| dc.description.abstract | The Western Ghats (WG) orography that runs along the western coast of peninsular India, is a long
and narrow mountain chain with an average height of about 1200 meters. The orientation of this
orography is approximately perpendicular to the mean low–level winds over the eastern Arabian
Sea during boreal summer season (June–September; JJAS). In JJAS, while western sides of this
orography receive high intensity of precipitation, a region to the lee side of the mountain, termed as
the Bay of Bengal Cold Pool (BoB–CP), receive very less precipitation. In this thesis, we have
investigated the role of WG orography in existence of the BoB–CP. In addition, we have shown the
influence of WG on climate around the globe as well as the intraseasonal variability over the Indian
region and the equatorial Indian Ocean.
In the first part of the thesis, we have documented the climatology of BoB–CP and how the region
is peculiar compared to other parts of south Asia. In boreal summer (JJAS), most of the Indian land
and its surroundings experience rainrates exceeding 6 mm/day with considerable spatial variability.
Over southern Bay of Bengal (BoB) along the east coast of the Indian peninsula (BoB–CP), the rain
intensity is significantly lower (<2 mm/day ) than its surroundings. This low rainfall occurs despite
the fact that the sea surface temperature in this region is well above the threshold for convection and
the mean vorticity of the boundary layer is cyclonic with a magnitude comparable to that over the
central Indian monsoon trough where the rainrate is about 10 mm/day. It is also noteworthy that the
seasonal cycle of convection over the BoB–CP shows a primary peak in November and a secondary
peak in May. This is in contrast to the peak in June–July over most of the oceanic locations
surrounding the BoB–CP. We use an Atmospheric General Circulation Model (AGCM) to
understand this paradox. Decade long simulations of the AGCM were carried out with varying
(from 0 to 2 times the present) heights of the WG. We find that the lee waves generated by the
strong westerlies in the lower troposphere in the presence of the WG mountains cause descent over
the BoB–CP. Thus, an increase in the height of the WG strengthens the lee waves and reduces
rainfall over the BoB–CP. More interestingly in the absence of the WG mountains, the BoB–CP
shows a rainfall maxima in the boreal summer similar to that over its surrounding oceans. The
redistribution of rainfall with the increase in height also resulted in the increase in Indian summer
monsoon rainfall (ISMR) by almost 15%. The WG also impacts the climate over the middle and
high latitude regions by modifying the upper tropospheric circulation.
In the second part of the thesis, we have investigated the role of convection over northern BoB in
controlling the rainfall over BoB–CP. Even after the removal of WG, the BoB–CP shows low level
divergence, which leads us to speculate the role of acceleration/deceleration of meridional winds by
convection over northern BoB. Intraseasonal variations (ISVs) over BoB–CP also depicts the
existence of the see–saw between precipitation over head Bay of Bengal and southern Peninsular
India, including BoB–CP. Based on these findings, we performed decade long simulations with
varying Sea Surface Temperature (SST) gradients over northern BoB. The SST gradient–
experiments reveal that convection over north BoB further reduces rainfall over BoB–CP by
intensifying the upper level lee–waves, causing down–draft and accelerating the low level winds
causing divergence near the surface. A combined effect of WG and SST gradients shows that even
though the SST gradients influence convection over BoB–CP, the effect is overshadowed by the
absence of WG indicating that the WG has dominant control on the convection over BoB–CP than
the other.
In the third part of our study, we analyzed the implications of the perturbations in WG orography on
ISVs over India as well as over the equatorial Indian Ocean region. The increase in height of WG
leads to the intraseasonal oscillations (ISO) to strengthen over the equatorial region. With the
absence of WG, the northward propagations have become stronger compared to the mean state.
These variations in ISVs also altered the ISVs over the equatorial Indian Ocean. Madden Julian
Oscillation (MJO) is the most important component of ISVs over the equatorial belt, which we have
investigated in this study. The model captures the MJO signal reasonably well with slight
underestimation in its strength and meridional extent. With the increase in WG height, there is a
change in circulation pattern around WG region, increasing the meridional as well as the westerly
component of wind over the equatorial region. This provides more moisture as well as an increase
in boundary layer convergence, eventually leading to the increase in convective activity associated
with MJO over the region. This also suggests that it is essential to represent the orographic features
near the equatorial region in order to simulate the MJO reasonably well in a model.
In the last part of the thesis, we document the observed changes in the variability of rainfall and
outgoing longwave radiation (OLR) associated with the MJO during 1998 to 2015, when reliable
satellite derived daily rainfall and OLR are available. Observations show recent weakening of
variance of convective activity with MJO across the equatorial Indian Ocean (EQIO) and Maritime
Continent (MC) during boreal summer as well as winter seasons. However, during boreal winter
MJO variance increased significantly over northern Australia and north–eastern Pacific. Using rain
gauge based observations we further show that the decreasing trend of 30–60 day intraseasonal
mode over MC is significant for an extended range of period (1958–2007). During northern
summer, the MJO variability in the POST (2007–2015) period display remarkable reduction in
convection for all the wavenumbers compared to PRE (1998–2006) period. During northern winter,
along with reduction of intensity, the maximum variance of MJO related activity is shifted from
lower to higher wavenumber in recent years. Thus, during the POST period, the convection
associated with an MJO is broken down into smaller scales, reducing the variability in rainfall along
the longitudes.
The multivariate MJO index (The Wheeler–Hendon Index) exhibits a shift from a higher probability
of stronger events to weaker events over EQIO, MC and Western Pacific Ocean in the recent years
both during summer and winter seasons. There is a southward shift in the location of maximum
variance from northern latitudes towards the southern latitudes either weakening the northern
branch of maximum variance or reducing (increasing) the asymmetry along those longitudes during
summer (winter). The relationship between OLR and rainfall has also modified from PRE to POST
between the Indian Ocean and Maritime Continents possibly due to increase in cloud top height and
recent sea surface warming over the Indian Ocean. These variations in MJO strength can have a
huge impact on the local and remote climate systems across the globe and can modulate the extreme
events across the globe.
This study highlights the importance of WG orography in modulating the convection over BoB–CP,
redistribution of rainfall over the subcontinent and the climate over the globe. These mountains can
impact the ISVs over India as well as the equatorial Indian Ocean. The results of this study
underline the importance of narrow mountains like the WG in the tropics in altering the global
climate and possibly calls for a better representation of such mountains in climate models. It also
provides insights into the recent weakening of MJO and its possible influence on the climate as well
as the extreme events across the globe. | en_US |
