Monsoon CPR Paper
by
The Professor
Introduction
The Indian monsoon is a weather phenomena that results in heavy rains during the summer months in most parts of India and many parts of SouthEast Asia.Causes of the Indian monsoon are explained as due to movements of the ITCZ (intertropical convergence zone) [:(Cmnt by william prothero: The ITCZ is an interesting part of the picture, but the wind directions from the data include this effect. ):] and differences in temperature on land and adjacent ocean region caused by uneven heating from the sun. In the summer, the land temperature rises above that of the surrounding ocean. The rising air draws in moist air from the ocean. This results in high winds and storms with large amounts of rainfall. In the winter, the land cools to a lower temperature than the surrounding ocean. So, the winds flow from the land to the ocean, and because there is little moisture in the air, there is very little rainfall.
The Indian economy and agriculture has grown to depend on a yearly monsoon and if it fails, the population suffers.
This paper will describe the effects of the Indian monsoon and show data that illustrates its causes and effects on the Indian sub-continent [:(Cmnt by william prothero: This is a quite adequate introduction. The importance of the monsoon is at least mentioned and the purpose of the paper is clearly described.):] .
Methods
The data for this paper will come from various web sites that display regional and global climate data. Some of the data will be "climatology" where the parameter is averaged over many years to suppress the effect of yearly variations and show major recurring effects. Data that is pertinent to this study is rainfall, temperature, wind speeds, and atmospheric pressure.The data are archived at IRI/LDEO Climate Data Library. at the Lamont Doherty research laboratories, affiliated with Columbia University. It numerous compilations of weather and temperature data acqired from individual weather stations, ship reports, and sattelite observations. Individual data sets will be noted when images are displayed [:(Cmnt by william prothero: The datat descriptions tell where the data come from and how it is collected, but it is a bit sparse on the details, which is ok for this class.):] .
Observations
{link: Mon_MapGd3.jpg}Figure 1 is a location map showing the study region. {link: Mon_MapRgn2.jpg}Figure 2 is a more detailed map showing the study region. {link: Mon_YrlyPrecip.jpg}Figure 3 shows a plot of the monthly rainfall, averaged over the years 1961 to 1990, for point A indicated in figure 2. The data are accessible in references 1 and 2. {link: Mon_YrlyTemp.jpg}Figure 4 shows a plot of monthly averaged temperature at the same location as figure 3.Figure 3 shows that the precipitation peaks in the late summer and is a minimum during the winter months. Figure 4 shows that the rainfall is highest when the temperature at location A is highest, as well. For that reason, I have chosen to display the regional precipitation and winds for the months of February and August to contrast the two seasonal regimes.
{link: Mon_PrcpRgnFeb.jpg}Figure 5 and {link: Mon_PrcpRgnAug.jpg}figure 6 show the precipitation in February and in August. The darker green color of the figure indicates higher precipitation. In the winter it is below 50 mm/month and at its peak, the precipitation reaches 350 mm/month in some locations. That figure is consistent with the plot of figure 2. Note that, from figure 6, the high rainfall extends East over Indonesia, Southern China and Korea.
The next piece of data in the puzzle is the wind data. {link: Mon_WindRgnFeb.jpg}Figure 7 and {link: Mon_WindRgnAug.jpg}figure 8 show the wind vectors for the region for the months of February and August. Notice that the wind direction is from the west in the monsoon season of August, but the wind is very weak and from the east in February.
The final data parameter that helps explain the Indian monsoon is the atmospheric pressure. {link: Mon_PresFeb2.jpg}Figure 9 shows the February averaged pressure at about sea level and {link: Mon_PresAug2.jpg}figure 10 shows the August averaged pressure at about sea leve [:(Cmnt by william prothero: In this section, we see the most important data, which is temperature, winds, precipitation, and atmospheric pressure, which all are part of the monsoon "picture."):] l.
Interpretations
The monsoon has high precipitation in the summer, caused by moist winds blowing from the sea, dropping their moisture on the monsoon region. In the winter, the wind direction changes and the rainfall is much less. The winds are caused by variations in surface atmospheric pressure. Winds tend to flow toward low pressures and away from high pressures, and are also influenced by the Coriolis effect. The low pressure region in the north is caused by summer heating of land regions, which causes air to rise, creating a low pressure region. In the winter, the land area cools more than the surrounding sea surface, so the winds tend to flow seaward (south). So, in this case, winds flow towards the region with higher surface temperatures than the surrounding regions.{link: Mon_WntrMdl.jpg}Figure 11 shows a sketch of the situation in the winter. It shows the cool temperatures in the winter, which creates dense air that flows outward from the resulting high pressure region. This air has already lost its moisture and is dry, so the rainfall over India is low.
{link: Mon_SumMdl.jpg}Figure 12 shows a sketch of the situation in the summer. The heating on land causes the air to rise, creating a low pressure region that brings air in from the surrounding regions. This air has blown over the sea surface, so is moist, and as it flows over India, it drops its moisture as rain.
The model and actual data correspond quite well. The switch in air pressure, change in wind direction, and precipitation amounts between summer and winter are shown in figures 3 through 10.
So, we have a model that illustrates that the Indian Monsoon is caused by the shift in wind direction, which is caused by changes in land temperatures in winter and summer [:(Cmnt by william prothero: Notice that there are descriptions of how the data support the explanations of why the monsoon occurs. The data are described and a few interesting facts are pointed out. The model figure also contains the most important features of the processes in the monsoon.):] .
References
1. Lamont data seb site:
Figure 1. World map showing the region of focus for this study. {fig: Mon_MapGd3.jpg}
Figure 2. Regional map of the study area. Point A indicates where the temperature series of figure 3 is located. {fig: Mon_MapRgn2.jpg}
Figure 3. Average monthly precipitation for the years 1961-1990. {fig: Mon_YrlyPrecip.jpg}
Figure 4. Averaged monthly temperature for the years 1961-1990 for point A in figure 2. {fig: Mon_YrlyTemp.jpg}
Fiugre 5. August precipitation for the study region. {fig: Mon_PrcpRgnFeb.jpg}
Figure 6. August precipitation for the study region. {fig: Mon_PrcpRgnAug.jpg}
Figure 7. Wind vectors for the month of February, in the study region. {fig: Mon_WindRgnFeb.jpg}
Figure 8. Average winds for the month of August, in the study region. {fig: Mon_WindRgnAug.jpg}
Figure 9. February averaged pressure. This shows that the higher pressures are in the north, and so wind velocities are expected to be in a southward direction. {fig: Mon_PresFeb2.jpg}
Figure 10. August averaged atmospheric pressure showing that the lower pressures are in the north. So, the expected wind direction is from south to north. {fig: Mon_PresAug2.jpg}
Figure 11. Winter situation. Cool temperatures over the Tibetan Plateau and northern Asia create a high pressure region relative to the surrounding sea. Dry winds blow over the Indian subcontinent and the rainfall is low. {fig: Mon_WntrMdl.jpg}
Figure 12. Moist summer winds flow toward the low pressure region north of India, causing high rainfall. {fig: Mon_SumMdl.jpg}