Gadgil, Sulochana

Krishnamurti, T. N.

Rajendran, K.

Tewari, Mukul

English

Publications of the IAS Fellows

A heavy winter monsoon rainfall episode
influenced by easterly waves, a westerly
trough, blocking and the ITCZ
T.N. Krishnamurti1, Mukul Tewari1, K. Rajendran1 and Sulochana Gadgil2
1Department of Meteorology, Florida State University, USA
2Centre for Atmospheric and Oceanic Sciences, Bangalore, India
A recent five-day cricket Test Match between
England and India held at Bangalore, India,
from 19 to 23 December 2001 ended in a dis-
appointing draw. England was leading with a
very respectable first innings score when heavy
rains started on Day 3 of the match and led to
the cancellation of the game. Just before that,
one of the authors of this paper had received an
e-mail from a Bangalore colleague inquiring
about the prospects of rain for the following
days. This led to our interest in the examina-
tion of the synoptic situation during that
period. Here we present a short account of our
findings, which turned out to be quite intrigu-
ing.
In this note we focus on the scenario that
preceded the heavy rainfall episode over south-
east India during the winter monsoon period in
December 2001. A persistent anticyclonic cir-
culation in the upper troposphere over east
Africa, centred near 58N, 508E, contributed to
the bifurcation of the strong westerlies near
208N into two streams. An intense and persis-
tent blocking situation was present near 608E.
One branch of the split flow in the westerlies
moved equatorward in the Arabian Sea, result-
ing in a trough in the Northern Hemisphere
and a ridge in the Southern Hemisphere, the
latter residing over the Southern Hemisphere
intertropical convergence zone (ITCZ) near
58S. This ridge± trough system provided a
moist stream of south-westerly flow over the
south-east coast of India where the frequent
westward passage of easterly waves near
850mbar provides the setting for heavy rain.
Another important feature within the easterly
trades are the frequent wind surges. These
surges move from east to west within the east-
erly trades at a speed of roughly 78 longitude
per day. The average strength of the trades is
around 12ms-1 and the surges often carry a
wind speed as high as 13 to 14ms-1. Com-
monly, convection and rain are found ahead of
the surge, due to the convergence of mass and
moisture. At the time of the heavy rain over
Chennai (13.138N, 80.258E) we could clearly
see the simultaneous arrival of an easterly wave
and a trade surge over the south-east coast of
India.
Winter monsoon rains over the coast of
south-east India usually occur during October,
November and December, with heavier rains
occurring in November and December. The
large-scale flow field over the region exhibits a
very strong vertical wind shear. At the surface
levels, the flow is generally from the north-east
(also called the north-east monsoon). As the
winds veer with height, at the 850 mbar level
the flow is more easterly. In the upper tropo-
sphere the winds generally tend to be more
westerly (Krishnamurti et al. 1997a,b). The
north-easterly monsoonal flow is very shallow,
occupying only the lowest 2 km of the atmos-
phere.
Importance of the study
The features described here are quite unusual.
Upper-tropospheric westerlies over the North-
ern Hemisphere normally do not penetrate
south of 108N. Typically, the region of the
ITCZ near 58S is characterised by a zonally
oriented anticyclonic belt that does not usually
367
Weather Vol. 57 October 2002
carry strong cross-equatorial flow. Wintertime
precipitation over the south-east coast of India
is generally attributed to westward-propagating
tropical waves near 108N over the Bay of
Bengal.
It is now being recognised (Krishnamurti et
al. 1997a,b) that, although this is indeed a
source of moisture for the rainfall over the
south-east coast, other possible moisture
sources exist as well. The trade wind easterlies,
the waves imbedded within these easterlies,
and the trade wind surges and associated mass
convergence also convey moisture zonally near
108N. In addition to these processes, this paper
highlights the possible role of a large-amplitude
extended westerly trough in trapping and
transporting moisture from the ITCZ region of
the Southern Hemisphere to the south-east
coast of India.
The formation of the extended trough was
evidently related to a persistent intense anticy-
clone over north Africa. An additional feature,
making this rainfall episode quite complex and
interesting, was the flow of the strong wester-
lies north of this anticyclone into a major
downstream block.
Analysis of the episode
The normal rainfall over Chennai for the
winter monsoon months of November,
December and January is 308, 157 and 24 mm
respectively (Rudloff 1981). A time history of
the daily rainfall at Chennai during the Decem-
ber 2001 episode of heavy precipitation is illu-
strated in Fig. 1. The rainfall totalled 184 mm
during 21, 22 and 23 December 2001. Rainfall
amounts as high as 84mm in 1 day were noted
on the 21st, mainly from widespread rainfall
over the province of Tamil Nadu. A better pic-
ture of the broader aspects of precipitation can
be seen from rainfall estimates derived from
the National Aeronautics and Space Adminis-
tration (NASA) TRMM satellite and the US Air
Force DMSP-SSM/I satellites. These datasets,
which were extracted from the archive at the
Florida State University (FSU) real-time multi-
model superensemble website (Krishnamurti et
al. 2001), are shown in Fig. 2 (p. 377). This
illustration shows the ITCZ rainfall near 58S
and the daily rainfall distribution over the
winter monsoon belt. We can see the rainfall
associated with the easterly wave and the large
increase in rainfall over Tamil Nadu on 21 and
22 December. An examination of our data files
on the real-time analysis since 1998 showed
that the scenario presented here is quite infre-
quent, with roughly one such example per
winter season noted during the past four years.
Figure 3 illustrates a sequence of 500 mbar
level geopotential heights for a 6-day period
covering 17 to 22 December 2001 at
1200GMT. Here one can see a persistent anti-
cyclone over Africa, centred around 508E. The
westerlies to its north exhibit a blocking bifur-
cation with part of the westerlies going as far
north as 558N. Of interest to this study is the
southern branch of the split westerlies that
carves out a trough at around 608E. The
related wind field exhibits an extended trough
into the Southern Hemisphere extending
directly over the ITCZ at 58S. In the geopoten-
tial field, because of the change of polarity of
the pressure field across the equator, this fea-
ture shows up as a weak anticyclone. This fea-
ture assists in the moistening of the upper
troposphere over south-east India, where the
easterly trades of the lower troposphere are
also active.
The trade wind easterlies at 850 mbar (Fig.
4, p. 378) extend between 58N and 208N all
the way from the date line to 508E. The trade
wind easterlies show an easterly wave (slanted
dark line) that propagates from roughly 908E
to 708E during a 6-day period between 17 and
Fig. 1 Daily rainfall totals for 19± 25 December 2001
at Chennai. These are 24-hour totals ending at
0000GMTon the respective days.
Weather Vol. 57 October 2002
368
22 December 2001. The yellow colouring
depicts a belt of strong easterlies where the
wind speed is in excess of 10ms-1. Within this
belt, wind surges were noted on 20 December
that moved westward and abated in intensity
by the 22nd.
The strong 200mbar subtropical jet over
Africa near 208N splits into two branches (Fig.
5, see inside back cover), one reaching as far
north as 558N, and the other extending
towards the Arabian Sea and eventually into
the Southern Hemisphere. The resulting
feature is a blocking situation between 608E
and 808E. The mechanisms leading to the
establishment of this block are not clear, but
the formation of the large-amplitude trough
near 608E that crosses over into the Southern
Hemisphere and taps moisture from the ITCZ
Fig. 3 500mbar geopotential heights for 17± 22 December 2001 at 1200GMT. The contour interval is variable in the
units of geopotential metres.
369
Weather Vol. 57 October 2002
at 58S appears to be related to the block’s pre-
sence. A search for insight into the origin of the
block and the splitting of the flow takes us to
the west of the Arabian Sea. An unusual fea-
ture of the upper troposphere at the time was a
very intense high pressure area near 258N over
north Africa. That high persisted for almost a
week after 17 December. To the north of this
high pressure area, waves in the strong wester-
lies, carrying winds of up to 60ms-1, moved
east at a speed of roughly 108 longitude per
day. The block appeared to be related to the
dynamical instability of this westerly jet.
The subtropical westerly jet, as it accelerates
from the Indian subcontinent towards southern
Japan, is generally oriented from roughly 258N
to 308N. In this example of the extended
upper-level trough over the Arabian Sea, the
location of the subtropical jet was much farther
south than its normal latitude (Fig. 5). This
may be a large-scale consequence of the block-
ing pattern at 608E that brought in the upper
westerlies to very low latitudes over the
Arabian Sea.
Ahead of the trough one would expect to
find a region characterised by positive vorticity
advection aloft and quasi-geostrophic rising
motions. We examined the vertical motions at
1200GMT on 21 December 2001 and con-
firmed the presence of large-scale upward
motions over the south-east coast of India
during this period of heavy rain.
Summary
A number of factors contributed to the heavy
rainfall episode over Chennai in southern India
during December 2001. These include the
westward passage of wind surges at the
850mbar level in the trade wind easterlies
around 108N. The speed convergence resulted
in mass and moisture convergence ahead of the
wind maxima, contributing to rising motions
and rainfall. A rapid build-up of an extended
trough around 608E over the Arabian Sea was
able to tap moisture from the ITCZ near 58S.
This feature contributed to the moistening of
the upper troposphere over south-east India
during the period of investigation. The trough
dynamics also provide a favourable environ-
ment for sustained rising motions ahead of the
upper trough over the south-east coast of
India. The extension of the upper-level trough
resulted in an anticlockwise flow and a prevail-
ing high pressure area above the ITCZ convec-
tion region. That upper-level high appeared to
provide a favourable environment for the main-
tenance of the ITCZ and its continuous supply
of moisture to the heavy rainfall episode.
This observational note on the features cited
above, i.e. the African anticyclone, the blocking
pattern, extended westerly trough interacting
with the ITCZ, the easterly waves and the heavy
rain over south India, is illustrative, but does
not provide rigorous explanations of the exact
mechanisms involved. Detailed modelling
studies can perhaps provide further insight.
Acknowledgements
We appreciate the editorial comments from
Drs Harry Cooper, Lydia Stefanova and
T. S. V. Vijaya Kumar. This work was partially
supported by NASA grant NAGS-9662 and FSU
research foundation grant 1338-831-45.
References
Krishnamurti, T. N., Jha, B., Rasch, P. J. and
Ramanathan, V. (1997a) High resolution global
reanalysis highlighting the winter monsoon ± Part
I: Reanalysis field. Meteorol. Atmos. Phys., 64,
pp.123± 150
Ð Ð (1997b) High resolution global reanalysis
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Correa-Torres, R. J., Vijaya Kumar, T. S. V.,
Williford, E., Kummerow, C., Adler, R. F.,
Simpson, J., Kakar, R., Olson, W. S. and
Turk, F. J. (2001) Real-time multianalysis±
multimodel superensemble forecasts of precipita-
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Rev., 129, pp.2861± 2883
Rudloff, W. (1981) World climates. Wissenschaftliche
Verlagsgesellschaft mbH, Stuttgart
Correspondence to: Dr T. N. Krishnamurti,
Department of Meteorology, Florida State
University, Tallahassee, FL 32306, USA. e-mail:
tnk@io.met.fsu.edu
# Royal Meteorological Society, 2002.
doi: 10.1256/wea.37.02
Weather Vol. 57 October 2002
370


A heavy winter monsoon rainfall episode influenced by easterly waves, a westerly trough, blocking and the ITCZ

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A heavy winter monsoon rainfall episode influenced by easterly waves, a westerly trough, blocking and the ITCZ

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