Lecture 20. Active-weak spells and breaks in the monsoon: Part 1

Size: px

Start display at page:

Download "Lecture 20. Active-weak spells and breaks in the monsoon: Part 1"

Job Norton
5 years ago
Views:

1 Lecture 20 Active-weak spells and breaks in the monsoon: Part 1

2 Although the summer monsoon season is the rainy season over most of the Indian region, it does not rain every day, at any place, during the season. Naturally, on the all-india scale also, there are large fluctuations in the quantum of the daily rainfall as seen in the next slide.

3 Variation of the all-india daily rainfall during the summer monsoon seasons of 2009 and

4 Advance of the monsoon-onset phase

5 At the end of the onset phase of the monsoon, around the beginning of July, the CTCZ gets established over the monsoon zone. During the peak monsoon months of July and August, the large scale rainfall occurs primarily over this zone.

6 Mean June-September rainfall Monsoon Zone: region between the red lines

7 Fluctuations between active spells and weak spells/breaks Even in the peak monsoon months of July and August, it does not rain every day over the monsoon zone. Rather, there are fluctuations between active spells with continuous rainfall for several days interspersed with weak spells with little rainfall, often lasting several days.

8 These fluctuations on the supersynoptic time-scales are an important feature of the subseasonal or intraseasonal variation of the monsoon. Perhaps the first illustration of such a fluctuation is by Ramdas (next slide) showing the variation of weekly rainfall over the region then known as Central Provinces and Central India. A long dry spell is seen to occur in the drought season of 1918.

10 The variation of daily rainfall over a central part of the monsoon zone ( E, N) during the drought of 1972 and the excess monsoon season of 1975 (next slide) shows that fluctuations between active and weak spells occur in both the seasons. However, a long dry spell occurred only during July-August of 1972

12 Blanford (1886) first described this fluctuation in the rainfall over the monsoon trough zone between spells `during the height of rains' and `intervals of drought', and elucidated the nature of the pressure distribution and circulation associated with these phases of contrasting rainfall conditions.

13 In his words The normal meeting place of the eastern and western branches of the monsoon is indicated by the trough of low pressure, which runs down from the Punjab to the southeast; the westerly branch prevailing to the southwest of this trough, and the easterly branch (more frequently in the Gangetic plain) to the northeast. This is seen in the next slide.

14 Seasonal variation of the direction of the mean pressure, surface wind July

15 In intervals of drought, when northwesterly and westerly winds interrupt the monsoon in North-Western and Central India, it (the trough of low pressure) is pushed northward to the foot of the hills (e.g. next slide).

17 On the other hand, during the height of the rains, at certain intervals, true cyclonic vortices, with closed isobars (barometric minima) are formed on or in the immediate neighbourhood of this trough".(p23)

19 Definition of breaks and active spells Blanford s (1886) intervals of droughts, during which the large-scale rainfall over the monsoon zone is interrupted for several days in the peak monsoon months of July- August, have been called as breaks in the monsoon by Indian meteorologists for over a hundred years (e.g., IMD 1888; Malurkar 1950; Ramamurthy 1969; Raghavan 1973; Krishnamurti and Bhalme 1976; Alexander et al 1978; Sikka 1980 etc.).

20 Traditionally, intense weak spells in July- August are referred to as breaks because a break is associated with a lull in the monsoon rainfall, after the CTCZ is established over the monsoon zone at the end of the onset phase. Active spells of the monsoon are the spells characterized by `height of rains' over the monsoon zone during the peak monsoon months of July-August.

21 Definition of breaks In Ramamurthy s (1969) comprehensive study of breaks during , a break situation was defined as one in which the trough of low pressure was not seen on the surface chart and easterlies were practically absent. (similar to the situation described by Blanford 1886). Subsequent to Ramamurthy's (1969) classic work, De et al (1998) have identified the breaks during I consider first what we know about these traditionally defined break spells.

22 Typical break day Note the absence of easterlies on the surface chart. Note also the presence of a ridge over the western part of the monsoon zone.

23 Also, no easterlies up to 3km

24 Note that there is hardly any rainfall over the monsoon zone and the west coast. Large positive pressure anomalies seen over the monsoon zone.

25 Typical active day

28 Ramamurthy identified breaks as spells in which the trough of low pressure was not seen in the surface chart and easterlies were practically absent in the lower tropospheric levels up to 1.5 km above sea level, provided they were 3 days or longer. Till about 1932, identification of breaks was done mainly with reference to surface charts; subsequently, upper wind charts were also consulted.

29 Ramamurthy described the morphology of breaks in terms of the surface pressure patterns, the rainfall distribution and lower middle and upper tropospheric wind circulation associated with breaks and compared them with those characteristic of active spells. Breaks are characterized by particularly large positive departure of pressure (from normal) and a large suppression of rainfall over the monsoon zone. On the other hand, active spells are characterized by very high rainfall over the monsoon zone.

31 This implies that the north south gradient of surface pressure is weaker during breaks:

33 Rainfall anomaly for Ramamurthy breaks Note that the largest rainfall anomalies occur over the monsoon zone

34 Ramamurthy s Break composite Note that the vorticity at 850 mb is anticyclonic for breaks; whereas it is cyclonic for active spells. Active composite Trough line Ridge Note the presence of a ridge at 850 mb over the western part of the monsoon zone for breaks

35 Ramamurthy s composites for breaks clearly show anticyclonic vorticity at 850 hpa (i.e. above the boundary layer) whereas that for active spells is cyclonic. In the break composite of 850 hpa, a ridge is seen over the western part of the monsoon zone, Such a ridge is also seen in the surface chart of a typical break day. Note that at 700 hpa, a trough is clearly seen in the composite for active spells but not in that of the break spells.

36 Simpson s observations For the period 1893 to 1899, the IMD collected observations taken on the ships in the Indian Ocean as far south as 8 0 S and from these, prepared daily weather map for the whole area. Using these maps, Simpson (1921) showed that on a typical break day viz. 18 August 1899, the conditions were similar to those in May. He showed that, on the break day, the winds over the southeast Arabian Sea blow against the southeast trades.

37 In consequence (because of the convergence), the weather is cloudy and rainy over the equator and dry over the Indian region, except for the northeastern parts. He pointed out that the pressure gradient between the south of the Indian Ocean and the Arabian Sea is small as a consequence of the relatively high pressure over western India. In his view, one of the most frequent causes of breaks is the establishment of an area of relatively high pressure over Western India.

38 Ramamurthy s Break composite Note that the vorticity at 850 mb is anticyclonic for breaks; whereas it is cyclonic for active spells. Active composite Trough line Ridge Note the presence of a ridge at 850 mb over the western part of the monsoon zone for breaks

39 Koteswaram (1950) has shown the presence of lows in low latitudes during breaks in the monsoon. This is consistent with Simpson s observations about the weather being cloudy and rainy over the equator.

40 Active spells, weak spells and breaks During weak spells, the strength of the cyclonic vorticity and the rainfall over the monsoon zone is much less than during active spells. Fluctuations between active spells and weak spells of rainfall occur every year. The break is a special case of a weak spell, with well defined circulation characteristics, which does not occur in every monsoon season (Ramamurthy 1969).

41 Active spells are characterized by a sequence of time-clustering, partly overlapping, development of monsoon disturbances (Murakami 1976) and cyclonic vorticity above the boundary layer (Sikka & Gadgil 1978). On the other hand, the break phase is characterized by a marked change in the lower tropospheric circulation over the monsoon zone, with the vorticity above the boundary layer becoming anticyclonic (Ramamurthy 1969, Sikka & Gadgil 1978).

42 Mean meridional velocity There is a marked change in the profile of the meridional velocity between active spells and breaks (next slide). Note that deep layer of southerlies over the Indian region characterizing an active spell, is replaced by a shallow layer north of about 15 0 N (i.e. over the monsoon zone) in breaks indicating a shallow cell.

43 8N N

44 In the next slide, the mean meridional velocity for Ramamurthy breaks (along about 75-80E) is shown along with that of the mean meridional velocity along 60-70E (i.e. across the heat low). The two profiles are rather similar, suggesting the presence of a heat low type circulation during breaks.

45 Latitude height section of meridional wind climatology (m/s) for E during July. 300hpa 500hpa 700hpa Southerly winds hatched

46 On the other hand, the mean meridional velocity distribution for active spells (along about 75-80E) is found to be rather similar to that of the mean July patterns along 78-88E, as well as the mean pattern around 80E(next slide). Thus the heat low type circulation associated with breaks is replaced by a dynamic low type circulation in active spells.

47 8N Latitude height section of meridional wind climatology (m/s) for E for July.

48 Special feature of intense long breaks During intense long breaks in the monsoon, the surface temperature increases rapidly and a heat-trough type circulation gets established over the monsoon zone with subsidence over most of the troposphere and the prominent trough at 700 hpa (associated with largescale monsoon rainfall) disappears (Raghavan 1973). An example is the break of 2002 (next slide)

50 The vertical variation of the meridional wind, averaged over E (after Rajeevan et al. 2008) for the peak break days during 2002 and for the active spell after revival from the break (next slide) shows that during the break, northerlies are prominent around 700 hpa, suggesting a shallow meridional cell. This is comparable to the circulation associated with the heat low in the July mean pattern.

51 Latitude Height section of meridional wind averaged over 78 88E for the break period (11 19 July 2002) active period (9 13 August 2002)

52 On the other hand, during the active spell, the meridional cell is deep with northerlies seen only above 300hpa. This is comparable to the dynamic low in the July mean pattern.

53 The occurrence of a heat trough type circulation over the monsoon zone in place of a TCZ in the peak monsoon months of July and August, implies a major transition. Revival from such breaks involves a transition to a moist convective regime with convergence up to the midtroposphere and northerlies aloft which is similar to the transition that takes place in the onset phase of the monsoon.

54 Revival from breaks It has been suggested that revival of breaks occurs with the formation of a low or depression over the north Bay of Bengal. However, Ramamurthy found that only 45-50% of the breaks were terminated in this manner. Sikka and Gadgil s (1980) analysis showed that out of 22 occasions on which the CTCZ disappeared from the monsoon zone for several days, only in 9 cases the revival occurred by this process, in the other 13 cases revival occurred with northward propagations of the TCZ.

55 I summarize the important features of the traditionally defined breaks (and active spells) emerging from the large number of studies over several decades. Important features of breaks 1. Absence of easterlies on the surface weather chart i.e. movement of the surface trough to the Himalayan foothills ( traditional definition of breaks).

56 2. Large positive surface pressure anomalies over the monsoon zone with a ridge over the western parts of the monsoon zone at the surface and 850 hpa. Absence of a trough at 700 hpa. 3. Anticyclonic vorticity at 850 hpa above the boundary layer. 4. Large negative rainfall anomalies over the monsoon zone, negative anomalies over the west coast, positive anomalies over the southeastern part of the peninsula and Himalayan foothills

57 5. Meridional Circulation: Shallow layer of southerlies over the monsoon zone, suggesting a shallow vertical circulation cell similar to that observed in a heat low. 6. In prolonged breaks, the surface temperature anomaly becomes large and positive over the monsoon zone and a heat trough occurs over the region.

58 Definition of breaks Although interruption of monsoon rainfall is recognized as the most important feature of the break, the criterion used by India Meteorological Department (IMD) and by several meteorologists for identifying a break, is the synoptic situation associated with such a rainfall anomaly, rather than the rainfall distribution itself (Rao 1976). It is implicitly assumed that a rainfall anomaly pattern of the Ramamurthy break type will always be associated with such a distribution of pressure.

59 Clearly, a definition of the break monsoon (and active monsoon as well) on the basis of rainfall and its distribution rather than in terms of the circulation or the pressure distribution would be useful. A definition of break/active monsoon in terms of rainfall distribution (and hence convection) can also facilitate analyses of the tele-connection with convection over other parts of the tropics, since global data on satellite derived convection are readily available for over three decades.

60 However, since a great deal is known about the traditionally defined breaks, the criterion for the definition on the basis of rainfall should be chosen so that the breaks based on rainfall (rainbreaks) have a large overlap with the traditional breaks. This has been accomplished by Gadgil and Joseph (2003) and Rajeevan et. al (2010)

61 On breaks of the Indian monsoon Sulochana Gadgil and P V Joseph Proc. Indian Acad. Sci. (Earth Planet. Sci.), 112, No. 4, December 2003, pp

63 Gadgil and Joseph (2003) and Rajeevan et. al (2008,2010)have defined breaks (and active spells) on the basis of rainfall over the monsoon zone, on the basis of rainfall criteria which are chosen so as to ensure a large overlap with the traditional breaks documented by Ramamurthy (1969) and De et al (1998).

64 Gadgil and Joseph analyzed daily rainfall during June to September at 273 well -distributed stations over the Indian region for the period obtained from IMD. Rajeevan et. al (2008,2010) analyzed the gridded data set prepared by Rajeevan et. al (2008). Gadgil & Joseph (2003) defined breaks (and active spells) of the Indian monsoon in terms of the rainfall over the western and eastern parts of the monsoon zone using thresholds for the maximum (minimum) rainfall for a spell to be considered a break (active spells).

65 Additional References Alexander GR, Keshavamurty N, De US, Chellappa R, Das SK, Pillai PV Fluctuations of monsoon activity. Indian J. Meteor. Geophys. 29: Blanford HF Rainfall of India. Mem. Ind. Met. Dept., 2 De US, Lele RR Natu JC Breaks in southwest monsoon, India Meteorological Department, Report No. 1998/3

66 IMD 1888 Indian Daily Weather Report, India Meteorological Department, New Delhi. Koteswaram P 1950 Upper air lows in low latitudes in the Indian area during southwest monsoon season and \Breaks" in the monsoon; Indian J.Met. Geophys. Vol. I, pp Krishnamurti TN, Bhalme HN Oscillations of a monsoon system. Part 1. Observational aspects. J. Atmos. Sci., 33,

67 Malurkar SL Notes on analysis of weather of India and neighbourhood. Memoris of 1. Met. D. XXVII Part IV pp Murakami T Cloudiness fluctuations during the summer monsoon. J. Meteorol. Soc. Jpn., 54, Raghavan K Break monsoon over India. Mon. Weather Rev, 101,

68 Ramamurthy K Monsoon of India: Some aspects of the break in the Indian southwest monsoon during July and August. Forecasting Manual 1-57 No. IV 18.3, India Met. Dept., Poona, India Sikka DR Some aspects of the largescale fluctuations of summer monsoon rainfall over India in relation to fluctuations in the planetary and regional scale circulation parameters. Proc. Indian Acad. Sci. (Earth & planet. Sci.), 89,

69 Sikka DR and Sulochana Gadgil, On the maximum cloud zone and the ITCZ over India longitude during the Southwest monsoon. Mon. Weather Rev.,108, Simpson G The South-West monsoon, QJRMS, 199, 17:

Lecture 14. Heat lows and the TCZ

Lecture 14. Heat lows and the TCZ Lecture 14 Heat lows and the TCZ ITCZ/TCZ and heat lows While the ITCZ/TCZ is associated with a trough at low levels, it must be noted that a low pressure at the surface and cyclonic vorticity at 850 hpa