Lecture 17 Seasonal Transitions

Lecture 17 Seasonal Transitions :part 2

Pre-­‐monsoon  rain  peak   Joseph and Pillai (1988) showed that there is a peak in the weekly rainfall of the average of four meteorological subdivisions of India south of latitude 13oN (Lakshadweep, Kerala, Tamilnadu and Andaman and Nicobar Islands) during March to May of the years 1960 to 1984, about six weeks before the date of MOK.

This pre-monsoon rainfall peak (PMRP) is clearly seen in the composite for the 25 years 1960 to 1984, of the weekly rainfall averaged over the four subdivisions, with the week in which the MOK occurs being considered the 0-week, shown in the next slide.

If x is the day of the rain peak (PMRP) in the period 1 April to 10 May counted from 1 April as day 1, and y is the day of MOK as declared by IMD also counted from 1 April, the straight line of best fit by the least square deviations method is, Y = 46.19- 0.75x The linear correlation coefficient between x and y is 0.87. The regression equation implies that if the PMRP occurs on 1 April (x=1), the monsoon onset should occur on 17 May and when PMRP is on 10 May , the estimated monsoon onset is on 15 June.

Since (y-x) is dependant on the calendar date, it is apparently controlled by the seasonal changes in the atmosphere and the ocean (higher SST and vertically integrated moisture in the atmosphere as the season advances). Further studies investigating the nature of PMRP to document the nature of the variation of the timing and magnitude of this PMRP from year to year, with the grid rainfall data as well as OLR data are needed.

Bogus Monsoon Onset In 1972 IMD had declared monsoon onset over Kerala in their official bulletins on 16 May in association with a spell of heavy rainfall and strong monsoon like westerly low level winds, but soon issued a correction calling it as temporary monsoon onset. Later that year IMD declared monsoon onset on 18 June.

A rainy event over Kerala which is comparable in intensity to that associated with MOK, but which is not succeeded by sustained rainfall over Kerala, (such as the one in May 1972 which was considered as a temporary monsoon onset), has been described as a “Bogus Monsoon Onset” by Flatau et al, (2001). They have reported six such occurrences in 1967, 1972, 1979, 1986, 1995 and 1997 during the period 1965 to 1997. Bogus onsets generally occur in May in years with delayed monsoon onsets.

•  A bogus onset is generally associated with the development of strong convection in the Bay of Bengal which is accompanied by the monsoon like circulation and appears over the Indian Ocean in May. •  This ‘‘bogus onset’’ is followed by the flow weakening or reversal and clear-sky and dry conditions over the monsoon region.

1995  onset  case   •  The best example of such a phenomenon is the development of the summer monsoon in 1995, when monsoon like perturbations that appeared in mid-May disappeared by the end of the month and were followed by a heat wave in India, delaying onset of the monsoon.

•  Strong convection (negative OLR anomalies) and strong winds over the Bay of Bengal BOB occurred in association with a tropical depression which formed over BOB (following slides). •  Associated with this, convection is also observed off the Kerala coast, which is termed as bogus onset

May 1-5, 1995

OLR anomalies

May 6-10, 1995

May 11-15, 1995

925 hPa Circulation : Winds

6-10 May, 1995

11-15 May, 1995

1-5 May,1995 Note the absence of cross equatorial flow and strong winds over Arabian sea

•  However, after the storm passed away / weakened, convection off the Kerala coast disappeared. Over south India, dry conditions leading to heat waves prevailed. •  After the passage of storm over BOB, the wind strength reduced substantially.

May 16-20, 1995

May 26-30, 1995

May 21-25, 1995

OLR anomalies

Real  onset   •  By the last week of May, cross equatorial flow started strengthening. •  Convection started re-organizing again in the first week of June with the actual onset phase of monsoon. Monsoon onset over Kerala occurred on 9 June.

21-25May

26-31May

16-20 May

6-10 June

11-15 June

1-5June

31May-5June

OLR anomalies

6-10June

11-15June Convection started reorganizing again in the first week of June with the actual onset phase of monsoon. Monsoon onset over Kerala occurred on 9 June.

•  During the bogus onset, even though there is strong westerly flow in the lower levels (say at 925 hPa), the vertical extent is limited. At 700 hPa, the winds are very weak and also easterlies off the Kerala coast (left). But with the actual onset, monsoon westerlies are deep up to even 600 hpa (Right). Another aspect is the lack of cross equatorial flow at 700 hpa during the bogus onset.

Wind circulation at 700 hPa

.

1-10 May

5-15 June

Daily OLR averaged over 700-750E Bogus ONSET

time

Actual ONSET

Rainfall associated with the bogus onset

Actual monsoon onset phase

Rainfall averaged off the Kerala Coast (710 -750 E)

PWAT increases sharply associated with the bogus onset

Later, with the actual monsoon onset phase, PWAT increases again

However, the increase is short-lived. Dry conditions set in quickly, once the convection over BOB dies off

Example  of  a  bogus  onset:  2004   •  In 2004 IMD declared the onset over Kerala on 18 May since good rainfall occurred over Kerala. •  However, this rainfall activity was associated, not with a seasonal transition of the monsoon circulation, but with a severe cyclone around the 3rd week of May over the Bay of Bengal which led to enhanced wind speed over the Arabian sea and enhanced moisture transport to the Kerala coast.

Satellite picture showing cyclonic storm on 18th May when the IMD declared the monsoon onset

•  The rainfall activity was short lived and rainfall decreased once the storm moved away (next slide). •  In fact, on 18th May there was neither significant cross equatorial flow over the west equatorial Indian Ocean nor westerlies over the Arabian sea in contrast to the circulation on 5 June (following slides).

•  Note that in the circulation pattern during 17-19May, there is no wind speed maximum off the African coast, which is associated with the monsoon onset. Strong winds are associated with the cyclonic storm. There was an enhancement of winds off the Kerala coast due to the storm.

11-15 May

16-20 May Winds at 925 hPa

OLR anomalies

•  The actual monsoon onset was on 5th June. •  When the actual onset occurs, strong cross equatorial flow is observed and the low level jet stream is positioned just below the tip of India. Strong vorticity and associated convection is observed just north of this wind maximum (low level jet).

Winds at 925 hPa

21-25 May

1-5 June

26-31 May

6-10 June

Daily OLR 65-75E

Rainfall due to bogus onset

Rainfall due to actual onset

Mean Daily precipitable water

Mean daily u wind Increase in wind speed associated with the bogus onset

Increase in wind speed associated with the actual onset

Typical  Onset-­‐2007   •  In 2007, the monsoon onset occurred on 28 May.

Mean daily OLR:700-750E Onset phase of Indian monsoon occurs in 2-3 spells not at one stretch. However, the gap between rain events is only 3 days.

Monsoon onset

Second northward propagation

Daily precipitable water

No gap after 28th

Mean daily u-wind

•  In developing methods for identification of MOK for operational purposes it is important to ensure that bogus onsets are eliminated. •  Such an operational method was suggested by Joesph et. al (2006). •  An important feature of the cloud bands during the onset phase noted by Sikka and Gadgil (1980) viz. the northward propagation from the equatorial Indian Ocean, is a key element of this objective method.

•  By analyzing 10 years which were selected on the basis of the MOK being between 28 May and 4June with the average MOK of 31 May i.e. very near the normal date, Joseph et. al (1994) showed that in the composite for these years, at 4 pentads before MOK, an elongated narrow band of convection forms close to the equator in the Arabian Sea longitudes. This convective band grows rapidly in area and intensity and moves north and culminates in MOK at 0 pentad (next slide).The composite OLR and 850 hpa wind for the onset date is shown in the following slide.

Composite of OLR for 10 specially chosen years

•  Thus they envisage the monsoon onset over Kerala to occur as a part of the northward moving epochs of the oceanic TCZ. •  They suggested that the timing of MOK is influenced not only by the timing of the annual cycle in the ITCZ (south to north movement) but also by the east-west oscillation (30-50 day mode) in convection between the Pacific and Indian oceans and by the 30-50 day mode of the Indian ocean.

•  Following the description of an “onset vortex” in southeast Arabian sea in association with MOK of the FGGE MONEX year 1979 by Krishnamurti et al (1981), several monsoon researchers have been searching for an onset vortex as a trigger for the monsoon onset. •  However, note that in the 850 hpa wind composite a fully developed low level jet (LLJ) is seen to the south of the peninsula. However, There is no onset vortex at 850 hPa in the southeast Arabian Sea but only large shear cyclonic vorticity north of the LLJ axis.

In the composite wind charts for 700 hPa given in Soman and Krishnakumar (1993) also there is no sign of an onset vortex in the southeast Arabian sea at MOK. Ananthakrishnan et al’s study of MOK during 1901 to 1968, showed that there is a prounced tendency for the formation of low pressure systems at the leading edge of the monsoon current (LLJ). In 45 percent of the years, a trough of low pressure or a more intense system (cyclonic storm in 8% of MOK) is present in the Arabian Sea at the time of onset of monsoon along the west coast.

Objective method of Joseph et. al. 2006 •  At MOK the area mean zonal wind of the box (70-85oE, 5-10oN) should reach 6 m/s at 600 hPa. •  If by this criterion, a possible MOK is found during the period 5 May to 25 May, whether it is a PMRP or MOK is checked by examining the spatial patterns of OLR and 850 hPa wind fields.

The slow and steady movement of organized convection (rainfall) from the equatorial area to the latitudes of Kerala to bring about MOK is checked in a Hovmuller diagram (as in the next slide) averaging OLR between longitudes 65oE and 80oE.

OLR

Zonal wind at 850hp a Composite of Hovmuller diagrams of daily OLR and 850 hpa zonal wind over 70-85E, for 9 specially selected years

Objective method of IMD •  In 2006, IMD reviewed its method and adopted new criteria for declaring MOK operationally (Pai and Rajeevan 2007). •  The criteria chosen were such that all the important facets which distinguish a real onset from a bogus one were incorporated. The bogus onsets are, therefore, eliminated when MOK is identified using this method. The criteria for declaring the MOK are based on rainfall, wind field and OLR.

1. If after 10 May, 60% of the available 14 stations enlisted viz, Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochim Trissur, Kozhikode, Talassery, Cannur, Kasargode and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the second day, provided the following criteria are also satisfied.

2. Depth of westerlies should be maintained up to 600 hPa, in the box equator to 100N, 550-800E. The zonal wind speed over the area 50-100N, 700-800E should be of the order of 15-20 knots at 925 hPa. The source of data can be RSMC wind analysis / satellite derived winds. •  3. INSAT derived OLR value should be below 200 Wm-2 in the box confined by 50-100N and 700-750E.

Mean OLR pattern associated with MOK (1988-2007)

Objective methods for MOK •  I have already discussed the first objective method for identification of MOK by Ananthakrishnan and Soman (1988). That method, as several other objective methods I shall next mention, retrospectively assess the date of MOK and are therefore useful only for research on the processes involved in the seasonal transitions and their variability.

Fasullo and Webster (2003) have derived the onset and withdrawal dates of the Indian monsoon from the variability in the large-scale hydrological cycle. They suggest that the method is proposed as an improved means with which to understand interannual variability in the monsoon transitions as compared to criteria that rely heavily on rainfall variability over limited spatial domains (e.g., individual Indian districts).

To diagnose onset and withdrawal of the Indian monsoon, they have used Vertically Integrated Moisture Transport (VIMT)

Where q is the specific humidity and U is the wind vector. They find that its variability particularly over the Arabian Sea is substantial during both onset and withdrawal of the monsoon.

An index named the Hydrological Onset and Withdrawal Index (HOWI) is derived from VIMT which is used to determine the date of monsoon onset / withdrawal over India. The onset so determined is highly correlated with MOK determined by IMD (subjective) and by Anathakrishnan and Soman (1988) Correlation coefficient

Wang,  Ding  and  Joseph  2009   •  This method is based on the Ocean Circulation Index (OCI) defined as the daily average 850 hpa wind over the South Arabian Sea (SAS) box 400E- 800E and 50N and 150N. •  The date of MOK in an individual year is defined as the first day when OCI exceeds 6.2 m/s (which is the average wind over the box for 1June which is the average date of MOK), with the proviso that the wind over SAS also exceeds 6.2 m/s in the following six consecutive days. • 

Xavier, Marzin and Goswami (2007): Monsoon onsets (withdrawal) have been defined by Xavier et al (2007) as the day when the tropospheric heat source shifts from south to north (north to south).Their objective definition of the large scale monsoon onset (over India) is based on the reversal of GrTT, (Gradient in Tropospheric Temperature as average of 600 to 200 hPa) between a northern box (40-1000E, 5-350N) and a southern box (40-1000E, 150S-50N) denoted by GrTT.

The onset date (GrTT onset) is defined as the date when GrTT changes sign from negative to positive and withdrawal date when the change is from positive to negative. A characteristic of the large scale onset of the Indian summer monsoon is an abrupt increase in the kinetic energy (KE) of the low level monsoon flow (Krishnamurti, 1985).

The day of abrupt increase in the KE of 850 hPa winds averaged over a large region (40-100cE,5-150N) above a threshold value of 40 m2s-2 and persisting for 5 consecutive days is taken as the KE onset. (Krishnamurti, 1985)

The day of abrupt increase in the KE of 850 hPa winds averaged over a large region (40-1000E, 5-150N) above a threshold value of 40 m2s-2 and persisting for 5 consecutive days is taken as the KE onset. Xavier et al (2007) have derived onset days by both GrTTand KE methods for the period 1950 to 2003. There is a strong linear correlation of 0.77 between these two onset days.

Joseph (2012) has compared the MOK derived by the old subjective method of IMD and the new objective method of IMD (Pai and Rajeevan, 2007), by the objective methods of Joseph et al (2006) and Wang et al (2009) and the objective onset dates for India derived by Fasullo and Webster (2003) and the onset dates by the GrTT method of Xavier et al (2007) for the 30 year period 1971 to 2000.This is shown in the next two slides.

•  Note that on the whole, there is considerable agreement between the onset dates determined by the different methods. However, there are large differences between the onset dats determined by Xavier et. al (2007) and others such as IMD (objectieve). About this, Joseph(2012) remarks :

“A question arises as to whether the GrTT onset of monsoon derived by Xavier et al (2007) is for onset over India or a much larger or different area of south Asia. A comparison with the objective IMD onset dates for Kerala shows that in the years 1972, 1979, 1983, 1986, 1995 and 1997 when MOK (IMD) was delayed by 10 to 19 days (mean delay of 12 days from the long term mean date of MOK), the onset by Xavier et al was two pentads earlier than the IMD’s objectively derived dates of MOK.

In the years 1985, 1990 and 1999 when IMD onset was two weeks earlier than normal, the onset dates by Xavier et al (2007) have very little difference from the IMD dates. Possibly the onset dates derived by Xavier et al define the beginning of strong convective heating in some part of the large monsoon area of south Asia which can also increase the kinetic energy of the monsoon flows through the area for the KE onset chosen by Xavier et al (2007).

Interannual  variation  of  MOK   •  There is considerable variation in the date of the monsoon onset over Kerala from year to year. •  The 100-year mean date of the onset is 1 June and its standard deviation is 7.4 days.

Interannual variation of the date of monsoon onset over Kerala

Frequency distribution of the date of the monsoon onset over Kerala

•  It is seen that in only about 25% the onset date is close to the mean (during 31May -2 June) and in about 50% of the years it is between 28May and 5 June. •  The earliest onset was on 11 May in 1918 and the most delayed onset was 18 June in 1972.

•  In India we consider the onset of the monsoon over Kerala which is the commencement of the rainy season as a very important event. •  It is pertinent to ask the question, to what extent does the performance of the monsoon over the country as a whole depend on when the season commenced.

Relationship of ISMR with onset date over Kerala

• It is seen that ISMR is not related to the onset date over Kerala. In fact, in the two cases of onset close to 20 days before and after the mean date, the ISMR was close to 65cm i.e. large deficit.

•  Fasullo and Webster (2003) suggest that the Hydrological Onset and Withdrawal Index (HOWI) they have proposed is a better index for the onset of the Indian monsoon because the magnitudes of the correlation coefficient with ISMR is larger than the IMD or Ananthakrishnan and Soman index (-0.31 as compared with -0.1).However, that still explains less than 10% of the variance.

•  Thus while the event is of great meteorological interest, when it occurs is not important for the Indian summer monsoon rainfall.

Advance of the monsoon-onset phase

References   •  Following papers, chapters and references therein •  Ananthakrishnan R and A Thiruvengadathan, ‘Thermal changes in the troposphere associated with seasonal transitions over India’ 1968, Current Science vol. 37, p 184-186 •  Ananthakrishnan R, J M Pathan &S S Aralkatti ‘The Onset phase of the southwest monsoon’ 1983, Current Science vol 52 p755-64 •  Ananthakrishnan, R. and Soman, M. K., 1988, “The onset of the southwest monsoon over Kerala”, Journal of Climatology, 8, 283-296.

•  Ding, Y., and Sikka, D. R., 2005, “Synoptic systems and weather in the Asian monsoon (Ed. B. Wang) Praxis”, Ehichester, U.K. 131-202. •  Joseph P V 2012 Onset, Advance and Withdrawal of Monsoon , in Monsoon monograph vol 1 p 284-329, India Meteorological Department New Delhi •  Pai, D. S. and Rajeevan, M., 2007, “Indian summer monsoon onset: variability and prediction”, National Climate Centre Research Report 4/2007, India Meteorological Department, Pune.

•  Rao Y P , 1976 ‘ The southwest Monsoon’ Meteorological monograph, Synoptic Meteorology No. 1/1976, India Meteorological Department. •  Soman, M. K. and Krishnakumar, K., 1993, “Space-time evolution of meteorological features associated with the onset of Indian summer monsoon”, Monthly Weather Review, 121, 1177-1194. •  Yin M T 1949,A systematic aerological study of the onset of the summer monsoon over India and Burma, J. Meteorology, vol 6 p 393-400