Saturday, 6 June 2015

Geography : Series 1

I will be writing articles for geography (GS and optionals) ,both from prelims and mains point of view. This is the first of the series of articles to be published in this column. It includes the basic things one has to know before understanding the concept of Monsoon in the Indian Sub-continent. Hope it is easy to comprehend and informative. 

Index:

1. Jetstreams (GS and Optionals)

2. Pressure system

3. ITCZ

4. Geographical features


Jetstreams

-     Narrow but swift moving streams of air sheering in the upper troposphere between 6-16 km
-     Formed at the boundary of two different air masses due to difference in their temperatures and consequent pressure difference
-     Width = few hundred km
-     Depth = around 5 km
-     At a velocity of upto 200mph
-     Following a meandering path
-     In a westerly (west to east) direction


Characteristics of Jet Streams
-     The greater the difference in temperature, the faster the wind velocity inside  the jet stream & velocity decreases with greater meandering
-     A Jetstream includes both clockwise and anti-clockwise circulations
o   Warm core anticyclones (due to warm tropical/sub-tropical airmass) and cold core cyclonic conditions (due to polar/sub-polar cold airmass) are found within the jets in upper troposphere
o   This is in contrast to the warm core cyclones (Low pressure) and cold core anticyclones (High pressure) at the Earth’s surface.



-     Jetstreams carry cyclonic vortices and move them along their path of flow
-     They help in meridional transfer of energy ie. Exchange of heat through meandering

Index cycle ( For Geography optionals)

It refers to the successive stages of growth, development and decay of jetstreams. They undergo changes in their velocity and other characteristics, represented by the index cycle. It has 4 stages :


Stage 1:
-     jet stream lies quite close to the Polar Regions
-     air mass exchange between the temperate and tropical regions is at its minimum
-     North-south pressure gradient is relatively steeper
-     High velocity
-     This is called the High zonal index of jetstreams
Stage 2 :
-      amplitude of jet stream waves increases.
-    The whole of the jet moves towards the equator
-     So there is an inroad of cold polar air southward.
-     The warm air masses from lower latitudes move towards higher latitudes.
-     Velocity less than the previous stage



Stage 3 :
-      bends in the jet stream become sharper.
-     amplitude of waves increases further.
-     tropical air masses move farther north, and the cold polar air moves farther south.
-   the jet stream is positioned nearer the equator and the exchange of tropical and polar air masses takes place on a much larger scale.
-     The temperature gradient is directed from east to west.
-     At this stage, they are called Rossby waves as they gradually meander.
Stage 4:
-     the giant size meanders of the jet are cut off from the main stream.
-     an immense pool of cold and dense polar air is isolated
-     in the upper troposphere of the lower latitudes where it is encircled by entirely different air masses.
-     In the upper atmosphere of higher latitudes the tropical air masses are entrapped by the colder air.
-     This is called the low zonal index of the jet stream.
-     The zonal character of the upper-level westerlies is no longer in existence.
-     They are fragmented into a number of cells.




When the Waves are well developed and cover a wide range of latitude they are said to have a low zonal index - which leads to the formation of ridges of blocking, high pressure systems and dry stable conditions. When they are almost straight and cover a narrow zone of latitude they are said to have  a high zonal index - which leads to a succession of low pressure systems and unsettled weather. 
The waves evolve then they straighten up and then meanders form again in an endless cycle.





Types :

1.     Polar front Jetstream (PFJ)

-     Formed at the polar front near the sub-polar low pressure belt between the latitudes of 50 and 60 degrees north and south of equator
-     Due to convergence of the polar cold air mass and the subtropical warm air mass
-     It sheers with high velocity due to high temperature and pressure gradient
-     Flows in westerly direction
-     It shifts position with apparent movement of the Sun







2.     Subtropical westerly Jetstream (STWJ)

-    Formed near the subtropical high pressure belt between 20 and 30 degrees north and south of equator
-     Due to convergence of the sub-polar cold air mass and the tropical warm air mass
-    It is weaker than PFJ and flows with a comparatively low velocity due to low temperature and pressure gradient
-     Flows in westerly direction
-     It also shifts position with apparent movement of the Sun.


3.     Tropical easterly Jetstream (TEJ)

-     It is an upper tropospheric easterly jet (east to west)
-     It extends across the tropics from the eastern Indian Ocean to western Africa and the actual flow direction is north-east to south-west
-     The latitudinal extent is approximately 20-30 degrees north of equator
-     Maximum wind speeds found between 5-10°N from southern India toward the east coast of Africa
-     It is found on the southern periphery of the anticyclone atop the Tibetan plateau (Tibetan high) associated with the Indian monsoon.
-     The jet becomes established once the monsoon starts and decays once the monsoon it ends for the season
-     So it is a salient feature of the tropics only during northern hemisphere summer 


4.     Findlater/ Somali Jetstream

-     It is a narrow lower atmospheric jet that develops during the Southwest monsoon
-     blows diagonally across the Indian Ocean, parallel to the coasts of Somalia and Oman
-     develops due to the temperature contrast and the consequent pressure difference between the warm air mass over the Somali oceanic current and the overlying cold airmass
-     Somali oceanic current shows seasonal reversal of direction 


5.     Polar night Jetstream

-     This jet meanders through the upper stratosphere over the poles.
-     It occurs only during the long winter night
-     The horizontal temperature gradient is strongly established between the equator and the pole, and the pressure gradient creates this westerly jet.
-     the jet is intermittent when temperature gradient fades during late winter



Pressure Belts (relevant in Indian Monsoon)

Equatorial Low Pressure Belt

-     extends from 5 to 8 degree North and South of Equator
-     Equitorial low pressure is of thermal origin
-     Due to the vertical rays of the sun and intense 
      heating, air expands and rises as convection 
      current causing a low pressure to develop.




Mascarene high pressure belt

-     Is a high pressure system found in the subtropical high pressure (STHP) zone at around 30 degrees south of equator
-     During northern summer, the dynamically formed STHP belt is strengthened to form a continuous belt stretching from the west coast of Australia to the east of Africa near Madagascar, across the Indian ocean




Inter-tropical Convergence Zone (ITCZ)

-     Extends from area encircling the earth near the equator 
-     Due to the meeting northeast and southeast trade winds from the northern and southern hemisphere respectively, and the consequent convective activity (upward movement)
-     ITCZ is of dynamic origin
-     It is less wide than the equatorial low pressure belt
-    It shifts its position with the apparent movement of Sun with respect to equator, like the other pressure belts
-     This low pressure belt is also called as doldrums, as it is a zone of total calm without any wind component
-     When it lies near the equator, it is called the near-equatorial trough.
-     Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough

Geographical features

1.     Himalayas
-     They act as an effective barrier to intercept the monsoon winds and confine them within the sub-continent
-     In the absence of the Himalayan system, monsoon winds will move far into the interior of Asia, without causing any rain

2.     Tibetan Plateau
-     Plays a crucial role in initiating the Monsoon circulation over the Indian subcontinent.
-     The summer time heating of the Plateau is the most important factor in the causation and maintenance of the Monsoonal circulation.  
-     It acts as a physical barrier
o   In the mid October, when the Plateau becomes very cold, proves to be the most important factor in causing the advance of the STWJ (westerly jet) south of the Himalayas by bifurcating it into two parts.
-     It acts as a high level heat source.
o   The summer time heating of the Tibetan Plateau makes it a high level heat source.
o   This heat engine produces a thermal anticyclone over this region during summer in the middle part of troposphere.
o   Winds coming out of this anticyclone in a clockwise movement over plateau give birth to tropical easterly jet streams (TEJ) which flow from East to West on the southern side of this upper air anticyclone.

3.  Western Ghats
-     They are close to the west coast of Indian peninsula
-    Provide an effective orographic barrier for the ascending monsoon winds laden with moisture from the surrounding oceans
-     Windward side receives 250-300 mm annual rainfall

4.     Indian Peninsula
-     Surrounded by oceans on three sides, there is a constant and adequate supply of moisture for the monsoon winds
-     They remain moisture laden upto large distances in the interior of the sub-continental landmass

5.     Eastern Ghats
-     For North-east Monsoon, they play the role of orographic barrier.



Books Referred:-

            1. Physical Geography by Savindra Singh
2. India : A Comprehensive Geography 

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