Evolution, Climate Change and Oriental Monsoon

By: Dr Nityanand Singh, Dr H N Singh and Ashwini Ranade
The evolution of Oriental Monsoon is a very complex phenomenon. Still more complex is the issue of climate change. It involves a whole range of climatic conditions which include solar radiation, temperature, pressure, wind, cloud and precipitation in a given period compared to the preceding periods and has a considerable impact on settled ecosystems.
Weather n Climate

The Earth’s atmosphere comprises of 78.08 per cent of nitrogen, 20.95 per cent of oxygen; rest are argon, carbon dioxide, water vapour and traces of various other gases. The total mass of the atmosphere is 5 x 105 metric tons, 80 per cent of which is confined to about 12 km. There is no definite boundary between the atmosphere and the outer space although the atmospheric effect is noticeable up to the altitude of 120 km. For practical purposes, the altitude of 100 km is the presumed boundary between the atmosphere and outer space, known as the Karman Line.

The following vertical layers are identified in a standard atmosphere – thickness and vertical temperature variation: troposphere (surface to 10 km; 17oC to -60˚C), stratosphere (10 to 30 km; 0˚C to -60˚C), mesosphere (30 to 80 km; 0˚C to -90˚C), thermosphere (80 to 400 km; 500˚-1500˚C to -90˚C), ionosphere (50 to 400 km) and exosphere (beyond 400 km).  Generally, atmospheric parameters display relatively stationary behaviour for a period of about 30 years; the mean of which is therefore treated as its normal value for any climatic parameter. The earth’s climate has a history extending over ~4.5 billion years.

Climate change is defined as a significant change in climatic conditions such as radiation, temperature, pressure, wind, cloud and precipitation during a specific period compared to the preceding period – capable of causing considerable impact on the settled ecosystems, its vigour and vitality. Processes in the atmosphere, oceans, cryosphere (snow cover, sea ice, continental ice sheets), biosphere and lithosphere (such as plate tectonics and volcanic activity) and certain extraterrestrial factors (such as the sun) cause the changes in climate which are gradual rather than abrupt and relative rather than absolute.

A provisional time scale to gauge climate change varies drastically from one scientific discipline to another. Therefore, to understand climate change and its impact on environment and society, background knowledge of numerous subjects such as meteorology, climatology, hydrology, geology, palaeoclimatology, palaeohydrology, anthropology, mythology, archaeology, Indology, history, economics, politics, psychology, religion, philosophy and spirituality is required.

Article 5 Figure 1
Inter-annual variation in column-area-annual mean of elected climatic parameters over the whole globe during 1949-2009. Blue curve indicates actual values, orange a third-degree polynomial smooth values and dotted black the mean value.

 

Recent Global Climatic Changes

The column area mean annual temperature of the global troposphere (1000-150 hPa; surface to ~15 km height) has increased by 0.58˚C, from -19.91 to -19.33 during 1979-2009 compared to the period 1949-2009 essentially due to increase in the net radiation balance at the surface. The mean sea level pressure (MSLP) has decreased by 0.13 mb, from 1011.69 to 1011.56 while the tropospheric thickness has increased by 33.58 m, from 13709.07 to 13742.65 and the resultant wind speed by 0.18 m/sec, from 7.29 to 7.47.

Due to warmer atmosphere, the precipitable water (PW) has declined by 0.05 mm, from 24 to 23.95. Condensed water vapour – clouds are visible from the earth’s surface as accumulations of water droplets or solid ice crystals floating in the atmosphere. From the space, clouds are visible as a white veil surrounding the planet earth. The total cloud cover (TCC) has decreased by 0.64 per cent, from 52.18 to 51.54, the high cloud cover (HCC) by 1.39 per cent, from 16.68 to 15.29 and medium cloud cover (MCC) by 1.25 per cent, from 19.26 to 18.01. However, the low cloud cover (LCC) has increased by 0.49 per cent, from 34.47 to 34.96.

Clouds continuously evolve with a variety of shape, size, distribution and appearance. Based on internal structure most of the clouds can be subdivided into mutually exclusive species, 14 in total. There is a further provision to describe the ‘supplementary features’ and ‘accessory clouds’ attached to the main body of certain clouds.  The original mass is called ‘mother-cloud’ and the extension as genitus of the mother-cloud. The global annual precipitation has decreased by 0.62 per cent, from 836.52 to 831.31 mm (Figure 1).

The Asian-Indian (Oriental) Summer Monsoon Circulation

The Asian summer monsoon is an atmospheric circulation (weather) that occurs during April through October over the region bounded by Afro-Asian landmass on the north and west, and Indo-Pacific Oceans on the east and south. The combined effect of temperature contrast between northern and southern hemisphere; between tropics and extra-tropics; between land and sea; between lower and upper troposphere; between the Tibet-Himalaya-Karakoram-Hindukush Highlands (THIKHIHILs) and the vast water bodies of the Indian and Pacific Oceans, is the main cause for the occurrence of the monsoon. During boreal summer, the temperature of the troposphere over the THIKHIHILs is ~10°C higher than that over the entire globe.  At the surface two ‘heat lows’, one over the Middle East and another over China-Mongolia known as the Asian Continental Low (ACL) develop. Intense large-scale low-level converge occurs over these lows and the rising airs are aligned to diverge from the upper tropospheric anticyclone over the THIKHIHILs. The outflows from the THIKHIHILs anticyclone are directed in varied proportion to subside over the eight deep highs: North and South polar, North and South Pacific, Australian, Mascarene, Marina and Azores-Bermuda (Figure 2).

Article 5 Figure 2
The Centres of Action (COA) invoked during active oriental monsoon circulation.

 

Location of the different COAs are approximate in an attempt to adjust them on a small map.

The divergence from lower layers of the deep highs flow as return current through a large variety of meander courses and converge into the ‘heat low(s)’ in the Afro-Asian dry province. The seasonally occurring, large-scale lower tropospheric converging air with imbedded various secondary circulations produce frequent rains/rain-spells over the Asia-Pacific region, popularly known an Asian summer monsoon circulation. The coriolis force, orography and diabetic heating affect large-scale monsoon flow.

The Asian summer monsoon can be divided into six interconnected monsoons: Indo-China peninsula, South China Sea, South Asian (Indian), East Tibet Plateau, East Asian (south China, lower Yangtze River) and Japan and Northeast Asian (north China and Korea) monsoon. Rainfall across India occurs through the following six convection-convergence processes: off shore trough along the west coast; trough in the low level westerlies along the east coast; line-cum-eddy convergence over the Indo-Gangetic plains; cyclonic convergence over head Bay of Bengal; cyclonic convergence over Gujarat and south Rajasthan; and, line convergence between extratropical northwesterly and southwesterly monsoon flow over extreme north India and northwestern Pakistan.

Relatively a small portion of the outflows from the THIKHIHILs anticyclone is directed towards the North polar high, the Azores-Bermuda High, the Marina High and the South polar high. The occurrence of the secondary circulations is highly dependent on:

  • intensity of the ‘heat low(s)’;
  • lower level convergence and rising motion over ‘heat low(s)’;
  • outflows from the THIKHIHILs upper troposphere anticyclone directed towards the eight deep highs across the globe; and

return flows from the lower layers of the deep highs through a variety of pathways converging into the ‘heat low(s)’.

Article 5 Figure 3
Inter-annual variation in column-area-JJAS (June-September) mean of selected climatic parameters over India during 1949-2009. Blue indicates actual values, orange a third-degree polynomial smooth values and dotted black the mean value.

 

Recent Monsoon Changes Over India

During June through September, the net surface radiation balance over India has increased over the years by 1.2 W/m2, from 148.1 to 149.29. The temperature of the troposphere has also increased by 0.19°C, from -8.93 to -8.74 and the thickness by 8.36 m, from 14311.55 to 14319.92, but the column-area mean resultant tropospheric wind decreased by 0.24 m/sec, from 1.53 to 1.29. Because of these and other changes in the atmospheric conditions, the monsoon rainfall has lessened by 2.47 per cent (from 948.53 to 925.12 mm) (Figure 3).

During monsoon period, the troposphere over the THIKHIHILs is thickest (14243.2 m) which provides a reliable measure of intensity of Indian monsoon rainfall. The troposphere thickness gradient (TTG) from the THIKHIHILs to the four subtropical highs over North Pacific, South Pacific, Australian and Mascarene Highs provide a still better measure of the monsoon rainfall.

During global warming (1979-2009), the troposphere over the deep highs has stretched by 22.8 m compared to the preceding cooler period (1949-1978). This resulted in the decline of the TTG from the THIKHIHILs to different deep highs. The correlation between the TTG and all India monsoon rainfall has thus weakened from 0.7 to 0.42. It is important to note that the tropospheric temperature over the oceanic and polar region has increased by 0.54°C, but over the Afro-Eurasian dry provinces the increase is a mild 0.02°C. In a nutshell, it may be said that the drivers of the general atmospheric and Asia-India monsoon circulations have weakened during the recent global warming epoch.

 

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Implications of Atmospheric Changes on Monsoon Circulation and Rainfall

In case of global warming, the spatial domain of Asian monsoon circulation would expand over Afro-Asian landmass, particularly during July-August; the Tibetan anticyclone would split, one would overlay the ‘heat low’ over Rajasthan-Middle East sector and the other over Asian Continental Low (ACL) over China-Mongolia sector; frequency and intensity of monsoon depression over head Bay of Bengal and line-cum-eddy convergence zone Indo-Gangetic Plains would decline; and Monsoon rainfall would increase over northwest India and eastern China and Korea, and decrease over central India and Indo-Gangetic plains.

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