Atmospheric circulation refers to the movement of air within the Earth’s atmosphere. This movement is driven by temperature and pressure differences, which create winds and other forms of weather. The Earth’s rotation also plays a role in shaping atmospheric circulation patterns. The most well-known patterns are the Hadley cell, ferrel cell and the Polar cell. Jet streams are also the part of atmospheric circulations. These patterns are responsible for weather patterns, such as the monsoons and the trade winds.
Atmospheric circulations can be classified into three categories:
- Primary circulations: consisting of primary winds present across the globe, Ex. Trade winds
- Secondary circulations: consisting of regional winds, Ex. cyclones and monsoon.
- Tertiary circulations: which flow over very small area, also known as local winds. Ex. Loo
They are also called as primary winds or permanent winds because they remain same throughout the year and are distributed all across the globe. These winds are related to thermally and dynamically induced pressure belts and rotation of the earth.
They blow from the sub-tropical high-pressure areas towards the equatorial low pressure belt. They flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere. The trade winds from two hemispheres meet at the inter tropical convergence zone, and due to convergence, they rise and cause heavy rainfall. Their off – shore nature on the western side of the continents are one of the reasons behind formation of deserts in those areas.
They blow from the sub-tropical high – pressure belts towards the sub polar low-pressure belts. They blow from southwest to north-east in the northern hemisphere and north-west to south-east in the southern hemisphere. These winds produce wet spells and variability in weather.
They blow from the polar high-pressure areas of the sub-polar lows. The Polar easterlies are dry, cold prevailing winds blowing from north-east to south-west direction in Northern Hemisphere and south-east to north-west in Southern Hemisphere.
General circulation of Atmosphere
The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. It refers to the large-scale, global movement of air that occurs in the Earth’s atmosphere.
Factors which affect general circulation: Latitudinal variation of atmospheric heatingThe emergence and shifting of pressure belts.The distribution of continents and oceansThe rotation of the earth
SIGNIFICANCE OF PLANETARY WINDS
- Balances the heat budget by transporting the excessive heat of tropics towards poles.
- Form the dynamic pressure belts – Sub Polar low-pressure belt is formed due to convergence and upliftment of sub-tropical westerlies and Polar easterlies.
- Cyclone formation and movement – Their convergence form the fronts at sub polar low-pressure belt and thus create extra tropical cyclone. Trade winds move the Tropical cyclones from East to West.
- Regional Climate- Monsoon in Indian subcontinent is caused due to eastward shift of SE trades after crossing the equator under the effect of Coriolis force.
- Movement of oceanic currents- North and South Equatorial currents move East to West under the influence of Trade winds. Gulf stream moves toward North-east and hit the NW coast of Europe under the influence of Sub-tropical Westerlies.
- Formation of gyres- Primary winds affected by Coriolis form the circulatory motion of current thereby forming the Gyres.
- Formation of deserts- Tropical Easterlies form the desert on the western margins of the Continents as they become dry when reach there and act as offshore winds.
- Their effect on Oceanic current movement allows transport of nutrients and thrive biodiversity in form of fisheries, planktons, and corals.
- Certain studies point out that some insects move in the direction of prevailing winds.
- Dust storms: These winds Carry particles from Saharan sand and dust storms can blow across islands in the Caribbean Sea and the U.S. state of Florida.
SHIFTING OF PRESSURE BELTS
Wind belts depend on temperature, so temperature changes can move the belts and change wind patterns.
Relative position of the earth with the sun changes within a year due to earth’s revolution and thus the position of all the pressure belts except the polar high pressure belts changes with the northward and southward migration of the sun.
- At the time of summer solstice, the sun is vertical over the tropic of Cancer (June 21) and therefore all the pressure belts belt shift northward. Thus, all the wind belts associated with the said pressure belts also shift northward.
- The sun becomes vertical over the equator at the time of equinoxes hence all the pressure belts which shifted to the north occupy their normal positions.
- After this there is southward migration of the sun which becomes vertical over the tropic of Capricorn at the time of winter solstice (23 December) and hence the pressure and wind belts shift southward.
These seasonal changes in the relative positions of the pressure and wind belts introduce the following typical climatic conditions:
(i) The Mediterranean climatic regions are found in the western parts of the continents within the latitudinal zone of 30°-45° in both the hemispheres. Subtropical belt shifts towards the equator at the time of winter solstice in the northern hemisphere, consequently, the zone between 30°-40° latitude is characterized by westerlies which give much precipitation during winter season because they come from over the oceans. This is why the Mediterranean regions are characterized by dry summers and wet winters.
(ii) The regions lying between 60°-70° latitudes are characterized by two types of winds in a year because of shifting of pressure and wind belts. With the northward migration of the sun at the time of summer solstice the polar easterlies are weakened during northern summer because the westerlies extend over these areas due to northward (poleward) shifting of sub-polar low-pressure belt while the situation is quite opposite in the southern hemisphere because the polar easterlies extend over much of the areas of the westerlies due to equator-ward shifting of sub-polar low-pressure belt.
The situation is reversed at the time of winter solstice.
(iii) Monsoon climate – Due to northward migration of the sun in the northern hemisphere at the time of summer solstice the north intertropical convergence (NITC) is extended up to 30°N latitude over Indian subcontinent, south-east Asia and parts of Africa. Thus, the equatorial westerlies are also extended over the aforesaid regions.
These equatorial westerlies, in fact, become the south-west or summer monsoons. The NITC is withdrawn from over the Indian subcontinent and south-east Asia because of southward shifting of pressure and wind belts due to southward migration of the sun at the time of winter solstice.
Thus, north-east trades are re-established over the aforesaid areas. These north-east trades, in fact, are north-east or winter monsoons.