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PROJECT 2 |
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THE 2006 MONSOON SEASON IN EAST CENTRAL INDIA
A PARADE OF DEPRESSIONS
by Philip Lutzak – October 2006
INTRODUCTION
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The word monsoon is familiar to people all over the world. Originally from the Arabic mausim, meaning season, its true meteorological meaning is a seasonal shift of wind, with the shift causing a change from dry to rainy or vice versa. Although the dry period brought in by the winter monsoon is also part of the monsoon, in this report we will use the term to refer to the wet period that arrives in the summer months in India. Although many areas of the world experience monsoons, the subcontinent of India has the distinction of having the most pronounced monsoon in the world. It is considered a gift from the gods by most of the Indian people because of its much needed rains that sustain their crops each year, and many cities and villages hold annual ceremonies to welcome its rains. But in years where it is excessive, it can produce severe flooding, property damage and casualties. This is exactly what happened in the Indian east coastal state of Orissa, on the Bay of Bengal, in 2006, when the monsoon arrived in a normal fashion to much fanfare and appreciation, only to continue at an accelerating pace until it became a disaster (figure 1 left). With its rains destroying their crops and washing away or submerging whole villages and their inhabitants, the monsoon at Orissa killed and injured hundreds of people and many thousands of livestock as well, proving that too much of a good thing can be as disastrous as too little. In this report I will focus on the factors that make up the normal development of the Indian monsoon and why the 2006 season unfolded differently for the state of Orissa. Because they experienced some of the worst flooding of the 2006 season, and they are in the center of the only state on the eastern coast that had above normal rainfall, I will focus on the coastal city of Bhubaneshwar, where the weather records and reports can give us a clear picture of the differences between a "normal" monsoon and the unwelcome deluge that occurred there. |
I will concentrate on the extended period of flooding rains that occurred from August 11th to the 18th, during which two tropical depressions passed over the city, and the first serious flooding began. As I present the normal statistics for an average monsoon year, I will also include the corresponding values for our period of study as a comparison. Before we delve into the origins of the monsoon and the reasons it was so intense at Bhubaneshwar and Orissa, let's begin with an overview of their 2006 monsoon season.
THE 2006 SEASON
THE MONSOON ARRIVAL & TOTAL RAINFALL AT ORISSA: Figure 2 below left is a chart showing the expected and actual arrival times of the Indian monsoon in 2006. We can see by the dashed lines how the monsoon usually starts in the southeast between May 25th and June 1st, covering all of south and east India by June 10th. From there it spreads north and west, covering all of India by the end of June except for the far northwest and Pakistan, which may not see its arrival until late July. In 2006 (red lines), the monsoon came early to all of southern and eastern India, while spreading to the rest of the nation and Pakistan closer to the normal times. It should be noted here that there were some rains in mid May at Bhubaneshwar, but generally speaking, it is the onset of very heavy, persistent rains that is considered the true beginning of the monsoon. In Orissa State and Bhubaneshwar, it began in earnest on June 3rd, a few days ahead of the normal date of June 7th. Figure 3 below right shows graphically the 2006 monsoon season results as of September 30, 2006. Note that Orissa state is colored in blue, denoting excessive totals, and indicating at least 20% above normal rainfall for the season. One noticeable aspect is that Orissa was the only east coastal state that reported above normal rainfall.
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| Figure 2. Chart showing the average start date of the Indian monsoon (dashed lines) along with the actual 2006 start date (red lines). Courtesy India Meteorological Department. | Figure 3. Chart showing the rainfall totals for the 2006 Indian monsoon. States colored in blue were the most above normal, with at least 20% or more above normal rainfall. Note that Orissa state was the only above normal state in eastern India. Courtesy India Meteorological Department. |
Looking at the chart of total rainfall at Bhubaneshwar for the 2006 monsoon season, we can see that when the season started in early June, it got going in a hurry. At that point 5 inches of rain fell in one day on June 3rd, giving the city a surplus, and as the summer wore on, they accrued an ever-increasing surplus for the rest of the 2006 monsoon season. The city remained well above normal, winding up with 19.7 inches (or 24%) more rain than average by mid-October. The entire state of Orissa and nearby areas also wound up with well above normal precipitation, with anywhere from 16% to 24% above the seasonal norms.
Although there were breaks in the rain in mid-June and mid-July, the month of August had many days of .5 to 4 inch daily rainfalls, virtually without a break, with one particularly large onslaught of heavy rains occurring in mid-month, from the 11th to the 18th. Although there were higher individual totals on other days, this period had so much rain that massive flooding began. As we will show in this report, all of the factors which can lead to a strong monsoon at Orissa were present in 2006, but it appears that the primary reason for the well above normal rains was the way the axis of the monsoon trough set up just north of the state of Orissa and allowed the center of a higher than normal amount of monsoon depressions and low pressure areas to pass over or just north of them.
THE ORIGINS OF THE MONSOON
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THE "GIANT SEA BREEZE" The monsoon that arrives in India every summer is primarily driven by the uneven heating of the land over India and nearby Asia and the ocean water surrounding it. It is quite similar to the sea breeze at any sea shore during a summer day, which is driven by the air over the land becoming much warmer than the air over the sea. Since water has a much higher heat capacity than land, it retains much more of the heat from incoming solar radiation than the land; the land radiates a lot more heat back outward to the overlying air than the sea. This heated air over the land then rises, leaving less mass, or lower pressure, over the beach surface below. The relatively cooler, higher pressure air over the cooler sea then rushes onshore to fill in this area of lower pressure over the land, causing the sea breeze. In the diagram of the sea breeze in figure 5 at right, note how the reverse flow takes place in the upper air, from the land to the sea, thus maintaining a constant circulation during the day. Although the basic mechanism is similar in the Indian monsoon, it should be no surprise that it is more complex, since the "giant sea breeze" of the monsoon occurs over months rather than just a day, and covers a very large land area. The following sections explain the main players in this annual "giant sea breeze" and how they worked out differently at Bhubaneshwar and Orissa in 2006.
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THE ITCZ: The Intertropical Convergence Zone (ITCZ), also known as the Global Equatorial Trough, is a belt of persistent clouds, showers, and thunderstorms that circles the earth at or near the equator. It occurs in the area where the trade winds of the northern and southern hemispheres converge; above this area there is upper level divergence and the release of latent heat of condensation, producing an area of low pressure elongated from east to west that we call a low pressure trough, or simply trough. As the sun's most direct rays move north of the equator in the northern hemisphere summer, this trough, with its band of clouds and showers, follows northward behind it. Likewise it travels south of the equator as the sun's most direct rays move south in the southern hemisphere summer. One can picture how, in areas near the equator (i.e. the Tropics) the ITCZ can cause a wet season as it approaches in the summer and a dry season as it moves away during the winter. Below in figure 6 is a chart showing the global climatological normal wind directions and rainfall during the summer (top - JJA) and winter (bottom - DJF) seasons. The ITCZ, with its classic band of heavy precipitation, is easily recognizable. Also note how the typical wind direction over India in the summer is west or southwesterly, from the ocean onto the land (wet) and becomes northerly, from the land outward to the ocean (dry) in the winter. This chart makes it easier to see that India experiences the most radical shift in precipitation from summer to winter of any significant, large land area in the world. In this typical January satellite image of the eastern hemisphere, we can see cloud-free India in its dry season, with the ITCZ's band of clouds well south of India.
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| Figure 6. The average position of the ITCZ in June, July & August (JJA) and December, January & February (DJF) is easily discernible in this chart. The streamlines are average wind directions at 850hPa or about 5,000 feet, and the colored areas represent total average precipitation amounts for the period. Courtesy UCLA Atmospheric Dept and CMAP. |
India experiences the effects of the ITCZ migration more severely than other land areas of the world because of very specific atmospheric conditions that develop every spring to enhance the ITCZ's influence, with origins from south of the equator all the way up to the great Tibetan Mountains on India's northern border.
Note: In many of the following charts, as a point of reference, I have denoted the position of Bhubaneshwar with an open circle.
THE MONSOON FLOW
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| Figure 7a. Surface temperatures over India from May 1st to 15th, 2006. Note the corridor of intense heat over the interior. Courtesy ESRL. | Figure 7b. Surface pressures over India from May 1st to 15th, 2006. Note the low pressure zone extending northwestward from Orissa state, corresponding to the heat over the interior. Courtesy ESRL. |
THE LOW-LEVEL HEAT SOURCE: As Spring begins and the dry westerly winds begin to retreat from their position over India, a subtropical high pressure ridge begins to build in from the west over north and central India at the surface and at 500mb, and with the accompanying relatively dry, relatively cloud-free air, the land begins to heat up rapidly under the strong April and May sun. In this increasingly hot, buoyant air, areas of relatively low pressure called "heat lows" start to form. In fact, in many parts of India April and May are the hottest months of the year, since the clouds and rain that come thereafter will keep temperatures lower. Figure 7a above shows the intense heat that set up over interior India in the first half of May 2006, and figure 7b shows the resultant areas of low pressure that arose with them. This is an important part of the setup for our giant "sea breeze", and many monsoon forecasters look at the intensity of the temperatures in Spring to get a rough gauge of how strong and how early the summer monsoon will be.
THE HIMALAYAN PLATEAU - A HIGH LEVEL HEAT SOURCE: As the Spring wears on and the incoming solar radiation intensifies over the Himalayan Plateau, this giant table of land in the sky begins to absorb more heat than the cooler free air surrounding it. As this large land area at 500mb (roughly 18,000 ft) heats up, it becomes what meteorologists call a high level heat source, as this chart of 500mb average temperatures in July illustrates. So we now have a large area of rising air over northern India and southern Asia acting as a further heat source that encourages air flow onto the Indian subcontinent. As far as the total "sea breeze" contribution to the Indian monsoon is concerned, most meteorologists consider this heat source as the primary one and the previously noted heat lows over interior India as secondary.
LOW-LEVEL MOISTURE:
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Figure 8a. Average DJF dewpoints for India. Dewpoints over 20 degrees Celsius represent very warm, humid air. Courtesy ESRL. Larger Version. |
Figure 8b. Average JJA dewpoints for India. Dewpoints over 20 degrees Celsius represent very warm, humid air. Courtesy ESRL. Larger Version. |
Figure 8c. Dewpoints for August 11-18, 2006 over India. Note the very humid air over Orissa state and Bhubaneshwar. Courtesy ESRL. Larger Version. |
At the same time that the land has begun to heat up in Spring, so does the Indian Ocean water surrounding the land. The northward progression of very warm water temperatures brings very humid air with it, causing dewpoints to rise significantly. Figures 8a and 8b above show the average dewpoints over India in winter (DJF) and summer (JJA), respectively. Note how dry the air is over
India in January, but how in the summer season there is usually a major encroachment of humid air onto the subcontinent from both the Arabian Sea and the Bay of Bengal. This is another clear indicator of the great reversal of India's air flow from offshore to inland. In figure 8C are the dewpoint values observed during the very wet period from August 11-18, 2006 over India. The high values along the east coast at Orissa are one clue that the monsoon season there was quite robust.
OUTGOING LONGWAVE RADIATION:
Another indicator of where precipitation is or has been falling is Outgoing Longwave Radiation, or OLR. Warmer areas on the earth will emit more OLR, while colder areas will emit much less. High cloud tops are basically black bodies to the frequencies of OLR that satellites measure, so that they will show much lower amounts of OLR than cloud-free land and ocean surfaces. Higher cloud tops are also associated with the tall thunderstorms and showers that produce heavy precipitation. Thus we can use values of OLR to tell us where the earth is (or has been) relatively cloud free but also where there is a prevalence of clouds and precipitation present. This chart of OLR over India for August 11th thru 18th, 2006, shows quite low OLR values over the north Bay of Bengal and east central India, an indication that thunderstorms and very heavy rains occurred that week over Orissa state. Also notice that states to their north & south had higher OLR readings, which tells us that they had less clouds and precipitation than Orissa.
THE PRIME PLAYERS AT INDIA'S EAST COAST:
THE MONSOON TROUGH & MONSOON DEPRESSIONS
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Figure 10a. The mean position of the ITCZ in January, annotated by the dashed line. Notice the distinct OFFSHORE wind flow over India (arrow directions.) Courtesy Penn State Meteorology Dept and ESRL. |
Figure 10b. The mean position of the ITCZ in July (portion over India is the Monsoon Trough), annotated by the dashed line. Notice the distinct ONSHORE wind flow over India (arrow directions.) Courtesy Penn State Meteorology Dept and ESRL. |
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MONSOON TROUGH: As the northern hemisphere heats up in the Spring and the ITCZ shifts northward, it eventually settles in along northern India, from the Bay of Bengal northwestward to northwest India and Pakistan, just south of the Himalayas. Note how far north of the equator it has moved from January to July, as seen in figures 10a and 10b above. The portion of the ITCZ lying over India and nearby countries in summer is known as the monsoon trough. The southeasterly winds that originally blew on the southern side of this trough start to blow more from the south and southwest due to the coriolis force (which turns winds towards the right in the northern hemisphere), bringing moisture in from the Arabian sea and Bay of Bengal over most of the subcontinent. Also refer back to the precipitation patterns in figure 6 to see how well they line up with the trough positions in these figures. In figure 10c at right we can see how the monsoon trough was configured in August of 2006, with the main axis of the trough quite close to and just slightly north of Bhubaneshwar. This is a favorable position for heavy rain in the area. The monsoon trough is very critical to the amount of rain that falls in a particular season, especially in the east and north of India, as most of the low pressure areas that bring their tremendous amounts of rain to India ride along it. Now that we are familiar with the monsoon trough, we can follow the major player that completes the picture of the monsoon in eastern India.
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Figure 10c. Mean Sea Level Pressures August 2006, with the mean position of the Monsoon Trough annotated by the dashed line. Compare to figure 10b and note the southward dip over the Bay of Bengal. Courtesy ESRL. |
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MONSOON DEPRESSIONS: A critical feature of the Indian monsoon at the east coast is the monsoon depression. These synoptic-scale low pressure areas are similar in size, intensity and appearance to warm core tropical depressions that form into tropical cyclones all over the world, but because they are mainly cold core systems, forming through a combination of primarily horizontal temperature differences (cold core) and secondarily from surface heat and moisture energy fluxes (warm core), they are more similar in structure to subtropical cyclones. Monsoon depressions also have closed height centers up to about 400mb, and a signature vorticity maximum (vort max) at 500mb. These depressions usually form over the Bay of Bengal and then move westward along the monsoon trough, often originating from low pressure areas within the monsoon trough that get an energizing boost from the divergence of an upper level 500mb shortwave trough or even the right entrance or left exit region of a jet streak. They also can develop from tropical lows such as decaying typhoons that have migrated overland from the western Pacific. Although they can form into a tropical cyclone, it is a rare event, since they don't have a very large expanse of water to travel over before they hit the coast of eastern India. But one did form in the 2006 season, crossing Orissa in early July, and is shown in figure 11 on the left (more). An average of four to six monsoon depressions form over the Bay of Bengal each season, with one or two forming over the Arabian Sea or inland as well. It is also important to note that because these depressions usually exhibit a tilt in 500mb heights towards the south and southwest in the upper levels, their heaviest rainfall usually falls to the south and southwest of the center, where the colder upper air will encourage stronger convection. |
Returning to figures 10b and 10c for a moment, notice the difference in the orientation of the monsoon trough during the August 11-18th period (figure 10c) as opposed to the long-term summer normal (figure 10b). Notice the southward dip in the monsoon trough over the northern Bay of Bengal. I believe that this was a major contributor to the unusually high number of monsoon depressions that formed during the 2006 season, since the base of the trough was quite close to the very warm Bay water rather than over the land masses further east that it is usually located over. Also, note in this chart of sea surface temperature anomalies from 06-01 to 10-01 2006 that there was a pool of above normal water temperatures in the northern Bay of Bengal during the monsoon season. This coincidence of low pressure and high theta-e (very unstable) air probably allowed more low pressure areas to form more often over the northern Bay of Bengal than we would normally see.
As previously noted, there was an especially intense week of rains from August 11th to the 18th, when two strong depressions crossed Orissa in just one week's time; please see this page on Two Depressions at Orissa, 08/11 to 08/18, 2006 for the details. That particular week of flooding killed 10 people, causing tremendous damage to crops and property and brought the death toll at Orissa to 24 since the monsoon started.
THE FINAL TALLY AT BHUBANESHWAR AND ORISSA
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THE 2006 MONSOON "PARADE": The 2006 season featured a much higher than normal appearance of monsoon depressions over east and north India, with nearly all forming in the Bay of Bengal east of the east central coast of India, moving in over the coast in an almost constant parade, especially in the month of August. Keeping in mind the location of the monsoon trough for the 2006 season (see figure 10c), and realizing that most of these depressions ran from east to west right along or near the axis of this trough, it's no surprise why there was a very wet season at Bhubaneshwar and Orissa. In addition, as previously explained, the 500mb heights of these depressions tend to tilt southward, meaning that the most prolific rain-producing sectors in these storms are to the south and southwest of the center. Almost all of these depressions passed over or just north of the center of Orissa state, putting them directly under the areas of highest precipitation rates. As documentation that the number of depressions that crossed the Orissa coast in 2006 was unusually high, I have included figure 14 below, which contains an excerpt from the 2006 Monsoon season summary issued by the India Meteorological Department (IMD):
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The season as a whole had been quite active in terms of the number of low pressure systems. In all, 16 systems (1 severe cyclonic storm, 8 depressions/ deep depressions and 7 low pressure areas/ well marked low pressure areas) formed during the season. All the systems formed over the Bay of Bengal except one land depression and one severe cyclonic storm over Arabian Sea. The systems formed over the Bay of Bengal generally had a west-northwesterly track causing heavy rainfall over central India, especially over Orissa, West Madhya Pradesh, Maharashtra, Gujarat, and West Rajasthan. The only low pressure area in June which formed over the North Bay and adjoining Gangetic West Bengal (6–8 June) was short lived and dissipated over Jharkhand and neighborhood. In July, one depression, 3 low pressure areas and one well marked low pressure area formed. All these systems moved west-northwestwards, except one which moved northwestwards. In August, one deep depression, 3 depressions and one low pressure area formed. All of them formed over the north Bay and crossed Orissa coast. They also had long tracks mostly in a westerly/west-northwesterly direction across central India and moved up to west Rajasthan as remnants. In September, one severe cyclonic storm formed over the Arabian Sea. It dissipated over the Sea itself due to large vertical wind shear and cold air advection. In addition, 3 depressions, including one land depression and one low pressure area formed. The last depression of the season formed over the Bay of Bengal in the afternoon of 28 September and crossed Orissa coast close to Gopalpur on 29 evening. It then moved westwards and weakened gradually. |
Figure 14. Excerpt from the IMD's 2006 Monsoon Season Summary. Courtesy India Meteorological Department.
So 14 of the season's 16 organized low pressure systems moved over Orissa, including 7 depressions. Note that four depressions crossed the Orissa coast in August alone, with 2 crossing the region in the one-week period we covered, when the average for the entire season is 4-6. It's easy to see why this season, and the very wet period in August in particular, ran high rainfall surpluses at Bhubaneshwar and Orissa state. Finally, once again, it appears at this point that the major reason for Orissa's much above normal rainfall was the aberrant orientation of the monsoon trough this season, which allowed a large number of monsoon depressions to form over the northern Bay of Bengal and then move in directly over Orissa state. It is important to mention that this pattern continued unabated all the way to the end of the season in late October. For further information I have included below, in the references section, additional articles on the effects of the monsoon at Orissa during the month of August and beyond to the end of the season. There are also short explanations of factors in the monsoon that do not come into play at the eastern coast of India.
REFLECTIONS ON WRITING PROJECT 2
REFERENCES
OTHER MONSOON PLAYERS
STORM REPORTS
The complete 2006 Monsoon Report from the IMD
The 2006 North Indian Cyclone Season from Wikipedia
NEWS ARTICLES
Infochange,India August 18, 2006
International Research Institute