1. Indian Ocean warming – its extent, and impact on the monsoon and marine productivity Roxy M. K. 1, K. Ritika 1, A. Modi 1, P. Terray 2, R. Murtugudde 3, K. Ashok 1, B. N. Goswami 1, S. Masson 3, V. Valsala 1, P. Swapna 1, S. Prasanna Kumar 5 and M. Ravichandran 6 1 Indian Institute of Tropical Meteorology, Pune 2 Sorbonne Universites, Paris, France 3 University of Maryland, Maryland, USA 4 National Institute of Oceanography, Goa 5 Indian National Centre for Ocean Information Services, Hyderabad Indian Ocean warming: o Strong, monotonous warming during the last century in western Indian Ocean o Links to asymmetry and skewness in ENSO forcing o Largest ‘in-phase’ contributor to global SST warming Potential Impacts: o Weakening monsoon o Reduced marine productivity

2. Ocean warming in a changing climate IPCC, AR5, 2013 Where has all the heat gone? Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence)

3. Indian Ocean warming - background Basin-wide / Warm-pool warming in recent years Chambers et al. JGR, 1999; Alory et al. GRL, 2007;Rao et al. Climatic Change, 2012; Swapna et al. Climate Dynamics, 2013 Studies note basin-wide warming over Indian Ocean in the last 50 years Warm-pool enlargement in recent years

4. Indian Ocean during the last century western Indian Ocean warmed up to 1.2degC, in 100 yrs Roxy et al. J.Climate, 2014 Significant warming over western Indian Ocean. In early 1900s, the western Indian Ocean was much cooler than the warm-pool. The monotonous warming over west nullifies zonal SST gradient - may influence monsoon dynamics.

5. SST > 28degC = enhanced convection Gadgil et al., Nature, 1984;Roxy, Climate Dynamics, 2013 Significant increase in precipitation with respect to higher SSTs. SST-precipitation relationship when the lag is considered Mean summer (June-Sept) SST:

6. ENSO – El Niño & La Niña El Niño and La Niña events generally have opposing features but … may be not so in the Indian Ocean?

7. Asymmetry in ENSO forcing Influence of El Niño > La Niña El Niño induce significant easterly anomalies and positive SST anomalies over w.Indian Ocean but... La Niña events does not result in significant anomalies over the Indian Ocean

8. Skewness in El Niño forcing Increase in Frequency and Magnitude of El Niños Detrended anomalies show increase in frequency and strength of El Niños. The warm events over Indian Ocean also has increased. Occasionally, they cross the El Niño criteria (1 S.D. = 0.77 degC). Indian Ocean warming (above) associated with positive skewness over east Pacific (below)

9. Largest contributor to global warming? Indian Ocean warming “in phase” with global warming

10. Land-sea contrast decrease in the past century Contradicts model/observations for Northern Hemisphere Roxy et al. Nature Communications, 2015, Revised Though models and observations suggest increase in land-sea contrast over Northern Hemisphere due to global warming, it is different over South Asia/Indian Ocean. The decrease in land-sea contrast reflects in tropospheric temperature gradients also. Observations suggest an increase in land-sea contrast over Northern Hemisphere during recent decades

11. Warm Indian Ocean, Weak south Asian Monsoon Indian Ocean warming well correlated with weak Precip. (a) & (b) Decreasing trend in precipitation from Pakistan through central India to Bangladesh. Significant over central Indian subcontinent (horse-shoe pattern) (c) & (d) Trend and correlation with western Indian Ocean warming has similar patterns!

12. Weakened Monsoon precip/winds due to warming Model simulations with Indian Ocean warming Decreasing rainfall over the south Asian subcontinent: horse-shoe pattern in model simulations with increased IO warming Model simulated warming of WIO Model simulated response to warming

13. Warming – Marine Primary Production western Indian Ocean is a highly productive region...

14. Reduction in Marine Primary Production Chlorophyll trends in observations and simulations Chlorophyll trends Observations: Merged Satellite (SeaWiFS, MODIS, and MERIS) 1998 - present Historical Simulations: (Best of CMIP5) 1950-2005 Roxy et al. PNAS, 2015, Under Review 20-30% reduction in marine primary productivity over the western Indian Ocean

15. Warming stratifies the ocean - and suppresses the mixing of nutrients from the subsurface, reducing chlorophyll

16. Reduced plankton might increase the fish stress Along with the stress from fisheries industries...

17. Indian Ocean warming – its extent, and impact on the monsoon and marine productivity Indian Ocean warming: o Western Indian Ocean experienced strong, monotonous warming during last century o Links to asymmetry and skewness in ENSO forcing o Largest ‘in-phase’ contributor to global SST warming Potential Impacts: o Weakening of the monsoon o Reduction of marine phytoplankton