👉ALL PAGES- LINKS INDEX👀
Introduction-
Clearing Dilemma?
- In most
geography textbooks, it is explained that the Southern Hill Complex of
Peninsular India divides the incoming Southwest Monsoon into two
branches—
- the Arabian Sea Branch and
- the Bay of Bengal Branch.
- For
beginners, this explanation provides a convenient understanding of the monsoon,
but it represents only part of the actual atmospheric process.
- In reality,
the Southwest Monsoon's differentiation begins well before the winds reach
the Indian peninsula. As the cross-equatorial monsoon current advances over
the northern Indian Ocean, it encounters the unique geographical configuration
of -
- the Indian Peninsula,
- flanked by the Arabian Sea on the west and the Bay of
Bengal on the east. Simultaneously, the broad monsoon current responds to-
- the
large-scale cross-equatorial pressure gradient,
- the Coriolis force, and regional contrasts in
- atmospheric pressure,
- sea-surface temperature,
- evaporation,
- moisture availability, and
- wind speed over the two ocean basins.
- These
combined factors gradually organise the incoming monsoon flow into two distinct
moisture streams. Thus, the bifurcation of the Southwest Monsoon begins over
the Indian Ocean itself, long before the winds make landfall over Peninsular
India.
- At this
stage, the Southern Hill Complex assumes its decisive role.
- As the
advancing monsoon reaches the southern tip of India, this integrated mountain
system—comprising the Western Ghats, Nilgiri Hills, Anaimalai Hills, and
Palani Hills—acts as a formidable topographic barrier.
- So, rather than
generating the two branches, these mountain ranges reinforce, channel, and
ultimately complete the separation that has already begun over the Indian
Ocean. This continuous north–south alignment of the Western Ghats obstructs the
westward moisture stream, while the Nilgiri, Anaimalai, and Palani Hills
further modify and channel the airflow across southern Peninsular India.
- Therefore,
the "Southern Hill Complex" should not be regarded as the origin of the
Arabian Sea and Bay of Bengal branches, but rather as the topographic
catalyst that strengthens and finalises an atmospheric bifurcation
initiated over the northern Indian Ocean through large-scale dynamic and
regional ocean-atmosphere interactions.
- One of the
defining characteristics of the Arabian Sea Branch is that it encounters
the Western Ghats almost at right angles, thereby forcing the moisture-laden air
to ascend rapidly along the windward slopes.
- This process produces intense "orographic
rainfall", making the western coast of India, which receives 18% of total
Indian rainfall while having only 3% of India's landmass Indian territory, one
of the wettest regions in the world, while simultaneously creating a pronounced
rain-shadow effect over the Deccan Plateau, considered a second arid
land of India after the “Western India Desert”.
- In this
blog, we shall examine in detail the mechanism of the "Arabian Sea Branch of
the Southwest Monsoon" and understand why it is the stronger, faster, and
more moisture-laden branch of the Indian monsoon system.
Role of
India's Topography in Guiding the Arabian Sea Branch of the Southwest Monsoon
- Now, study the above infographics in detail and track how the movement
and inland penetration of the Arabian Sea Branch of the Southwest Monsoon
are profoundly influenced by the physiographic features of Peninsular India, piloted
by the Western Ghats, which constitute the most important topographic
barrier, stretching almost continuously along the western coast. The average
elevation of the Western Ghats, within the range of 900-16-00 meters, gradually
increases from north to south, where the southern section collectively forms the Southern Hill Complex, which represents the
highest and most formidable orographic barrier encountered by the advancing
Arabian Sea Branch.
- As we have already discussed, the bifurcation of the
Southwest Monsoon begins over the northern Indian Ocean under the
combined influence of multiple geographical factors and the configuration of
the Indian Peninsula. So, by the time the monsoon winds make landfall, two distinct
moisture streams have already begun to emerge, where the southern Hill Complex does
not create these branches; rather, it reinforces, channels, and completes
their separation.
The First
Surge- Instrumentality of Southern Hill Complex
- In the first
surge, the Arabian Sea Branch strikes the lofty mountain wall of the Southern
Hill Complex almost perpendicularly.
- As the moisture-laden air is forced to
ascend the steep windward slopes, it undergoes rapid adiabatic cooling, leading
to condensation and exceptionally heavy orographic rainfall.
Consequently, Kerala, Coastal Karnataka, Goa, and the Konkan Coast
receive some of the highest annual rainfall in India in the range of 250-350 Cm.
👀
The Second
Surge- Western Ghats “Wind Gaps”
- However, not
the entire Arabian Sea Branch is lifted over the highest mountain ranges. A
considerable portion of the airflow spreads northeastward along the Western
Ghats and penetrates the interior through several important mountain passes and
natural gaps, as shown in the following infographics.
- Among these, the Palghat Gap (Palakkad Gap) is the most
significant because of its exceptional width, allowing a substantial volume of
moist air to enter the interior of Peninsular India. Other important passes,
including-
- Thal Ghat,
- Bhor Ghat,
- Thamarassery Ghat,
- Shencottah Pass, and
- several regional passes, provide additional corridors, known as "wind gaps", through which localised branches of the monsoon penetrate the Deccan Plateau.
- These topographic
openings moderate the rain-shadow effect and distribute rainfall over parts of
the interior plateau with an average precipitation range of 40-80 cm, which are responsible for-
- mango,
- coffee,
- cardamom, and
- cashew cultivation.
Third
Surge- Inland Penetration through River Valleys corridors
- Beyond these
mountain passes, another important mechanism facilitates the inland propagation
of the Arabian Sea Branch.
- A significant portion of the remaining moist airflow
advances, propagating through “frictional force” through the east-west
oriented river valleys of the Tapi, Narmada, and Mahi, which act as natural
lowland corridors connecting the western coast with the interior of Central
India.
- The Tapi
Valley guides one branch of the monsoon towards the Betul Plateau
and the Mahadeo Hills of the Satpura Range. Owing to the combined
effects of moisture transport and orographic uplift, these regions generally
receive 100–150 cm of annual rainfall, with the southern slopes of the
Satpura Range receiving comparatively higher precipitation because the natural wind-blow direction is northeast.
- A second
branch enters the Narmada Valley, transporting moisture eastward towards
the Amarkantak Plateau, where the Vindhya, Satpura, and Maikal hill
systems converge. Although this branch contributes to moderate rainfall over
the Amarkantak region, the elevated terrain restricts further northward
movement of moisture; as a result, districts situated immediately north of the
Vindhya escarpment, including Mirzapur in Uttar Pradesh, receive
comparatively lower rainfall than the adjoining plateau regions because of the
rain-shadow influence of the Vindhyan uplands.
Note: In one of our upcoming blogs, we will discuss the reasons why we have 6 most arid districts in India.- A third
inland pathway follows the Mahi Valley, carrying moist air into western
Madhya Pradesh and southeastern Rajasthan. From here, the remaining moisture
gradually merges with the broader southwest monsoon circulation over
northwestern India before advancing towards the Indo-Ganga Plain.
- One of the important extensions of the Arabian Sea Branch reaches the "Saurashtra
Peninsula of Gujarat." Here, the advancing monsoon encounters the Gir
Hills, resulting in comparatively heavy rainfall over the southern and
southeastern parts of the peninsula. In contrast, districts such as Surendranagar
and Jamnagar, located to the north and northwest of the Gir Hills,
receive considerably lower rainfall because they lie in the local rain-shadow
region and are relatively distant from the principal moisture source.
- Further
north, the Arabian Sea Branch encounters the "Aravalli Range." Since the
Aravallis are aligned from southwest to northeast, almost parallel to
the prevailing southwest monsoon winds, they provide very little orographic
obstruction. Consequently, the moist air is not forced to ascend, and rainfall
over much of western Rajasthan remains limited. This situation is further intensified
by the extreme summer heating of the Thar Desert, which creates a strong "continental thermal low and vigorous atmospheric turbulence." This we have already discussed. For more information, click over here.
- the
parallel orientation of the Aravalli Range and
- the intense heating of the Thar
Desert significantly reduces condensation and precipitation over western Rajasthan,
allowing the monsoon winds to continue their northward movement with much of
their remaining moisture intact.
- Now, in the last leg when the
Arabian Sea Branch advances towards northwestern India, irrigation systems such
as the Indira Gandhi Canal locally increase atmospheric humidity.
Although these irrigation networks do not control the large-scale monsoon
circulation, they slightly enhance near-surface moisture availability.
- Ultimately, the Arabian Sea Branch merges with the broader southwest monsoon
system over the Ganga Plains from thereon; when reaching the Himalayan
foothills, the moist air is once again forced to ascend, producing
widespread rainfall across
- the Punjab Plains,
- the Himalayan foothills, and
- the
middle Himalayan regions, including Shimla and adjoining hill stations.
Conclusion-
- The study of the
Arabian Sea Branch advancement mechanism demonstrates that the distribution of monsoon rainfall over
India is not governed solely by atmospheric circulation but also by the
country's remarkably diverse physiography. The
- Southern Hill Complex,
- the Western Ghats,
- their passes and gaps,
- the river valleys of
the Tapi, Narmada, and Mahi, the Gir Hills,
- the Aravalli Range,
and
- finally the Himalayas together form an interconnected topographic
framework that channels, modifies, redistributes, and ultimately determines the
inland propagation of the Arabian Sea Branch from the western coast of
Peninsular India to the foothills of the Himalayas.
Comments
Post a Comment