India, being an agricultural country, has a demand for accurate long-range forecast of the monsoon rainfall. Besides attempting to understand the physics of the monsoon problem it is also valuable to assess new parameters that can be used as predictor to improve forecasts.
Sea surface temperature (SST) is routinely used for predicting whether the total amount of rainfall that India receives during the monsoon season will be less or more than the long-term mean of 887.5 mm. However, Ocean Mean Temperature (OMT) has better ability to predict this than the sea surface temperature.
How Ocean Mean Temperature (OMT) can better predict the monsoon than the Sea Surface Temperature (SST)?
Compared with SST which has 60% success rate of predicting the Indian summer monsoon, OMT has 80% success rate.
In addition to better predictive success, the information on whether the amount of monsoon rainfall will be more or less than the long-term mean will be available by beginning of April, two months before the southwest monsoon can set in. This is because OMT is analysed by measuring the ocean thermal energy during the period from January to March. Southwest monsoon sets in around June 1 each year in Kerala .
The SST is restricted to a few millimetres of the top ocean layer and is largely influenced by strong winds, evaporation, or thick clouds. In contrast, OMT, which is measured up to a depth of 26 degree C isotherm, is more stable and consistent, and the spatial spread is also less. The 26 degree C isotherm is seen at depths varying from 50 –100 metres. During January –March, the mean 26 degree C isotherm depth in the Southwestern Indian Ocean is 59 metres.
External link: https://www.aoml.noaa.gov/phod/docs/omt-ismr-asl.pdf
PRACTICE QUESTIONS
QUES . With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct? UPSC 2020
1 . OMT is measured up to a depth of 26°C isotherm which is 129 meters in the south-western Indian Ocean during January–March.
2 . OMT collected during January–March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long-term mean.
Select the correct using the code given below:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Ans (b) The SST is restricted to a few millimetres of the top ocean layer and is largely influenced by strong winds, evaporation, or thick clouds. In contrast, OMT, which is measured up to a depth of 26 degree C isotherm, is more stable and consistent, and the spatial spread is also less. The 26 degree C isotherm is seen at depths varying from 50–100 metres. During January–March, the mean 26 degree C isotherm depth in the Southwestern Indian Ocean is 59 metres. Hence statement 1 is not correct.
