Indian Forum for Water Adroit

[Thesis Colloquium] CAOS: 23 February 2018: "Influence of Western Ghat orography on temporal and spa


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PhD Thesis Colloquium

Title : Influence of Western Ghat orography on temporal and spatial distribution of rainfall over southeast Asia

Student: P. V. Arushi

Date/Time: Friday, 23 February 2018, 4:00 PM

Venue: CAOS Seminar Hall

The Western Ghat (WG) orography is an important link between the Indian landmass and surrounding ocean, compelled us to study the orographic influence over ISM circulation. Decade long simulations of a general circulation model (GCM) were carried out with varying (from 0 to 2 times the present) heights of the WG to understand the influence of this orography in spatial distribution of rainfall over the country. In boreal summer, most of the Indian land and its surroundings experience rainrates exceeding 6 mm/day with considerable spatial variability. Over southern Bay of Bengal along the east coast of the Indian peninsula (referred to as the Bay of Bengal cold pool or BoB-CP), the rain intensity is significantly lower (<2 mm/day ) than its surroundings and exhibit contrasting annual cycle of rainfall. We find that the lee waves generated by the strong westerlies in the lower troposphere in the presence of the WG mountains causes descent over the BoB-CP. This descent disappears in the absence of WG, resulting in a rainfall maxima in the boreal summer similar to that over its surrounding oceans.

Observations show that acceleration of meridional wind around the southern tip of peninsula in response to vigorous rainfall over north BoB reduces surface moisture convergence over BoB-CP. For further analysis, couple of simulations with varying SST gradients across the BoB are done for the same time
period. Results show that reduced convection over head BoB reduced inhibition over BoB-CP. However, WG stands out to be the major driver in suppressing convection over BoB-CP. Change in height of WG has contrasting affect on the western and eastern sides of Indian peninsula by modifying the total rainfall
budget over the country. Mid-latitude westerly jet shows a shift, which in turn impacts the climate over the high latitude regions as well. Change in circulation and associated moisture convergence over the equatorial region strengthened MJO activity in the Indian ocean basin with increase in WG height. These results underline the importance of narrow mountains like the WG in the tropics in determining the monsoon climate and calls for appropriate representation of such mountains in climate models.

In the last part of this thesis, we demonstrate some observed changes in MJO related convective activity over equatorial Indian Ocean and Maritime Continent during the period 1998-2015 using multiple data sets. We show that variance of MJO has decreased significantly during this period over most region where its intensity is high climatologically. However, the most probable location of highest MJO variance has been shifted southward, even increasing variance over northern Australia. We attribute this to the changes in sea surface temperature over southern Indian Ocean. These results have significant importance to understanding and predicting decadal scale changes in MJO activity.


Dr Arindam Chakraborty

Associate Professor
Centre for Atmospheric and Oceanic Sciences
& Divecha Centre for Climate Change
Indian Institute of Science
Bengaluru - 560 012, INDIA
Thanks & Regards

Sonali Pattanayak
Research Associate
Divecha Center for Climate Change