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IOD Holds The Key Amidst El Nino Scare During Monsoon

January 21, 2023 9:09 PM |

Official Climate Prediction Centre (CPC) ENSO probability forecast , based on observational and predictive information from early in the month of January, projected  evolving El Nino conditions during the Indian Summer Monsoon. 

Now, International Research Institute (IRI) ENSO forecast of mid January, based on real time analysis, corroborates earlier findings. These forecasts are based on 2 different data sets and therefore, may differ by margin of 0.5 Degree Celsius. 

A strong +VE Indian Ocean Dipole (IOD) can meet the challenge of El Nino and offset the ill effects.  It is on record that +VE IOD weakened the impact of El Nino in 1994 and 1997. Also, an extremely strong +VE IOD occurred in 2019 and diluted the impact of evolving El Nino. Southwest  monsoon nearly ended close to excess rainfall during that year. IOD had grown rapidly from May onward, peaked in Oct-Nov and decayed in December. This is the normal pattern which the IOD cycle follows. 

Strong +VE  IOD increases precipitation to above average over East Africa and Indian region. However, it decreases the quantum of rainfall in Indonesia and Australia.  Even with the latest simulation techniques, IODs are difficult to predict several months in advance. For Indian monsoon, IOD prediction is needed from summer to autumn (Northern Hemisphere).  However, it faces 'Winter Prediction Barrier' like the El Nino forecast encounters 'Spring Barrier'. 

On an average, the ratio of +VE, -VE and Neutral IOD is 1:1:2. It means , one out of 4 IOD events will be +VE and -VE each and the rest 2 will remain neutral.  Also, unlike El Nino/La Nina, the occurrence of consecutive positive IOD or negative IOD is rare.  Successive neutral IOD is  observed commonly and such incidents leave the fate of monsoon to El Nino/ La Nina and partly to the positioning and timings of transient MJO. 
The intensity of IOD is represented by anomalous SST gradient between Western Equatorial Indian Ocean and South-Eastern Equatorial Indian Ocean. This gradient is termed as DMI(Dipole Mode Index). When DMI is positive, the phenomenon is reflected as +VE IOD and vice versa. Threshold value of IOD is +/- 0.4Deg Celsius.  For an IOD event, these values need to be sustained for 8 weeks. 

 The current forecast of IOD is available up to June 2023. There is no clear consensus amongst various models on the likely state of IOD around the time of commencement of southwest monsoon. Reliability diminishes towards the far end of the forecast. The latest value of IOD index for the week ending 15th Jan was +0.01deg. Fate of the monsoon hangs and is subject to the IOD verdict after the' Winter Prediction Barrier'.

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