Figure: Coefficient of variability (relative magnitude of the standard deviation to the average value) of SST and OMT during 1993–2017 (Courtesy: SCIENTIFIC REPORTS | (2018) 8:12092 | DOI:10.1038/s41598-018-30552-0).

Currently, sea surface temperature (SST) is the only oceanographic input for the atmospheric prediction models for Indian Summer Monsoon Rainfall (ISMR).  However, the thermal energy needed for maintaining and intensifying monsoons comes from the upper ocean, not just from the thin layer represented by SST alone. Here, Ocean Mean Temperature (OMT) down to the 26o C isotherm depth was analyzed in the southwestern Indian Ocean (SWIO) along with ISMR. It was found that the OMT of SWIO during January-March is a better “qualitative” predictor of ISMR (whether total rainfall will be above or below 887.5mm), with 80% success rate, compared to SST, NINO3.4, Indian Ocean Dipole Mode Index and El Niño Southern Oscillation Modoki Index, having success rates of 60%, 52%, 48%, and 56%, respectively. The higher skill for OMT may be because it better represents the upper ocean thermal energy conditions whose variations are mainly responsible for ISMR activity, whereas SST accounts only for the temperature of a very thin layer influenced by meteorological factors such as strong winds, evaporation, or thick clouds. In addition, compared to SST, OMT variations are more stable and consistent with less spread (Figure above).  The failure of predicting the sign of ISMR using OMT in 20% of the cases illustrates the complexity of the monsoon system, which is not always controlled by SWIO thermal energy alone. An important next step is to identify physical evidence for the importance of OMT using dynamical models. 
The ISMR prediction for 2018 is below 887.5 mm, with 80% probability.  

(Summary written by M M Ali)

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Statistical Evidence for the Role of Southwestern Indian Ocean Heat Content in the Indian Summer Monsoon Rainfall

T Venugopal1,2, M.M. Ali*3,4, M. A. Bourassa3,5, Y. Zheng3, G. J. Goni6, G.R. Foltz6 and M. Rajeevan7

1 Department of Physics, Novosibirsk State University, Novosibirsk, Russia.
2 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, India
3 Center for Ocean-Atmospheric Prediction Studies, Florida State University, USA.
4 Indian Institute of Tropical Meteorology, Pune, India
5 Department of Earth, Ocean and Atmospheric Science, Florida State University, USA
6Physical Oceanography Division, AOML/NOAA, USA.
7Ministry of Earth Sciences, Government of India.

The details of the study are available at  https://rdcu.be/4vHw