Influence of Western Ghats Orography on Temporal and Spatial Distribution of Rainfall over South Asia

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.