The Indian Monsoon
By Marie Smith
Well, I began my discussion for project 2 with an account of my trip into the northern Australian rainforest. I was puzzled about the guide's references to the "beginning of the rainy season", and I thought I could address that within the project but found that it really had nothing to do with the subject matter (well, it does in a way, but that wasn't the gist of the lessons).
I will open again with this question about northern Australia's "rainy season". I have since learned that the Australian monsoon, which is grouped with the Asian monsoon, occurs from October to April every year. As in other monsoon areas, the dates are not etched in stone and there are variable "dry" periods within that time frame. There's a theory out there that the spread of Kelvin waves might have something to do with the onset of the Australian monsoon, but I am no expert on the matter and it seems to be an unproven theory. I am currently putting a page together on Kelvin waves; feel free to check it out, but it's still under construction. Here's the missing photo for that page.
My tour guide was talking about the "late" monsoon. Northeastern Australia tends to see the monsoon later in the season than other parts of Australia, and this period begins anywhere from late January to mid-March. It's also the "wettest" period of the season, as indicated on this schematic from a website geared towards fishermen in the Reef area. This isn't the only source I've found on the climate in Queensland. You can go to the website for the Australian Bureau of Meteorology to check out the annual rainfall rates for Queensland. The cities and towns I visited included Cairns, Port Douglas (no Brad Pitt sightings), and Daintree. I arrived in Cairns on February 5th. Bingo!!!! How was that for timing??
Anyway, enough about that. It certainly relates to my topic for project 3, but project 3 won't be about the Australian monsoon (but stay tuned...I might do a separate project on it later). Instead, this will be about the monsoon as it affects the central parts of India.
India rests on an area of the planet that sees the most magnificent monsoons that exist. More intense in many ways than the "rainy season" of Queensland, it consists of stark contrasts between severe hot, dry weather and profuse, wet, sometimes lethal flooding weather.
The Indian monsoon is a complicated issue for someone with my level of knowledge. Saturated with so many new facts and knowledge, my brain has difficulty deciding which of the many features of the monsoon emerges first and which is the most important. The monsoon is a seasonal process, closely tied to contrasts in temperature between the land and the sea. An outstanding feature of the Indian monsoon is that it doesn't happen all at once across the subcontinent. Rather, it progresses over the course of May to July over central and northern India then ends in a similar reversed progression throughout October. Like tornado season in the United States, we can estimate the "normal" start dates and progression, but nothing in meteorology is completely predictable.
I have chosen to talk about the city of Allahabad/Bamrauli in north central India for this discussion, but I will first try to explain (in a long nutshell) my understanding about how the monsoon forms and moves about. I chose to talk about dates in August from this year's monsoon season, mainly because the heaviest rainfall occurred during the middle of that month. Later in the discussion, I will look at two specific days.
As I said before, many factors play into the onset and progression of the monsoon. Under "normal" conditions, the easterly trade winds blow away from and to the south of central India. During the dry season (akin to the northern hemisphere winter and early spring), north easterlies dominate. As spring progresses, however, this pattern changes very dramatically.
The subtropical jet and its associated front play a significant role in the "monsoon shift". As the STJ encounters the Himalayan Plateau, it is diverted. This leads to a ridge to the west and a positively tilted trough to the east of India. As spring progresses, the STJ pushes northward and the subtropical high nudges eastward over the subcontinent. Although this high acts to suppress convection, it allows for substantial heating of the land.
I am still unclear about what happens to the subtropical high I mentioned in the previous paragraph as the season progresses, whether it moves over the Himalayan Plateau to become the Tibetan High or moves elsewhere. I do know that at some point, near the onset of the monsoon, this area weakens to a point where significant convection can take place. Storm chasers in the Plains in the United States gripe and complain whenever such a ridge is present, again because highs suppress convection. They often find themselves "baking under the cap" on days when clouds don't even form. Optimists say that if the land gets hot enough, rising air parcels will reach the LCL so that clouds can form, and that they then start to agitate the inversion enough to "break" the cap. I want to say that the intense heating of the land over India during the early spring (with temperatures soaring to an excruciating 40 degrees C plus in some areas), combined with displacement of the ridge, provides a setup for intense convection. Indeed, heat lows form over the hot land, and when the warm air rises air is pulled in from all directions to replace that rising air. Allahabad/Bamrauli is located in an area that sees these soaring temperatures. The Himalayan Plateau itself acts as a high-level heat source and "helps" the air to rise.
The water in the surrounding ocean heats up significantly as monsoon season approaches. Sea surface temperatures increase in a northward fashion as the season progresses. Air traveling over warm sea is moist. This air is "pulled ashore", partly because of the large heat lows (and also by the ICTZ, or monsoon trough, as it migrates over the subcontinent), where it encounters orographic lift first at the shoreline and again at the Plateau. High theta e values contribute to the instability.
Another high pressure system in the southern hemisphere is a major player in the formation of the Indian monsoon. As air circulates around the Mascarene High, it approaches the equator from the southeast, crosses it, and takes on a southwesterly component once inside the northern hemisphere. This occurs over warm sea, and the parcels have high theta e because they have traveled great distances over these warm seas. Another "surface player", the low level Somali jet (LLSJ), is a relatively fast atmospheric current that runs up along Somali and is deflected to the right in the northern hemisphere (again over warm sea), heading towards India from the southwest.
The inter tropical convergence zone, which I talked about in project 2, migrates northward in the northern hemisphere summer. It reaches northern and central India during the late spring and provides another source of lifting (which in turn causes more moist air from the surrounding seas to be drawn over the land). This is probably one of the most significant factors in the development of the monsoon. Finally, there are upper-level players in the monsoon game. The Tibetan High becomes strong during the monsoon season, and there is a height reversal in the upper levels of the atmosphere (higher columns to the north, lower towards the equator). Air parcels coming southward from the Tibetan High "fall" along the height gradient, gaining speed and leading to the formation of the Tropical Easterly Jet. Allahabad/bamrauli lies beneath the right entrance area of this jet. Storm chasers love the right entrance/left exit areas of a jet streak because these areas favor convection (divergence occurs in these areas, which means that air must converge at the ground and then rise--this was a critical ingredient in the storm that produced the tornado in Campbelltown, PA, last summer). Monsoon depressions develop under the right entrance area of the TEJ (the left exit area tends to be over the ocean, not over land).
I hope my explanation of the ingredients of the Indian monsoon is accurate and that you can understand it. Now I will turn your attention to Allahabad/Bamrauli in mid August of this year and try to point out some of the things I just talked about.
Allahabad/Bamrauli, located in north central India, appears to be some kind of military airport or base. The following is a precipitation chart from the last 365 days for that area:
You can see that the monsoon came upon this area during the latter half of June. Estimating based on what we had learned about five day rainfall averages, I will say that the season officially got under way around June 20. Based on the estimated onset dates shown to us in lesson 5, this is slightly late. Allahabad/Bamrauli lies between the June 10 and June 15 lines on those schematics, closer to June 10. Significant rainfall ceased in the middle of September, with a little burst in mid-October. The average date of departure for the monsoon in this region is between October 1 and October 15. There is a significant "dry spell" between the middle of September and the middle of October. At first, I was inclined to say that the season ended abruptly and early in September, but the mid-October burst suggests that the monsoon may still have been present at that time. It is common for a few breaks in the heavy rainfall to occur during the season.
I chose August 1-August 24 as my dates to analyze for this project. Although July was very robust, I wanted to see both "normal" monsoon rainfall and a "break". A significant break seems to have occurred around roughly August 10-15. I also wanted to capture the enormous spike in rainfall near the middle of the month.
One goal I had for this project was to look at hourly differences in rainfall. I will attach some of the NCEP maps for these times. Although I have yet to learn about precipitation patterns as they vary throughout individual days, I think I see a hint of these patterns. Rain fell between certain hours each day and then didn't fall during others. I will leave that for the very end.
The following is a daily mean composite taken from NCEP's website, and it shows precipitation rates for August 1 to August 24. Here, you can get a glimpse of the sea level pressure in north central India for those days, and the LLSJ was very robust during that time frame. Speaking of jet patterns, the tropical easterly jet is evident on this map. I wanted to show you a map that describes the lifted index for those days too, but was unsuccessful using NCEP's page for this. The lifted index is a measure of atmospheric stability, and is defined here. I have not been officially taught about the lifted index as of yet, but have learned what it means through my "freelance" meteorology studies over the years. I would like to say that the lifted indices in the regions of the monsoon trough, or ITCZ, are very high, so they should be high over Allahabad/Bamrauli for this time frame (I could be very wrong, though, since I have my own ideas about this and have likely formed many bad habits through my independent learning!). During monsoon season, a lot of lifting goes on, from convergence in the ITCZ to divergence at 200mb in the right entrance of the jet streak to the orographic lifting against the shore and the Himalayan Plateau to the "giant sea breeze" mentioned in lesson 5. My theory is that all of these things might affect the lifted index somehow, but I am getting beyond the point of the project now.
It is interesting to note on the image of the tropical easterly jet daily mean composite that the right entrance area doesn't exactly correspond to Allahabad/Bamrauli's ground position. Rather, I would expect the heaviest rains to be over eastern and southern India. This IR satellite image, courtesy of Meteo France, was taken on August 4th and, indeed, it shows dense cloud cover over that area. Rain from these clouds may have been affecting Allahabad/Bamrauli, but I can't be sure of that. It does, however, show a monsoon depression. Note the tropical cyclone spinning off the coast of Taiwan (FYI). Here's a better view of the Indian Ocean from that day.
I also looked at images that were taken on August 18 (IR) and August 19 (visible, noon) in an attempt to correlate the appearances of the cloud cover with the "spike" on the CPC Precipitation chart. I had initially guessed that the heavy rainfall started a few days beforehand, but the August 18th image clearly shows a lot more clouds than images from a few days earlier (which were likely in the "break" denoted on the CPC chart).
Here are some additional images that I would like to share with you. They are from various days throughout my chosen time frame, from various 6 hour time blocks. I observed that the 06z-12z blocks seem to have more rain than other times of the day (except on August 14, which fell in the "break").
Aug 1 Aug 16
Aug 2 Aug 17
Aug 14 Aug 24
Aug 1 Aug 16
Aug 2 Aug 17
Aug 14 Aug 24
I looked at almost every 6 hour block on these dates and more, and I want to say that more rain falls between 06z and 12z over Allahabad/Bamrauli than is does between 12z and 18z. The data from August 20th which, for the sake of brevity, I have not included, show little rain. There is even less for August 24 for all times, which suggests that August 20 and 24 were beyond the spike on the CPC map. The changes in the rainfall patterns throughout the day still interests me, but I won't dare touch it here. Diurnal patterns, maybe??
I hope I have covered all of the bases for this project. Gathering all of the maps and trying to reason about what I'm looking at has taken a considerable amount of effort (while trying to learn to create the page, to boot). New interests have cropped up throughout this assignment, such as the rainfall patterns I mentioned above, and Kelvin waves as they might relate to the monsoon in Australia. One of the biggest things I have taken away from this, however, is the knowledge of what the Australian guide meant when he was talking about the "rainy season". It's been almost two years since my excursion and, although that question has been on my mind for all of this time, I never really found the time to research it and find the answer. Serendipity....finding that answer by total accident...was present here. When I set out to learn about the Indian monsoon I discovered the Australian monsoon.