last lecture we have seen that the system
basic system responsible for the summer indian
summer monsoon is a tropical convergence zone
and today we will talk about tropical convergence
zone and the indian monsoon
now summer monsoon the monsoon or the rainy
season commences with the monsoon onset over
kerala mok which is recognised as a rapid
substantial and sustained increase in rainfall
so we have already seen this this is the monsoon
onset over kerala composite for several years
80 years and 0 is the onset date and what
you see is a very sudden increase in rainfall
which is afterwards sustained this is a very
important facet of the onset
now after the onset over kerala what happens
in the onset phase is that the monsoon actually
the onset occurs more and more northern parts
so it goes across the peninsula and then the
onset over this part also is thereby 10th
of june so after this then there is an advance
of the monsoon in this direction so there
is an advance reaches northward and advance
that is westward and by first of july you
can see most of india is under the sway of
the monsoon so these are the mean dates of
course of onset from india met department
now the retreat begins in september and slowly
the monsoon retreats by october 15th it has
retreated from most of india except for the
southern peninsula and south-eastern peninsula
and to retreat from these regions it takes
till december so october to december is the
post-monsoon season
but we will first see the summer monsoon season
so the entire country comes under the sway
of the monsoon at the end of the onset phase
around the end of june the monsoon begins
its retreat from the north-western parts around
the beginning of september and so july and
august are considered to be the peak monsoon
months by first half of october the monsoon
is restricted to the peninsula south of 15
degrees north
now for the country as a whole most of the
rainfall occurs during the summer monsoon
season june to september which we have seen
already june july august september so these
are the 4 major rainy months
and the focus of most of the studies has been
the summer monsoon however we should remember
that southern and south-eastern peninsula
receives substantial fraction of the rainfall
during what we call the post-monsoon season
so in fact the mean monthly rainfall if you
look at different places in this northern
part which is the monsoon zone most of the
rain in fact does occur from june to september
as you can see here but in the south-eastern
part you see chennai for example more rain
in october-november than in any other months
and then bangalore we have rain from the summer
monsoon as well as rain in post-monsoon
palayamkottai at the tip of the peninsula
here we have rain primarily in october-november-december
but on the west coast we get rain most of
the rain in the summer monsoon but the rain
continues in october-november as well so this
part whatever is theory we have for the basic
system responsible for the monsoon must not
only explain the fact that over this part
the monsoon zone the monsoon most of the rain
occurs from june to september but we should
also understand from that basic system that
a large amount of rain occurs here in october-november
and so on
so now what is the mean rainfall during the
summer monsoon and post-monsoon seasons? so
this is the mean rainfall during the summer
monsoon and this is the monsoon zone over
which most of the rain occurs and this is
really the seed of the tropical convergence
zone that we talked about last time in addition
to that there is heavy rain on west coast
which is partly accounted for by orography
and heavy rain in the north-east as well the
post-monsoon on the other hand we get most
of the rain south of 15 north over the southern
part of peninsula to some extent over the
east coast as well okay
now we have seen from the first study of satellite
imagery that sikka-gadgil showed that the
maximum clouds on mcz associated with an active
monsoon day resembles that associated with
the canonical itcz the intertropical convergence
zone which was known to tropical meteorologist
as the system that gives rain over tropical
pacific for example now the mcz over the indian
region is associated with cyclonic vorticity
at 850 and 700 millibar we have already seen
that
furthermore sikka-gadgil showed that there
is a high correlation between the axis of
the mcz maximum clouds zone and that of the
700 millibar trough which is known to be associated
with intense convergence in the lower troposphere
and maximum non-orographic rainfall so it
was known to indian meteorologist that the
key element in the dynamics to look for is
where the 700 millibar trough is because that
is the axis of the non-orographic rainfall
but sikka and gadgil showed that what you
saw from the satellite as maximum cloud zone
axis of that zone and axis of the 700 millibar
trough which is the axis of the non-orographic
precipitation which is to say axis of the
large-scale rainfall in the monsoon are in
fact highly correlated this was shown by sikka
and gadgil so the intertropical convergence
zone as conceived by charney is a prominent
zonally oriented region of moist convection
in the tropics associated with high cyclonic
relative vorticity and convergence in the
lower troposphere these were the major dynamical
attributes of the itcz
so putting these 2 together putting the fact
that the axis of the maximum cloud zone was
highly correlated with the 700 millibar trough
and had all the dynamical characteristics
of the canonical itcz sikka and gadgil concluded
that it becomes clear that the organised moist
convection associated with the monsoon may
be attributed to a continental itcz over the
region so they said that the monsoon mcz could
be considered as a manifestation of a continental
itcz and the secondary mcz and oceanic one
now the term intertropical convergence zone
comes from the fact that convergence in the
zone is between air from the 2 hemispheres;
however we have seen already that on indian
longitudes very there are 2 cloud bands one
over the equatorial indian ocean and one over
the monsoon zone so it is to be noted that
since on occasion there are 2 mcz simultaneously
present over the same longitudinal belt it
is clear that convergence in only one of them
can be intertropical from the 2 hemispheres
and so we have dropped the eye from itcz and
we only use the term topical convergence zone
to denote such a system and the monsoon is
attributed to continental tcz or ctcz over
the indian region so this was the conclusion
so now today we are going to talk about the
variability of ctcz ctcz is the basic system
responsible for the large-scale monsoon rainfall
so large-scale rainfall over the indian region
during the summer monsoon is associated with
the tcz and this tcz is called the continental
tcz as it occurs over land to distinguish
it from the more common tcz observed over
tropical ocean
so first let us consider the summer monsoon
season and this is the monsoon zone that we
have seen before over which the large-scale
rainfall occurs
now since we are going to look at the ctcz
in this lecture we will look at first and
foremost what the satellite shows us so what
we now look at is the climatology or mean
or the average outgoing longwave radiation
map for june to september averaged over 4
months and what you see here is the darker
shade means higher the clouds the lower the
olr okay so these are clouds with very very
high tops here the dark ones and the entire
region is marked so that only olr below 240
watts per second is shaded
so there are deep clouds almost everywhere
here but highest frequency of deep clouds
occurs in this region here this dark region
here so from june to september you can see
this is what gives the rain over the monsoon
zone here you have this kind of a gradient
that you saw also in rainfall but you will
see a lot of rain occurs over the head bay
during june to september you notice that there
is another tongue here over the equatorial
indian ocean these comes from the oceanic
tcz that we have seen that occurs simultaneously
sometimes with the continental tcz
so during the summer monsoon low values of
olr occur over the indian land mass the bay
of bengal and the equatorial indian ocean
over the indian region the ctcz occurs over
the monsoon zone that is between 15 and 25
north low values of olr also occur along the
west coast of peninsula and the north-east
that we have seen before okay
now the ctcz gets established over the monsoon
zone at the end of the onset phase that is
the spring to summer transition the summer
to autumn transition is the retreat phase
of the ctcz
so consider first the variation of the ctcz
from spring to autumn on a monthly scale okay
so april-may are supposedly the spring months
and first we see the picture in april and
what you see is this of course is the mid-latitude
rain that you see here this has nothing to
do with the tropical systems what you see
is a tropical convergence zone if you wish
over the eastern equatorial indian ocean so
there is rain over eastern equatorial indian
ocean on the average no rain at all over the
western equatorial indian ocean and so the
rain is restricted to this region
the deep clouds are restricted to this region
in april now in may you see that it has expanded
northward and a little bit westward right
so the region extends from 5 south to 10 north
see it extends a lot over this region over
70 to 90 from 5 south to about 10 north see
tip of india has also come under the low olr
region and it is much farther northward over
myanmar and this part of the bay okay
now this is the june mean june rainfall remember
the mok or monsoon onset over kerala typical
date is 1st of june so entire june the monsoon
should have been here and furthermore you
know that by 10th of june it covers this much
of the continental tcz and then later on progresses
so almost the entire peninsula is now under
the sway of the monsoon when we consider the
mean monthly olr picture but notice the high
rainfall or low olr region of the west coast
and also head bay near myanmar coast
so now june is over now we come to the peak
monsoon months of july and august and here
you see are very typical pattern this is the
continental tcz here now this is the continental
tcz but remember that you it does not end
with the indian region in fact as you go eastward
it dips into the bay where it is even more
intense okay so if you like you can say the
ctz here changes to an oceanic tcz here
so we have an oceanic tcz and a ctcz here
and notice we have a full-fledged oceanic
tcz here okay this is the equatorial indian
ocean this is for the months of july and august
and if you look at august sorry the last one
was for the month of july this was for the
month of july and you see already the ctz
is well established and this is the month
of august and again you see the equatorial
indian ocean itcz here the tcz here and this
is the ctcz with convection over the bay
this is the month of september remember 1st
of september the monsoon already starts retreating
from here so september the rain is restricted
more or less to the eastern part here or the
monsoon zone and of course the peninsula gets
a lot of rain in september now the excess
or to speak or where the deepest clouds are
the lowest olr is has already shifted towards
the equator but it has spread a little bit
across the bay as well this is the september
story
now during october and november the tcz is
most intense over the eastern equatorial indian
ocean remember that the south-eastern peninsula
comes under the sway of the tcz in these months
and consequently receives a large fraction
of its annual rainfall
this is the picture we saw earlier that you
know places like chennai and palayamkottai
and so on gets a very large fraction of the
annual rainfall in october and november and
what you see is that in october-november the
centre of the ctcz is over the equatorial
region okay and it is spread way up to about
15 north here you see so the centre is here
and we are looking at mean monthly picture
and it is spread right up to here
so associated with it we get rainfall over
india as well but you have to remember that
the centre is here this is different from
the summer monsoon case that you saw earlier
where see for example july where there was
a prominent band of cloud right over the land
here okay which was connected to the band
over ocean no doubt but it was prominently
here and this was where the deepest clouds
were if you looked at the entire 80 degrees
east the deepest clouds largest rainfall would
be here
so it is not that is the system has spread
to that the system axis is here on the other
hand when you look at the post-monsoon it
is a different story the axis is around here
and this is the periphery of the system which
is giving us rainfall in october-november
same story even more so in november you see
the system has become more intense and shrunk
a little more and only a small part of the
peninsula is getting deep clouds in november
now an important question to address is why
is there hardly any convection on the arabian
sea in these months we can just note this
if you look at october and let us look at
the earlier seasons say july or august if
you look at augusta there is quite a bit of
a olr here over the arabian sea okay that
is east of 70 degrees east or so but even
then there is hardly any low olr here but
in the equatorial region you see that this
is extending right up to 60 east or so in
august
now what happens in september also you see
it up to 60 east but october now it has begun
to shrink but the equatorial thing is fine
in october but november you see it has shrunk
even further and there is absolutely nothing
over the arabian sea in november where as
in october also you did not have much over
the arabian sea but there was a little bit
here but in november it has totally disappeared
therefore an important question to address
is why is there hardly any convection on the
arabian sea in these months okay and in fact
i shall address this question at a later stage
in his lecture series when we understand a
little more about what determines where the
tcz will occur over the oceans okay we are
going to look at that a little later and after
we understand that we should be able to explain
why there is hardly any convection over the
arabian sea during october-november why is
it restricted to the bay okay
now in december which is the last month of
the october to december season you can see
that in the mean there are no deep clouds
at all over india
of course there would be events in which they
occur here but other times there are no clouds
at all so the average olr does not reflect
few days of deep clouds here so again you
see here december is very much concentrated
over eastern equatorial indian ocean and this
part of west pacific with no deep clouds anywhere
else
so climatology of olr for october to december
is that the tcz if you wish the axis of the
tcz is on the equator it extends basically
from about 90 east which is here to about
110 east which is here but on the periphery
you know south peninsula and so on which gets
rain during the post-monsoon season okay
now question is what is the system which gives
us rain in the post-monsoon season? in the
summer monsoon we have seen that it is a tropical
convergence zone and we have called it a continental
tropical convergence zone because it occurs
on the continent now we should see what is
the system that gives us rain in the post-monsoon
season
now this is an example that we have seen before
of what happens in the summer monsoon season
this is 7th of august 2007 and what you have
seen is a very bright cloud band stretching
across here and you can see this is india
i hope you can see that and that is sri lanka
right there and this cloud band which we would
call mcz or maximum cloud zone is over the
monsoon zone here but it does not stop here
it stretches right across almost to the west
pacific okay
so this is the tropical convergence zone which
is responsible for a rainfall in the summer
monsoon
this is another picture where we have 31st
july where again you see this and interestingly
at the same time we have an oceanic tcz here
as well with a link between the 2 so there
are 2 tropical convergence zone and this is
what we meant you see we cannot have intertropical
convergence in both this and this it has to
be only in one or the other that is why we
call them tropical convergence zone
and an 15th of october you see this is again
a the tropical convergence zone and beginning
to look more like the classical canonical
itcz and what you see here is again very similar
to what you saw earlier but now to the south
what you saw occurring here in the summer
monsoon is now occurring here at the foot
of the peninsula so to speak toe of indian
is dipping into the cloud band here and what
you see is the cloud band very characteristic
of a tropical convergence zone on 17th october
so we are now in the post-monsoon season
and as far as the satellite is concerned the
picture is very similar now this is in fact
a global picture for the next day which is
18th october and what you see is the cloud
band you saw here corresponding to our tropical
convergence zone what is interesting is across
the globe you see this itcz here see this
is the atlantic one and this is the pacific
one you see ours is more intense and somewhat
larger in latitudinal extent but you can see
that it is the same animal which is sort of
girdling the earth in the equatorial regions
which is responsible for a post-monsoon rain
and this is a case of a tropical convergence
zone from which cyclone was born and now the
cyclone is taking off you can save this very
beautiful outflow region of the cyclone and
the cyclone is taking off from this region
and the same picture here now you see because
the cyclone has become so strong the itcz
horizontal extent has decreased but the itcz
persists over the pacific
so the system responsible for organised rainfall
in the summer monsoon as well as the post-monsoon
is the same system it is a tropical convergence
zone; however the summer monsoon june to september
season it is called the south-west monsoon
season because the low level winds are from
the south-west and the post-monsoon season
october to december is called the north-east
monsoon season because the low-level winds
are from the north-east
so you can see here this is a slide you have
seen before that in july the winds are from
the south-west and in november they are from
the north-east you can see that very clearly
here this is the south-west monsoon because
winds are from the south-west and this is
the north-east monsoon because the winds are
from the north-east okay
but i believe these are misnomer why are they
misnomers because they give the impression
that the system responsible for the rainfall
in the 2 seasons is different which is clearly
not the case as we have seen that the same
system namely the tcz is associated with the
rainfall in both the seasons also the term
south-west north-east gives the wrong impression
to lay people that they will come from the
south-west in the summer monsoon whereas it
comes from the north-east in the post-monsoon
season
see simply because the wind is in that direction
does not at all means the rain comes from
that direction but by calling a rainy season
by name which conveys the direction of the
wind people who do not know the system which
includes a vast majority of the people in
the monsoonal region assume that in south-west
monsoon rain must be coming from the south-west
and in north-east monsoon it must be coming
from the north-east
i therefore suggest that the term summer monsoon
and post-monsoon and not south-west and north-east
monsoon be used for the 2 monsoon seasons
that we experience in india okay
now note that the ctz so the lesson so far
is that right from the onset of the monsoon
over kerala on 1st june to the retreat around
mid-december from the entire indian region
the system responsible for the large-scale
monsoon rainfall of our country is the tropical
convergence zone okay note that the ctcz is
established over the monsoon zone at the end
of the onset phase towards the beginning of
july and the ctcz primarily fluctuates over
the monsoon zone during the peak monsoon months
of july and august the retreat of tcz from
the western part of the monsoon zone commences
in september okay
consider first the spring to summer transition
now let us see how the transition occurs northward
shift is accomplished and here i would like
to go to the picture first then we will come
back to the description
what you see here is for a particular year
these are bands now this is latitude going
from 10 south to 25 north and we are sitting
over 90 degrees east which is the latitude
of around calcutta okay so sitting at that
longitude we are seeing where are the deep
clouds okay so on this specific day this is
march on this day in march there is a very
deep cloud here and shallower clouds here
now you see below 180 watts per second is
the darkest colour okay
lighter then that is 18-160 i think below
160 is the darkest 160-180 is the middle one
180-200 is the lighter one so everything coloured
in green corresponds to deep clouds because
they involve on a daily scale olr or outgoing
longwave radiation of the order of 200 watts
per second so what you see now this is time
and this is latitude so what you are seeing
here is a northward surge which went from
10 south to about 5 north
then it hung around for a while another northward
surge here which went further northward okay
and then finally another surge here which
took it to its final destination around 20
north okay so this is the spring to summer
transition it is made up of several northward
surges one after the other and that is what
you saw here northward shift of the tcz from
its mean location near 5 degrees north in
april-may to 20 degrees north in july-august
is accomplished by successive generation of
northward moving epochs over the equatorial
region
as you have found that the average rate of
northward progression is about 1 degree latitude
per day and this is the rate of progression
that we saw in the next slide okay so what
is the average rate of this northward progression
that you say they found that it was about
1 degree latitude per day but remember this
process is not linear it is not as if it will
cover 10 degrees in 10 days rather it is a
very jerky process because the basis is all
non-linear instabilities and so it is only
an average we are talking of
order of magnitude of the rate sometimes you
know it will go up to some point and hang
around there and then go northward as you
see here this happens very very often so even
here you see it started then it went here
very fast and then it kind of slowed down
and then continued so it is not a uniform
progression but typically it is about 1 degree
a day that is the progression
the maximum latitude attained by each such
band is generally farther northward than that
of the preceding one and after one or more
surges of this type the ctcz gets established
over the monsoon zone towards the end of june
or beginning of july so you saw here as you
can see very clearly in the envelope we have
drawn that the maximum latitude attained in
each surge is further and further northward
till it comes to a month of july okay and
then it is going here
now this is the retreat of the monsoon okay
where if you look at the envelope it is going
southward but are the bands moving southward?
no not at all in fact what is happening is
the bands are still generated over the equatorial
indian ocean and they are moving northward
but the latitude they reach is becoming more
and more southward so this thing has gone
right up to beyond 20 north but the next one
here has gone only up to about 7 or 8 north
okay so this is what happens
so actually it is interesting that about the
spring to summer transition we had surges
that went more and more northward with every
succeeding surge it is interesting that this
mode of northward shift of the tcz is similar
to northward progression of monsoon rains
over india which is also accomplished in such
surges so monsoon rain onset also occurs in
surges and sometimes one surge will take it
up to a certain point and the next surge will
take it further northward and so on and so
forth
very similar thing we see in cloud bands now
when we see the summer to autumn transition
again the transition begins with the final
disappearance of the tcz from the region north
of 20 degrees north in september note that
this transition also comprises northward moving
epochs just like the spring to summer transition
and not southward moving ones as would be
expected from the monthly location of the
tcz
during this transition the maximum latitude
attained in each northward moving epoch decreases
progressively we have seen this already in
this picture so we have a situation in which
we have northward movements throughout but
the final destiny of the northward epochs
actually becomes further and further northward
in the spring to summer transition and further
and further southward in the summer to autumn
transition that is how it occurs okay
thus in the summer monsoon as well as the
post-monsoon season an important feature of
the inter-seasonal variation is a northward
propagation of the oceanic tcz onto the subcontinent
so this is a very very important feature of
the variation of the cloud bands or the intra-seasonal
variation or variation within the season perhaps
the most dominant feature if we start looking
at clouds is the northward progression
and this is a picture that you have seen earlier
from sikka and gadgil and what you see here
is that these northward progressions occur
from april right up to october and they occur
year after year we have already seen that
and this is from 70 80 90 degrees east in
1975 and you see that the northward progression
is coherent across the indian longitudes you
see particularly here for example 80 70 and
90 which means that a band stretching right
across from the arabian sea to the bay of
bengal is going all the way from the equatorial
region to 20 north or so as a band that is
why these northward movements are coherent
that is why these are movements of an mcz
so they are coherent across
now the question is what is the contribution
of these propagations to the maintenance of
the ctcz okay see we call it a continental
tropical convergence zone because it is on
the continent but we also notice that all
the time we are getting bands from the equatorial
indian ocean which come and end up in the
ctcz so obviously ctcz is maintained partly
by these northward moving tcz from oceanic
side so question is to what extent the ctcz's
existence depend on that?
what is the contribution of this propagations
to the maintenance of the ctcz
and in fact in that paper sikka and gadgil
also addressed this now how did they address
it? to do that we have to again define a few
things properly first of all we have already
noted that there are 2 favourable locations
for mcz one over the heated subcontinent and
another over the equatorial indian ocean
and remember they assume that the boundary
between the 2 is a kind of no man's land where
the frequency of occurrence was minimum around
7 degree north or so to the south of that
is what they call a secondary band to the
north of that is what they call a primary
band or the monsoon band which we can call
the ctcz so variation of the mean location
of the axis of the primary mcz and at this
point this is the primary mcz so we should
not call it the ctcz at this juncture becomes
the ctcz only here when it is over land
so this is the primary band and this is the
main location of the axis of the primary band
moving northward and moving southward but
throughout the season june to september intermittently
we saw secondary band appear over the equatorial
indian ocean and that is what you see here
and that axis remains more or less fixed standard
deviation is higher here when it is to the
north and the standard deviation is roughly
same all around here so these are the 2 bands
we are talking of what is the contribution
than of the oceanic tcz to the maintenance
of the ctcz
now they also derive the standard deviation
of the axis
and this is the standard deviation of the
axis okay and what you see generally it is
below 7 degrees okay the spread is a little
large in july-august but it is all less than
6 degrees or so where as standard deviation
here is about 4 degrees for the secondary
band
so because the standard deviation of that
band that is how much the axis vary just due
to fluctuations in situ is less than 6 degrees
sg considered northward movement greater than
7 degrees latitude within the lifespan of
an epoch is significant since the standard
deviation of its axis is less than 7 degrees
for every month okay so remember epoch is
the starting of an mcz epoch or an event
it started at a certain latitude on a certain
date and then after a lifespan of a few days
it dies within its lifespan it may propagate
it may not propagate okay so consider first
the latitude and date of origin of an mcz
epoch that is latitude and date at which it
first appears at the given longitude now we
have to remember that we are looking at the
band and as such we are not worried about
what happens east-west progression and so
on at this point we are focusing on north-south
so the appearance of a band at a given longitude
may be the result of a generation of a new
cloud band at the location or an advection
of a band generated from east or west if we
are sitting at say 90 degrees east and we
say mcz epoch started at 90 degrees east on
a certain date it could have started because
something more there from 100 degrees east
or it could have started because a cloud system
got generated there at 90 degrees east this
is something that we have to bear in mind
now look at the variation of the number of
mcz epochs with latitude of origin okay so
how many epochs are generated at different
latitudes going from 0 and all the way here
till 30 north and solid lines indicate epochs
with significant northward movement okay these
are epochs with significant northward movement
and these you can see up to here and what
you see from this picture is that vast majority
of the epochs are generated over the equatorial
ocean relatively few are generated over the
continent
this is something very clear that most of
the mcz epochs are generated over the equatorial
region 0 to 10 north from april to october
now we have to remember that in the sikka-gadgil
study the data they had available was hemispheric
imagery or a hemispheric cloud mosaics so
their data was restricted to 0 to whatever
you wanted in the northern hemisphere restricted
in the northern hemisphere so they could not
go south of the equator but later studies
have found that equator is not a laxman rekha
at all
in fact equatorial indian ocean mean stands
out to 10 north or so so actuality it is 10
south to 10 north but sikka-gadgil’s data
is only from 0 to 10 north so they found that
most of the epochs are generated 0 to 10 north
also mcz epochs without significant northward
movement dominate over equatorial region so
what we saw was okay over the equatorial region
that the dashed lines are much much longer
than the solid lines
so mcz epochs which do not dominate which
do not move northward are dominant here and
there are dominant almost everywhere and north
of 18 there are no northward moving epochs
at all because for it to be northward moving
it has to move at least 7 degrees and there
is not that kind of a span left for them to
move so without significant northward movement
dominate over equatorial region while north
of 18 degrees only such epochs are generated
which is natural because of minimum displacement
of 7 degrees is considered necessary for significant
moment okay
now this is a table showing percentage of
epochs starting in different latitudinal belts
1-3 4-6 and so on in different months and
what you can see here is that in april vast
majority are generated right near the equator
in may the more shifts are little bit here
but in june it is more flat right up to here
and july it is still equatorial but you are
getting some more being generated in 10 to
12 and so on
now you see longer and longer tails develop
here so when you come to june to september
you do get quite a few epochs being generated
even north of 20 degrees these are generally
the systems generated on the head bay and
so on and so forth which we will come to later
but note that the percentage of epochs generated
north of 15 degrees north in the peak monsoon
months of july and august
so if you want to look at how many epochs
are generated north of 15 north that is over
the monsoon zone in the peak monsoon months
of july and august when ctcz fluctuates primarily
over the monsoon zone they are only 30% in
july and 10% in august so even in the peak
monsoon months when the ctcz fluctuates primarily
over the monsoon zone most of the epochs are
still generated south of the monsoon zone
this is an important point to remember thus
even when the ctcz fluctuates primarily over
the monsoon zone the contribution of the northward
moving epochs is very large
now frequency of epochs with different life-spans
so if you look at frequency of epochs with
different life-spans than it is very interesting
in the april they are very very short-lived
epochs typically 3 to 4 is the maximum number
have life-spans between 3 to 4 day several
have between 5 to 7 days none last longer
than 13 days now long-living epochs longer
life-span epochs characterised the major peak
monsoon months here july and august
they are the longest ones long duration epochs
occur in july and august primarily but rest
of the months they are typically shorter with
vast majority being less than 5 days or so
and you can see that here now this is of course
combined for all the months and here you see
separately the northward moving epochs from
the non-northward moving epochs and you see
that the longest epochs are those with northward
movement see solid bars here which indicate
the epochs which move north actually dominate
in fact there only epochs that survive up
to about 15 days and beyond are solid are
the northward moving ones and later on actually
here the ones that have shortest life-span
are the ones that do not move northward at
all
so you can see all epochs with life-span beyond
18 days that is here onwards and almost all
with life-span beyond 10 days that is beyond
here almost all of them are northward moving
epochs this is an interesting thing to note
so already we see the dominance of northward
moving epochs in july and august now what
sikka-gadgil did was to systematically divide
the epochs into 3 types type i is generated
in the equatorial region which do not cross
over onto the monsoon zone and this had a
mean life-span of about 4 days see these are
cloud bands that appear over the equatorial
region and disappear in about few days type
ii were generated in the equatorial region
which crossover onto the monsoon zone which
had a mean life-span of 22 days and type iii
are generated within the monsoon zone as i
said these are typically systems generated
over the bay of bengal which then move westward
on to the monsoon zone
the type iii epochs generally involves synoptic
scale systems generated over the bay or appear
over the bay by westward propagation of synoptic
scale systems generated over west specific
moving onto the monsoon zone now sg found
that the northward moving epochs that is type
ii epochs contributed 82% of the days on which
mcz was present over the monsoon zone and
78% of the days on which mcz was present over
the equatorial region
so whether we consider the equatorial region
or over the monsoon zone the maximum contribution
seems to be coming from epochs which are northward
moving epochs which are generated over the
equatorial indian ocean and which move northward
onto the monsoon zone so it is a very very
convincing proof of how important this is
these epochs are
thus the convective days associated with the
ctcz over the monsoon zone in july-august
are dominated by epochs generated over the
equatorial indian ocean which move northward
hence the contribution of northward moving
epochs of the oceanic tcz to the maintenance
of the ctcz is substantive so we expect the
variability of the ctcz on different scales
to be linked to the variability of convection
oven the equatorial indian ocean
that is very clear because convection over
the equatorial indian ocean now has turned
out to be a lifeline of the monsoon a lifeline
of the continental tropical convergence zone
which is the basic system responsible for
the monsoon so naturally variability over
the monsoon will be linked with variability
of convection over the equatorial indian ocean
which is the source of this lifeline
now synoptic systems generated over the bay
of bengal which move westward onto the indian
region also contributes significantly to the
maintenance of the ctcz an example is the
systems of the summer monsoon that we see
here
and see in 1999 summer monsoon for example
so many monsoon depressions were born here
and a low-pressure system was also born and
you can see they are all generated here and
move along the monsoon zone this way okay
so these are tracks of lows and depressions
so we do get contribution from such systems
also and these will not be type ii because
type ii means those systems come from here
these are type iii which means what we call
in situ which is to say within the latitude
of the monsoon zone the system is getting
generated and giving rain so this is the other
system
the tracks of all cyclonic storms and severe
cyclonic storms from july onwards
this is from imd atlas attack of all cyclonic
storms and this is the mean july rainfall
and you can see that there is an association
of the july rainfall with the tracks of these
storms and part of the large-scale rainfall
certainly can be attributed to genesis and
propagation of synoptic scale systems
for the season as a whole the maximum number
of low-pressure systems are generated over
the head bay of bengal
see this we have seen before this is the frequency
of genesis of low-pressure systems in different
places and the maximum occurs here this is
the 40 and they all move across and give us
this kind of june to september rainfall this
is the monsoon zone okay so these systems
do contribute a great deal these are what
sikka-gadgil called the type iii events
so i think now we have to also see how often
does the propagation from within the system
occur type iii and how often type ii occur
and in fact sikka-gadgil have also looked
at how typically in a season the ctcz gets
established how it fluctuates and when it
fluctuates between active spells and weak
spells how does it revive after a weak spell
and that is what we are going to look at next
time
so what we have seen this time is that in
fact the tropical convergence zone is the
basic system responsible for the summer monsoon
rainfall as well as the post-monsoon rainfall
and the rainfall that we experience the variation
of the monthly rainfall that we saw across
india is consistent with this as we saw from
the olr data how the tcz moves from month
to month secondly we saw that northward propagations
which is the most important feature of inter-seasonal
variation of the maximum cloud zone in fact
dominate the entire picture
they are the key element of the seasonal transition
with the spring to summer seasonal transition
occurring with northward propagations which
take the monsoon further and further northward
and summer to autumn transition which is the
retreat again in was northward propagation
is not southward which takes the band the
culmination of the northward propagation is
that more and more southern latitude that
is how that has occurred and we have also
seen that if we look at the contribution of
these northward propagations to ctcz over
the monsoon zone and to the oceanic tcz
then that maximum contribution occurs from
northward propagating mczs more than 80% for
the ctcz and very close to 80% for the equatorial
oceanic itc so northward propagating epochs
of mcz are a very critical element of the
monsoon or the seasonal evolution of the monsoon
as well as the inter-seasonal fluctuation
of the monsoon this is what we have seen by
the analysis done so far now we will see how
the monsoon evolves and how the revival by
different processes takes place and so on
and so forth in the next lecture thank you
