DYNAMICS OF INDIA'S SOUTHWEST MONSOON
The primary driving force of monsoons is the unequal heating of the earth's surface by the Sun. Over the course of a year, the Sun's heating power is at its maximum between latitudes 30° south and 30° north, the region defined as the Tropics. Because the Earth is tilted on its axis, even the Tropics are not equally heated on a given day. The earth's orbit around the Sun, combined with the tilt in its axis, results in the changing seasons and the Sun's apparent northward and southward progression in the sky during the course of a year. The Sun's apparent alternating poleward journey reaches 23 1/2° latitude in each hemisphere before reversing itself. This boundary is referred to as the Tropic of Capricorn in the Southern Hemisphere and the Tropic of Cancer in the Northern. This alternating cycle of the Sun results in the area of maximum solar heating drifting back and forth across the Equator as summer passes from one hemisphere to the other.
The chart (below left) of long-term surface temperature vividly displays the result of solar heating over northern India and Nepal during summer. The splotches of yellow and red represent surface temperatures in excess of 36° Celsius (approaching 100° Fahrenheit). The geography of northern India, and particularly Nepal, enhances the impact of solar heating. With a specified amount of solar radiation, soil tends to reach a higher temperature than the ocean's surface and it increases in temperature at a more rapid pace. The elevation of the Himalayan Plateau is approximately 5,500 meters (approximately 18,000 feet). In atmospheric terms this is about the half-way point of our atmosphere (hence the almost universal need for oxygen on Mt. Everest). Above 18,000 feet anywhere else on Earth, the air would be quite cold.
Naturally, the air over the region of maximum solar heating tends to become relatively warmer than the air surrounding it. Over time this warm air becomes less dense and therefore more buoyant, setting the stage for air parcels to assume the tendency to rise. These rising air parcels, if supported by elements of the upper atmosphere, promote the development of a region of relatively lower sea level pressure. As air parcels rise, they also cool, and any water vapor present begins to condense forming clouds and precipitation.
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A side-by-side comparison of the long-term mean of surface temperature (Figure 2.1) with sea level pressure (Figure 2.2) during summer shows the strong correlation between maximum solar heating and areas of relatively low barometric pressure. The atmosphere strives to maintain balance, so air parcels from areas of higher pressure in the south flow northward to replace the rising parcels in the areas of lower pressure. This flow sets the stage for the southwesterly breezes associated with India's summer monsoon.
The barometric pressure gradient between northern India and the Indian Ocean, although promoting the southwesterly summer breezes evident on the chart of mean surface winds speeds (Figure 2.3), simply can't account for the stiff breezes over the Arabian Sea. The elongated area just off the coast of Somalia, shaded in orange and red, has a mean wind speed of 12 to 14 meters per second (approximately 30 miles per hour). Note how quickly the mean surface wind speed decreases as it reaches India's southwest coast.
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Chart of mean surface winds (in meters per second) for the months of June through August 1968-1996 (full image). The relatively high surface winds are clearly evident over the Arabian See. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado. |
Chart of mean winds at 850mb (in meters per second) for the months of June through August 1968-1996 (full image). The core of the Somali Low-level Jet is clearly shown by the dark red shading. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado. |
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This surface wind over the Arabian Sea is associated with a feature located approximately 1,500 meters (5,000 feet) above the earth's surface called, appropriately enough, the Somali Low-level Jet. The chart of mean wind speed at 850mb (Figure 2.4) shows that the core of the jet contains wind speeds in the neighborhood of 16 meters per second (approximately 35 miles per hour). Although the wind associated with the Somali Low-level Jet diminishes over India, wind speeds of approximately 10 meters per second (22 miles per hour) persist over much of the subcontinent.
Although there are competing theories regarding the formation of the Somali Low-level Jet, one is that the Jet forms from the interaction of wind flowing from the persistent area of high pressure at approximately 30°S latitude and the geography of the Somalian coast. Just inland from the coast, the topography of Somalia transforms into a rocky, highland plateau at an elevation of approximately 5,000 feet. In simple terms, wind from the southern hemisphere flows northwest toward Somalia and while doing so it is shifted to the right by the Coriolis force. The Somalian coast essentially creates a barrier that intensifies this shift to the right and overall wind speed. An annotated chart (Figure 2.5) showing the location of this region of high pressure and its relationship to the Somali Low-level Jet appears below.
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Chart of mean sea level pressure, in millibars, for the months of June through August 1968-1996 from NCEP. Annotations are provided to identify the Somali Low-level Jet and its influencing region of high pressure in the Southern Hemisphere. Image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado. |
The Somali Low-level jet both enhances the speed of the surface wind and delivers moist maritime air to the Indian subcontinent. If you wish to create an active Indian summer monsoon, you need an abundance of moist air, and the Somali Low-level jet certainly delivers it.