Monsoon as such is a complex phenomenon in terms of its arrival, withdrawal and variability. No two Monsoon seasons have followed the pattern.
The in-built complex characteristics of Southwest Monsoon are further influenced by external oceanic atmospheric phenomena. The most prominent ones being the El Nino, Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO).
El Nino
El Nino is an oceanic atmospheric phenomenon wherein the equatorial waters in the Eastern Pacific warm up abnormally due to weakening of trade winds. This weather system re-emerges after a gap of about two to five years and its effects last for about 12 months on an average.
El Nino started evolving last year and crossed the threshold value in the spring stoke fall of 2014. El Nino has been the most-talked about phenomenon this Monsoon. Its effects are expected to continue throughout this year and spill over to the first few months of 2016 as well. El Nino is invariably linked with poor Monsoon rains in the Indian sub-continent. And, this El Nino is possibly the strongest in the recent decades. All Global weather models have consensus about El Nino being strong this season.
Increased number of typhoons in the recent past is also aiding in the spread of warm waters across the Pacific, accentuating the El Nino affects.
Here’s a look at the Nino indices in the last four weeks:
The composite picture of extensive study of Monsoon rains in last 11 years shows that, during an El Nino year North and South India remains at risk of below normal Monsoon rains. But, Central and East India performs relatively better.
Indian Ocean Dipole (IOD)
IOD is defined as the difference in the sea surface temperature between the two equatorial areas of the Indian Ocean – the western Indian Ocean and the eastern part. IOD is simply the periodic oscillation of sea surface temperatures, from ‘positive’ to ‘neutral’ and then ‘negative’ phases.
Whenever the sea surface temperature of the western Indian Ocean rises above normal (0.4°C) and becomes warmer than the eastern end, it leads to a positive IOD. This condition is favorable for the Indian Monsoon as it causes an apparent barrier in the eastern Indian Ocean and diverts all the southwesterly winds towards the Indian sub-continent.
Since the beginning of the Monsoon season this year, the IOD remained +/- 0.5, which is considered to be neutral IOD. Throughout June and July, IOD never exceeded the 0.5 threshold value. This week, IOD exceeded the 0.5 value for the first time. And, Skymet believes that it will remain positive for rest of the season.
The image above suggests that a weak positive IOD event will persist and will not do much to neutralize the impact of El Nino.
Madden-Julian Oscillation (MJO)
MJO is again an oceanic-atmospheric phenomenon which affects global weather activities. The MJO can be defined as an eastward moving 'pulse' of clouds, rainfall, winds and pressure near the equator. MJO is different from El Nino and IOD. It is a transient feature and that typically recurs every 30 to 60 days.
As its periodicity extends up to 60 days at times, it is also known as ISO or Intra Seasonal Oscillation.
This traversing planetary scale phenomenon travels across the equatorial region of the Globe carrying a pulse of clouds and rains. It amplitude or powers is invariably large over warmer waters of the Indian Ocean and relatively less over the cooler waters of the Pacific Ocean.
During a normal Monsoon season (bereft of El Nino) its amplitude is less over the Eastern Pacific Ocean. In case of an El Nino year, when the Eastern Pacific warms up, MJO gains amplitude there as well. This Monsoon MJO remained outside the Indian Ocean throughout June, July and up to the first week of August.
Below is the image displaying the daily MJO index from July 2 to August 10
Monsoon 2015 – Performance and Forecast
IOD on its own does not have the potential to significantly alter the rainfall pattern, especially in an El Nino year. IOD normally finds it difficult to neutralize the negative effects of a strong El Nino, as is the case this time around. So, here comes the role of MJO.
IOD along with MJO gains the strength to subdue the effects of El Nino on Indian Monsoon, but not for a long period of time.
This season MJO is following a disturbed pattern and its cycle has become highly unpredictable. Its behavior is likely to remain erratic in the coming days. MJO has spent the entire Monsoon season so far in the unfavorable zone and El Nino in any case is very strong. Given the situation, IOD has not been able to counter the effects of both El Nino and MJO. August will thus, witness below normal Monsoon conditions. Cumulative rainfall for August and September is forecast at 92% and 112% (error margin of +/-9%) of their monthly LPAs, respectively.
Image credits - Climate Prediction Center, .jamstec.go.jp, Keralaayurveda
