Reevaluate the reason of Ocean warming

For the past 30 years, it was believed that there is a threshold of temperature (29 degrees C) beyond which any increase in sea surface temperatures (SSTs) doesn't significantly affect the variability of rainfall over the Indian Ocean . Now, this classic hypothesis supported a study published in 1984, on the connection between Indian Ocean and the monsoon has been challenged by a publication in the journal Climate Dynamics authored by Dr. Roxy Mathew Koll, a climate scientist at the centre for Global Climate Change Research, Indian Institute of Tropical Meteorology, Pune.

The study expresses that there is a direct, factually straight connection between SSTs and rainfall, and that rainfall increases along side increase in SSTs, over the whole range of possible SSTs over the Indian Ocean (26-32 degrees C). This helps in quantifying the connection , with a rate of increase of up to 2 mm/day for a rise of 1 degree C in SST.

This understanding is significant during a changing climate scenario, where the SSTs are increasing. The previous belief was based on studies in which SSTs and rainfall were dissected simultaneously and it was found that beyond 29 degrees the rainfall showed no substantial variability.

This method is insufficient as SSTs affect rainfall and vice versa and the two cannot be separated when analysed simultaneously, and result in a wrong perception. However, Dr. Roxy’ s study utilizing satellite information over the Arabian Sea , Bay of Bengal and South China Sea found that there is a time delay between SST rise and rainfall increase. The delay is 5 days for Arabian Sea , and around 12 days for Bay of Bengal and South China Sea.

The difference within the lag is attributed to the spatial variance in surface convergence and uplift over these regions. Surface convergence is the convergence of air/wind at an area over the ocean surface due to low pressure formation or ascending branches of tropical air circulation.

The converging warm moist air near the surface has got to rise or uplift because it is less dense (lighter) than the air above. The uplift of the warm moist air leads to the convective activity (cloud formation).

"The winds and the convergence associated with the summer monsoon are stronger over the Arabian Sea , while they are relatively weaker over Bay of Bengal and the South China Sea. This probably results in a faster uplift and cloud formation over the Arabian Sea while delaying the response over the Bay of Bengal and the South China Sea", Dr. Roxy pointed out,in the journal.

Climate model experiments were conducted to see the effect of greenhouse warming (carbon dioxide, C0₂) on the ocean atmosphere interaction and the climate. The results of the experiments indicated that rainfall over the monsoon basins will still increase in a global warming scenario.