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Edmund Halley viewed the monsoon as a glorified sea breeze circulation fueled by different heat capacities of land and ocean.

The resulting meridional temperature gradient between warmer air of the land and cooler air of the ocean fueled by heating caused by the Sun were thought to cause precipitation in the Indian subcontinent. However, there has been tremendous progress in recent times and a broad consensus has been reached on the climatology of the the Indian Summer Monsoon. Several groups of scientists are converging on the land - ocean - atmosphere coupling model as an explanation of the causes of the Indian Summer Monsoon.

What are the salient features of the new understanding of the land sea atmosphere coupling model?

The question does NOT refer to causes of the interannual variability of the Indian Summer Monsoon but the climatological causes of the Indian Summer Monsoon.

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The Indian Summer Monsoon(ISM) has several layers of complexity as it involves interactions between land, sea and atmosphere. This answer will focus on the state of the art overall understanding of the ISM and provide a summary of the salient features of the interactions between land, sea and atmosphere. In general it should be noted that while some references may talk of Land - Atmosphere interactions and Ocean - Atmosphere interactions it is almost impossible to differentiate between the role of land - atmosphere interaction and that of the ocean -atmosphere separately and the two processes should be considered to be part of the overall land - atmosphere - ocean interaction especially in the context of Asian Summer Monsoon[1]

1) Seasonal transition from boreal spring to summer causes heating of the Asian landmass faster than the surrounding oceans owing to the difference in heat capacities in land and sea leading to the establishment of a meridional temperature gradient [2]. The land retains a shorter climate "memory" than the ocean [1].

2) The presence of the Himalayas(oriented east west) and Tibetan plateau ensures that sensible heating occurs aloft as well. The implications of the elevated sensible heating are that not only is the meridional temperature gradient present at the surface but aloft as well and thereby "tethering" the monsoon onset [2] [3]

3) Formation of a surface heat low pressure over the Indian subcontinent during the late boreal spring that ensures a cross equatorial flow due to the north south pressure gradient at the low level and return cross equatorial flow aloft.

4) Thermodynamic conditions caused by the solar heating in boreal spring and early summer cause the seasonal migration of the Inter Tropical Convergence Zone also known as the Monsoon Trough(MT) [4]

5) Seasonal migration of the Mascarene High from it's December January February(DJF) position over the south eastern Indian Ocean at 85 E 30 S to June July August(JJA) position over the southern Western Indian Ocean near 55 E 30 S[5]

6) The role of East African Highlands(oriented north-south) in blocking the low level easterly flow and steering the winds across the equator into the south westerly monsoon flow. Without orographic influence of the East African Highlands the south easterly winds that blow across the equator would blow over East Africa and the Atlantic rather than being transformed into the Somali Jet[5]. A GCM simulation study in which the orographic influence of the African Highlands were removed showed that the Highlands have a huge influence on the monsoonal flow[6]

7) The Findlater Jet otherwise known as the Somali Jet gathers momentum due to the Earth's rotation [7]

8) A wind evaporation feedback mechanism is responsible for rapid onset as well as intensification [8]

9) Initial monsoon onset is attributed to sensible heating but after the monsoon has set it is latent heat that drives the monsoon.[9]

10) Interactions between extra tropical eddies and meridional overturning circulation modelled as feedback. This study is remarkable as it shows that the onset of monsoon can be modelled in the absence of surface in-homogeneities as well as hydrological cycle [10]

11) The poleward extent of the monsoonal precipitation has been attributed to the "ventilation effect". At mid latitudes westerly winds advect air with low moist static energy from the cool oceans of the continent which tends to balance incoming solar radiation [11] [12]

12) Oceans contribute by changing SSTs and the Indian Ocean sees a large surface positive heat flux during the boreal spring and summer which is balanced by net southward transport of heat across the equator and increase of heat storage across the upper ocean [13]

Finally it must be mentioned that not all of the coupled feedbacks have been fully explored in general circulation models or in observations [14]

References

[1] Yasunari, T. Role of Land - Atmosphere Interaction on Asian Monsoon climate. J.O.M. S. of Japan, Vol. 85 B, pp.55-75, 2007

[2] Li, C. F. & Yanai, M. The onset and interannual variability of the Asian summer monsoon in relation to land sea thermal contrast. J. Clim. 9, 358–375 (1996).

[3]Fasullo, J. & Webster, P. J. A hydrological definition of Indian monsoon onset and withdrawal. J. Clim. 16, 3200–3211 (2003).

[4]Prive, N. C. & Plumb, R. A. Monsoon dynamics with interactive forcing. Part I: Axisymmetric studies. J. Atmos. Sci. 64, 1417–1430 (2007).

[5]Slingo, J., Spencer, H., Hoskins, B., Berrisford, P. & Black, E. The meteorology of the western Indian Ocean, and the influence of the East African highlands. Phil. Trans. R. Soc. A 363, 25–42 (2005).

[6]Rodwell, M. J. & Hoskins, B. J. 1995 A model of the Asian Summer Monsoon. II. Crossequatorial flow and PV behavior. J. Atmos. Sci. 52, 1341–1356.

[7] Findlater, J. A major low-level air current near the Indian Ocean during northern summer: Interhemispheric transport of air in the lower troposphere over western Indian Ocean. Q. J. R. Meteorol. Soc. 96, 551–554 (1970).

[8] Boos, W. R. & Emanuel, K. A. Annual intensification of the Somali jet in a quasi-equilibrium framework: Observational composites. Q. J. R. Meteorol. Soc. 135, 319–335 (2009).

[9] Chou, C. Land-sea heating contrast in an idealized Asian summer monsoon. Clim. Dynam. 21, 11–25 (2003).

[10] Bordoni, S. & Schneider, T. Monsoons as eddy-mediated regime transitions of the tropical overturning circulation. Nature Geosci. 1, 515–519 (2008).

[11] Chou, C., Neelin, J. D. & Su, H. Ocean-atmosphere-land feedbacks in an idealized monsoon. Q. J. R. Meteorol. Soc. 127, 1869–1891 (2001)

[12] Chou, C. & Neelin, J. D. Mechanisms limiting the northward extent of the northern summer monsoons over North America, Asia, and Africa. J. Clim. 16, 406–425 (2003).

[13] Loschnigg, J. & Webster, P. J. A coupled ocean-atmosphere system of SST modulation for the Indian Ocean. J. Clim. 13, 3342–3360 (2000).

[14]Turner, A. G. and Annamalai, H. Climate change and the South Asian summer monsoon. Nature Climate Change, 2. pp. 587-595.(2012)

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