1. Study of intra-seasonal variation of moisture using precipitation isotopes of the Andaman Island, Bay of Bengal Nitesh Sinha, S. Chakraborty, R. Chattopadhyay, S. Sengupta, PM Mohan*, A. Datye MONSOON WORKSHOP-2015 Indian Meteorological Society Indian Institute of Tropical Meteorology, Pune-411008, India * Department of Ocean Studies and Marine Biology, Pondicherry University, Port Blair-744112, Andaman and Nicobar Islands, India

2. Keywords:  18 O &  D of rain water  SAMPLE = 1000((R SAMPLE /R SMOW )-1) where R is 18 O/ 16 O or 2 H/ 1 H Global Meteoric Water Line (GMWL)

3. D-excess =  D – 8.  18 O D-excess is a measure of the relative proportion of  18 O &  D. -It is correlated with the physical conditions of the oceanic moisture source area. -It reflects the prevailing conditions during evolution and interaction or mixing of air masses en route to the precipitation site. -Sub cloud evaporation is one of the major processes responsible for the spread of the d-excess. -High d-excess values usually represent secondary moisture source (such as, continental source).

4. Mean Monsoonal (JJAS) wind pattern over BoB at 850 hpa  The Bay of Bengal strongly responds to Intra seasonal oscillation.  The secondary sources of moisture is expected to be low. Objectives How the isotope ratios in rain are controlled by moisture source as well as by the atmospheric process. To understand the isotopic variation in precipitation on intra-seasonal time scale. Why ??

5. The oxygen isotope variation and rainfall time series for the year 2012 (upper panel) and 2013 (lower panel)

6. Composite wind pattern (2012-13) normalized d-excess > 1 normalized d-excess < -1 *Shaded: Monsoon Season *Non-Shaded: Pre & Post Monsoon Season

7. The d-excess anomaly for the year 2012 (upper panel) and 2013 (lower panel)

8. Moisture convergences calculated using zonal and meridional wind anomaly relative to the JJAS mean for the year 2012 (left panel) and 2013 (right panel) respectively.

9. For 2012, the integrated values of OLR and Rain correlated with daily  18 O variations R max > ±0.8 at 17-20 days For 2013, the integrated values of OLR and Rain correlated with daily  18 O variations R max > ±0.5 at 26-28 days * Box: 7.5-12.5N, 90-95E

10. Power Spectrum of  18 O variations with confidence curve (red) of 90% for the year 2012 (left panel) and 2013 (right panel) respectively

11. Conclusions: The isotopic composition of rainwater seems to be controlled by the dynamical aspects of the moisture rather than the individual rain events. Early to mid monsoon, significant amount of moisture generated locally (from ocean and rain drop re- evaporation). In the later phase of monsoon, moisture comes mainly from the Indian Ocean. The isotopic ratios also respond to the integrated effects of convective activities which appear to be linked to the monsoon intra seasonal oscillation. Hence,  18 O of rain may be used as recorder of ISO.

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