B Nandi

1. Introduction

We start by introducing the basic wind in meteorology known as geostrophic wind and then by introducing anticyclonic curvature we like to explore various properties of an anticyclone. Next step will be to go over the genesis of different types of anticyclone, gradually.

2. Geostrophic Wind

Where Pressure gradient force is balanced by the Coriolis force, it gives a balanced wind called geostrophic wind.

f Vg = −∂ Φ /∂n  (in natural coordinate) , Φ denoting geopotential heightof a pressure surface.

This balanced wind give rise to following two laws as the characteristics of the wind.                      

  • Buys Ballot Law States that if you are standing with your back to the wind in the Northern Hemisphere, low pressure will be on your left.
  • Ferrel’s Law  Wind is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

3. Gradient wind

Now, we introduce the anticyclonic curvature, bringing in the Centrifugal force. So, Coriolis force is required to balance centrifugal force besides the pressure gradient force and thus it has to be more in magnitude than in the case of geostrophic wind.

Hence, in anticyclone, winds are super geostrophic.

The balance equation for these three forces can be written

This implies (as left-hand side is positive) that V > Vg, super geostrophic wind results when anticyclonic curvature is introduced. Or in other words, for anticyclonic curvature Coriolis force is required to balance two forces, pressure gradient force and centrifugal force. Hence needs to be stronger. Coriolis force can be stronger only when wind speed is higher.

Rewriting the equation

For radical to be non-negative

Thus, the pressure gradient in a high must approach zero as |R|      0. It is for this reason that the pressure field near the center of a high is always flat and the wind is gentle compared to the region near the center of a low.

Righthand side of the above equation should be positive. This provides the minimum radius a high pressure should have for particular pressure gradient (given in terms of geostrophic wind).

Smaller size anticyclone possible at higher latitude as Coriolis parameter value is higher. Closure to the equator anticyclone needs larger radius.

Recollecting vorticity in natural coordinate as

curvature vorticity can be written as

So curvature vorticity is always less than planetary vorticity, Coriolis parameter f. But the shear vorticity can be of any value. Usual relative vorticity in anticyclone is negative and less than f making absolute vorticity positive.  The shear vorticity can make relative vorticity more than f and thus negative absolute vorticity in Norther Hemisphere is possible. This negative absolute vorticity gives rise to inertial instability. The outflow cloud from cloud cluster moves away northward mainly due to inertial instability.

Another important corollary follows that, when Sub Tropical Jet Stream shifts southward and say 50mps wind is at 200N we will not have the requisite space to provide the radius (2049km) of the anticyclone and westerlies may prevail right up to equator in that region. There must be a compensation by expanding easterlies somewhere in the globe.

4. Summary

  • anticyclone has super geostrophic wind for same pressure gradient.
  • Close to center pressure gradient must be flat meaning light wind. So, air remains in contact with same type of surface for longer duration to acquire the characteristics – like temperature moisture etc. Hence, low level high pressure over ocean becomes a source zone to develop moist air mass and over land becomes a source for warm dry airmass.
  • For higher wind speed around an anticyclone larger radius is needed.
  • Size of the anticyclone is lager at lower latitude.
  • For surface anticyclone, since pressure is high at surface level, air has to go out from the central zone (known as divergence) driven by pressure gradient force and to fill the void air must descend from top to surface. Thus, anticyclone is an area of subsidence.
  • Subsidence has three effects
    • Air gets compressed as it moves down and temperature increases.
    • In descending motion layer cloud does not form, as moist air gets heated up and if, any water droplet present will evaporate. So, anticyclone is usually associated with clear/fair weather.
    • Due subsidence the lapse rate increases towards DALR and this increased lapse rate aids better convective motion which can easily overcome the subsidence velocity. Especially, applicable for upper tropospheric anticyclone.

5. Moving anticyclone

Relative motion with respect to the anticyclone

In the forward zone of a moving anticyclone, air is caught up by the higher curvature (less radius) thus turns inward. This inward turning of air allows it to remain within the anticyclone for longer duration to pick up airmass characteristics.

In the rear zone of the moving anticyclone air is caught up with the lower curvature (higher radius) thus turs away from anticyclone

So, the end result is in forward zone when moving over from land to ocean pick up moisture and when the anticyclone turns back it throws the moisture from its rear area which has accumulated moisture earlier. This happens quite often during winter months. Low level anticyclone is pushed by the WD from central India to Bay of Bengal and starts retuning after he passage of WD and at this time it throws the moisture to peninsular India.

6. Anticyclone in general circulation model    

In general circulation model heat, angular momentum and moistures are redistributed to achieve a balance between pole and equator. The basic model is that the heated air over the equatorial region, especially over ocean, rises to upper troposphere as cumulonimbus cloud. The latent heat liberation due to cloud formation creates a mesoscale low in the middle troposphere and meso high at the top of the cloud. This high start spreading out (diverging out) the wind. The major spreading out is northward due inertial instability. This wind will be subjected to two enforcing.

  • One is Coriolis force which will turn the wind to right
  • Secondly, As per simplified Vorticity theorem

Diverging air must suffer reduction in its “absolute vorticity”.  That is, should develop anticyclonic relative vorticity as

So, the rising air turns anticyclonically and move towards higher latitude. Here, is the reason why we have upper tropospheric anticyclone. But this explains the one side (that is the western side) of the anticyclone.  Example is provided with the NWP chart of 00UTC/01Jan2024 for 200hPa level and satellite image for equatorial ITCZ.      

To complete the story, the air moving poleward undergoes three things

  • (i) It gets cooled by radiational cooling.  
  •  (ii) It gets subjected to convergence as the ring of air starts gaining latitude the radius of the atmosphere reduces (refer diagram), in other words the area of the parcel of air reduces leading to convergence.
  • (iii) As the air moves northward radius of rotation around the earth’s axis reduces, thus, angular momentum increases and wind becomes stronger westerlies and the increase in winds provides speed divergence. Atmosphere always keeps a check and breaking mechanism. In(ii) wind convergece and this cannot continue to infinite convergence and that is why there is a speed dvergence to keep the check. Ultimately, subtropical westerly Jet stream forms. A stage comes when the belt of wind becomes completely westerlies and further poleward movement stops. Usually this cold air converges around the latitude of 300N. This convergence of cold air leads to increasing in air density as per equation of continuity.

The weight of cold dense high density air directly shows as belt of high pressure at surface at horse latitude (300N). namely Azores/Bermuda High and Pacific high. There is no counterpart of upper tropospheric anticyclone for these surface highs. Dense air subsides due weight and gets warmer as it moves down and creates warm high. Descending air need not be vertically downward, which will be opposed by general static stability of the atmosphere. they usually follow slant path to descend. This part explains the formation of surface high or surface anticyclone. Global image of high pressure is shown below.

 With northward movement, the increase in wind speed due conservation of angular momentum of air ceases when wind becomes westerly, leading to formation of wind maximum. Air must get subjected turning right due high Coriolis force and gain anticyclonic vorticity. Thus, the return journey of NWly to NEly wind and ultimately strong easterly at equatorial region takes place. This happens many degrees of longitude away from the originally northward moving branch and may give 40 to 90 degree of diameter (in terms of longitude) and elliptical shape as North south minor axis is just about 30 degree of latitudes. This is demonstrated in NWP chart of 12UTC/28Jan2024 for 200hPa level.

Conservation of angular momentum gives the following when air moves from latitude φ1 to φ2

Ω = Angular Velocity of earth

ω = zonal angular velocity of air relative to earth

φ = Latitude

a = radius of earth

Zonal velocity u can be given as

If we consider that the air started from nil zonal velocity from equator and reached latitude φ, then

(at 300 latitude u=135m/s), which is unusual indicating that there must be zonal torque due pressure gradient force and eddy torque to reduce the speed or something else yet to understand.

7. Features of the upper tropospheric anticyclone

Refer the WV image of 12UTC/28Jan2024 for African and Asia region

(a) The western branch of this upper tropospheric anticyclone over Africa ( in fact the ITCZ lies south of equator) carries lighter moist air pole ward, hardly gets a chance to subside as it keeps meeting colder and denser air on its journey and rather rises (WV image of 12UTC/28Jan2024 for African region).  Reaches Indian region almost from west to east direction. After creating maximum wind, it starts turning anticyclonically at the same time losing westerly angular momentum. Cloud also carried by the NWly wind and its weak descend is evident from the cloud (WV and cloud top alert image of 12UTC/28Jan2024 for Indian region) not evaporating but temperature is increasing.  This NWly zone has two issues

  • (i) NWly wind speed is reducing with the reduction in westerly angular momentum, thus converging.
  • (ii) From the Northern most part to west-east ridge PVA (positive vorticity advection) is taking place giving rise to divergence.

Due to this combination of convergence and divergence there is a weak descending motion not enough to evaporate the clouds.

(b) When wind turns NEly curvature vorticity starts reducing and convergence starts providing subsidence motion. So, the SE quadrant is in favor for subsidence.

(c) On NWly wind cloud is sustained but temperature is increasing indicating a weak warming with weak descend. So eastern half has descending motion and comparatively more in SE quadrant.  Peripheral part of anticyclone is explained. The central part of anticyclone is a region where atmospheric Pressure tries to adjust the wind pattern to form high pressure with weaker shear gradient, wind is usually less than 25mps and this central zone is characterized by descending motion (yellow colour) (WV image of 12UTC/28Jan2024 for Indian region). This adjustment has life more than a week before wind pattern changes again.

8 Summary of upper tropospheric anticyclone

  • (a) It starts with the convergence at ITCZ in the equatorial region which leads to towering cumulonimbus cloud cluster rising high into upper troposphere.  Latent heat liberation causes mesoscale high at upper troposphere and this forces air to diverge.
  • (b) Diverging air forms a corridor where moist air and Cirrus cloud start moving to North.
  • (c) Northerly motion is subjected to Coriolis force and deflection to right forming SWly wind
  • (d) Diverging air is subjected to gain anticyclonic curvature as per vorticity theorem. But increase in Coriolis parameter keeps a check on gaining anticyclonic curvature, since absolute vortiocity is sum of relative vorticity and coriolis parameter.
  • (e) In this corridor angular momentum conservation increases the wind speed creating speed divergence allowing vertical motion in the neighboring layer immediately below and seen as cloud top temperature decreasing
  • (f) After reaching maximum latitude and becoming westerly after the formation of wind maxima it is subjected increasing Coriolis force to turn NWly. In this part wind losses westerly angular momentum and westerly component reduces causing speed convergence and weak descending motion but good enough to carry cirrus cloud with gradual evaporation and cloud base lowering. A stage comes when wind becomes weak and loses entire westerly angular momentum and starts gaining easterly angular momentum forming the anticyclonic ridge there after it returns to the equatorial ITCZ region as easterlies completing the whole anticyclonic circulation at the peripheral region.
  • (g) Over the central zone, Pressure starts adjusting to wind (geostrophic adjustment) to from high pressure and starts subsidence nearly entire central parts defined by the boundary of wind less than 25mps.

9 Mid-tropospheric anticyclones

So far in our discussion we discussed two types of anticyclones that is

  • (i) Upper tropospheric anticyclone
  • (ii) Surface anticyclone

Yet we have third type of anticyclone, which is mid tropospheric anticyclone.

  • (a) This form due to high heating over the surface. Rate of decrease of pressure over heated zone is much less than the surrounding cooler region or may be due to the release of organized latent heating column of air is kept warm creating higher pressure aloft. Same can be explained with thermal wind concept. A heat low forms due to excessive heating over the surface creating lighter air and less pressure. Cyclonic circulation can be seen with the heat low and thermal wind is anticyclonic keeping the warm zone to the right. So, the cyclonic circulation of the lower level gets changed into anticyclonic circulation at the mid troposphere. Very common feature in summer season.
  • (b) It can be part of upper tropospheric anticyclone seen up to mid troposphere.

10. From fundamental of weight

Pressure at any level is defined by the weight of the column of air above it. Meaning


If the density of the air above a level is more than the surrounding, it will give rise to high pressure at that level. In order to have o high pressure only in mid troposphere we need high density of air only in the middle atmosphere. It can be achieved in the following ways

  • (a) By cold air getting trapped in mid troposphere (say by evaporation cooling of cloud). Cold advection will cause a ribbon cold air zone giving rise to ridge rather than centralized high pressure.
  • (b) By compression/convergence to produce high density air. This can be achieved if there is a low-pressure area aloft.
  • (c) The weight of middle atmosphere should not get reflected in lower level or surface level, needs warm column or moist column beneath the high pressure.

So, with above characteristic mid tropospheric high should be zone high lapse rate facilitating convective development, especially if there is warm moist air is in the lower parts. Although mid tropospheric divergence likely to create stratified middle layer clouds.

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