It has been observed since 1979 that there is a heterogeneous increase in the tropospheric temperature across the globe. The rise in tropospheric temperature over selected climatic zones during the period 1949-1978 is as follows – the equator +0.45°C, the north polar +0.60°C, the north subtropic +0.32°C, the south subtropic +0.77°C and the south polar region +0.66°C, recording a higher increase in the polar regions as compared to the equator. Consequently, a decrease in the tropospheric temperature gradient from equator to poles is also observed causing a weakening of the general atmospheric circulation. Further, there is a decrease in the boreal summer tropospheric temperature gradient in Tibet causing weakening of the Asia-India monsoon circulation and all-India monsoon rainfall, which during the 1999-2012 has reduced by 6.1 per cent (857mm) compared to the period 1949-1998 (913mm).
Monitoring Global Warming
The difference in the 12 month mean of the tropospheric temperature between equator and the different climatic zones has indicated a general decline in the thermal contrast from mid-1970s and more sharply in recent decades (Fig 1) – the highest being between the equator and south polar region in the past decade. In fact, the whole depth of the troposphere is warming across the globe though at different rates. In general, the southern hemisphere is warming at a faster rate than the equator and the northern hemisphere, 0.87°C and 0.32°C, respectively since 1979. Global, regional, subregional and local climatic changes are widely recognised as manifestations of global warming.
In a generalised sense monsoon is like the sea breeze, except the latter is a diurnal phenomenon confined to coastal areas, while monsoon occurs for a longer duration (limited to part of the year) over larger areas. Numerous quantitative approaches have been attempted to understand the monsoon. During summer, weather systems over the northern tropical region associated with warmer atmosphere intensify and that over northern extratropical regions associated with colder atmosphere, weaken. The monsoon is a thermally driven tropical tropospheric circulation which occurs due to seasonal poleward spreading of hydrometeorological conditions (high temperature, low mean sea level pressure, lower tropospheric convergence and upper tropospheric divergence, moist winds, clouds and rains) on either sides following north-south oscillation of the sun between Tropic of Cancer and Tropic of Capricorn. For poleward spreading of the equatorial condition increase in tropospheric temperature at the rate of 1°C per 5° latitude seems essential. During June through September, largest and most intense monsoon occurs over the Asia-Pacific region (Fig 2). During the peak period (July) of the monsoon, the global distribution of the tropospheric temperature and mean sea level pressure is as follows:
TROPOSPHERIC TEMPERATURE: Along northern boundary of the spreading, from northern Africa to China-Mongolia and western-north-western North Pacific Ocean, the tropospheric temperature is more than 1°C higher compared to that over the equator. The tropospheric temperature is lower than 12°C over north polar and 30°C over south polar region. In fact, it is the upper tropospheric temperature structure across Tibet-Turkey region that controls the monsoon intensity and rainfall activity; the role of other parameters i.e. mean sea level pressure, land surface processes, sea surface temperature (SST) and north-south thermal contrast follows the temperature structure.
MEAN SEA LEVEL PRESSURE: With respect to the equator, the mean sea level pressure (MSLP) is higher than 10mb over the North Pacific and the North Atlantic Oceans, 5 to 10 mb over the south subtropic and more than 30 mb over the south polar and lower than 5 to 10 mb over the AfroAsia landmass. Demarcation of the Asia-Pacific monsoon regime reveals that the dry and high lands of southern Asia experience intense heating, low atmospheric pressure, convergence and upwelling in the lower troposphere, and anticyclonic circulation and divergence in the upper troposphere. The troposphere in the Asian cordillera region is warmer by 5°C as compared to the equator. In the backdrop of this global tropospheric thermal setting and large contiguous landmass in north and west and vast water bodies in east and south, the cordillera acts like an elevated heat source and the area experiences a strongest troposphere upwelling. The outflows from the anticyclone are scattered in all directions and subsides over eight deep highs across the globe – six great oceanic subtropical highs (North Pacific, South Pacific, Australian, Mascarenes, St. Helena and Azores-Bermuda), north polar high and south polar high – located just outside the monsoon regime. A combination of return flows from the lower layers of these highs through a variety of courses converging into the low-pressure area produce frequent rains and rain spells across the Asia-Pacific region. As the return flows travel long distances over oceans, therefore, they are moist. Hence, the monsoon is a type of secondary atmospheric circulation that develops seasonally within the ambit of the general atmospheric circulation.
The convergences between dry airflows from north and northwest and moist airflows from east, southeast, south and southwest, and secondary convergences in the moist airflows create 18 rain-producing weather systems, e.g. cyclone, trough, meander, eddies, squall-line, channelisation, waves etc. The rain producing weather systems intensify during orographic ascent along the west coast and the Himalayas. The rainfall data spanning 200 years in presented in Fig 3. Coriolis force, diabetic heating and orographic barriers affect large-scale monsoon flows. The 18 convergences can be grouped into five types – equatorial, tropical, subtropical, temperate and polar. These are secondary circulations in respect of monsoon circulation but tertiary circulation in respect of the general atmospheric circulation. There could be quaternary circulations (meso- and mico-scale systems, eddies and thunderstorms) embedded in each of the tertiary circulations.