| dc.description.abstract | This study is for part of the First Garp
Global Experiment (FGGE) period. The onset of 1979
summer monsoon over Southwest India is studied in
relation to pressure surges in Southwest Indian Oceano
Furtnerl fluctuations in the intensity of the monsoon
system over the Southwest Indian Ocean and neighbourhood
were studied in some detail for the period of about
two months from the middle of June to the middle of
August 1979. This period is hereafter called the
"selected monsoon period". The region of study lies
between latitudes ISoN and 350S, and longitudes 3loE
o
and 85 E. Southern India is part of this regiono
Stations whose data were used for analysis
are Rodrigues, Mauritius, Fort Dauphin, Durban, Diego
Suarez, Saint-Denis, St. Brandon,' Dar es Salaam,
Seychelles, Mombasa and Garissao Mauritius, Saint-
Denis and Fort Dauphin are near the mean location
oooS, SOoE) of the centre of the Mascarene high
while Diego Suarez, Mo~basa and Garissa are close to
the low level jet systemo Seychelles is affected by
easterly disturbanceso The northern boundary of the
region.of study is near the mean location of the
tropical easterly jet. In addition to the individ~al
station data mentioned above, objectively analysed
data at grid points available at the British
Meteorological Office, Bracknell were also usedo The
data used were the once-a-day (1200 GMT) values of sea
level pressure and winds (at both low and high levels)
Using Mauritius and St. Brandon surface
pressure values, two epochs were selected for intensive
study:
(i) Intense Mascarene high (IMH) which was
of seven days duration (3 July to
9 July 1979). During this period the
surface p r essu r e , p, at the two stations
exceeded the mean for the "selected
monsoon period", p, by more than one
- standard deviation, 0, that is, p>p+o.
(i i) Weak Mascarene high (WMH) which was of
eight-Aays duration (27 June, 23 to
25 July, 3 to 5 August and 10 August
1979) 0 During this period the surface
pressure, p, at both stations was less
than the mean for the "selected monsoon
period" by more than one standard
deviation, that is, p<p-o.
Mean sea level pressure, horizontal velocity
divergence and relative vorticity at 850 mb level, and
wind fields for th~se periods showed the following
features:-
(a) the southeast trade winds in the region
between the Mascarene high and the
equator were relatively strong or
weak during the periods of intense or
weak Mascarene high respectively.
(b) East African low level jet stream
was associated with horizontal velocity
convergence at the core and horizontal
velocity divergence on its two sides.
(c) the orientation of the horizontal
velocity divergence isopleths was in
the direction of the low level streamlines.
This pattern was better organised
during the period of intense Mascarene
high than during the period of weak
Mascarene high.
Cd) the negative values of relative-vorticity
at 850 rob level to the right of the low
level jet stream were large over the
Southwest Arabian Sea. The flow in this
region, particularly between Equator and
lOoN, had negative absolute vorticity
values. The flow in this region may
therefore be inertially unstable.
(e) the tropical easterly jet stream
oscillated ln north-south direction with the
oscillations in the intensity of the
Mascarene high, the jet stream being
furthest south during the period of
intense Mascarene high and furthest
north during the period of weak Mascarene
high.
The time series of individual station and
grid point data were subjected to spectral analysis.
It was seen that the 4-6 day and 11-14 day periods
were invariably dominant in all the elements analysed.
These will hereafter be referred to as 5-day and
quasi~biweekly periods respectively. These periods
were significant at 95% confidence limit for almost
all the elements.
Cross spectrum analysis was also performed
for different paramenters at the same station and for
stations at differen~,locations.
analysis showed:
The results from this
La) 5-day oscillation
li) The surface pressure and meridional
wind component at 900/850 mb level
showed 5~day oscillation over the
southwest Indian Ocean.
(ii) The period of this oscillation
increased from about 4.5 days to
about 6.5 days as one moved
eastwards along latitude 300S,
from longitude 3loE to longitude
850E.
(iii) Near latitude 300S, the S-day
oscillation had zonal wavelength
of 600 longitude and its phase
speed was l~J ms -1 f rom west to east.
The troughs and ridges of this wave
were orientated in nearly north -
south direction between 300S and the
near-equatorial region.
Civ) Statistically significant coherence
existed for this oscillation
between the Mascarene high surface
pressure and the meridional wind
component at 900/850 mb between
the equator and latitude 2S
o
So
(b) Quasi-biweekly oscillation
Ci) In addition to the S-day oscillation
mentioned above, a quasi-biweekly
oscillation was also observed in
surface pressure, and in the zonal
and meridional components of wind
at 900/850 mb level.
(ii) Statistically significant coherence was
observed between Mascarene high
surface pressure and zonal wind
component in the region between the
equator and latitude 2S
o
S.
Ci
o
Along 51 E, the meridional wind
component showed a wavelength of
1700 latitude and a phase speed of
-1
18m s.- inth e d ire c t ion sou th t0
north.
The 5~day oscillations ln the surface pressure
is associated with the trough/ridge movements in the
southern extra-tropical westerlies while the quasibiweekly
period appears to be due to the Hadley cell
circulation. The Southeasterly trades display major
peaks at periods similar to those of pressure. However,
the meridional wind component, V, shows a relatively
stronger peak at the 5-day period and the zonal wind, U,
has a relatively stronger peak at the quasi-biweekly
period.
The oscillation in the meridional wind at
850 mb level with observed quasi-biweekly period and
wavelength of 1700 latitude is considered to be
associated with the Hadley cell. | en |
