North East Monsoon will be here soon, its the lifeline of Tamil Nadu. So lets see whats in store for us this year. Many would have already seen outputs from other weather bloggers who have already given very good articles on the upcoming NEM. As I am getting old, it is getting very tough to put value added posts, apart from the ones already putup by the young energetic weather bloggers like Hemachander, Jaswanth Weatherman, Rainperal Marimuthu, Veeraraghavan, Sri Hari, T.Raja, Samuel Weather, Monsoon of Chennai, Gopinath, Kabir etc, I can go on and on. So I am not going to take class what is North East Monsoon and bore you guys, but will giving a different perspective. The simple questions of common man.
1. Which places get North East Monsoon rainfall in India and Tamil Nadu ? Why it is ignored in our Country and why it is crucial to Tamil Nadu.
2. North East Monsoon Rainfall wrt Lanina, neutral IOD and Negative IOD ?
3. Why MJO is important for NEM Rains irrespective of the Elnino or Lanina or IOD and How 2020 NEM is going to be ?
4. Will delayed NEM onset give less rains ?
5. Why North East Monsoon Prediction is difficult and is 50% based on guess ?
1. Which places get North East Monsoon rainfall in India and Tamil Nadu
Only one state (Tamil Nadu) is dependent on North East monsoon with nearly 50% rains of its annual quota. This is one reason this North East Monsoon is not given much importance and ignored. The rains are mostly associated with Low Pressure Systems & Cyclonic storms. Its the season where each Low Pressure captures headline space in Tamil Nadu, Pa groups are in holiday mode already due to Covid-19, they wont be too keen this time to see Collector announcement. Govt will be on their toes with elections nearing and they dont want to take any chances. Farmers keenly watching, it can go on….No where else in our country monsoon rains will be watched so closely than in Tamil Nadu.
In Tamil Nadu, coastal districts like Chennai to Thoothukudi are dependency on NEM varies from 65% to 85%. So in these places if North East Monsoon fails we automatically go into drought zone. And you can see the rainfall reduces as we go inland into Tamil Nadu. Krishnagiri, Dharmapuri, Tirupattur gets the least rainfall during North East Monsoon in the State. So a person in North West Interior Tamil Nadu should not compare their rainfall with that of coastal belt. Most of the rains in interiors happen during October month when transition period is on and after that they depend on rains from cyclone which move from Bay of Bengal via Gulf of Mannar or Comorin Sea towards Arabian Sea.
Only two districts are exception in Tamil Nadu who get balanced rains from NEM and SWM and they are Kanyakumari and Tiruvannamalai. All coastal districts are highly dependent on NEM while the two districts in western ghats Kovai and Nilgiris are dependent on SWM.
2. North East Monsoon Rainfall wrt Lanina, neutral IOD and Negative IOD ?
Elnino or Lanina or Neutral ?
I dont want to go too much into Elnino and Lanina or Neutral. The images will explain how it affects the walker cell and in turn affects Global Climate. When the sea surface temperature near the dateline to Peru in Pacific Ocean is warmer than normal we call it as Elnino and when it is colder than normal we call it as Lanina and when it is normal, we call it as ENSO Neural. We follow the ONI Index to determine whether it is Elnino or La-nina or Neutral and how strong the event is – NOAA Elnino / Lanina years . If the Nino 3.4 index three month average crosses above +0.5 we call it as Elnino year and if it is below 0.5 we call it as Lanina. We can see 2015, 1997, 1982 (red color) are strong elnino events in history. The blue ones indicate Lanina years. Elnino happens once in 4-5 years like 2009, 2006, 2002 these are called moderate elnino years. It has nothing to do with Global Warming and its part of climate cycle and have been there for 100s of years. Lanina is opposite and cooling of the nino 3.4 region and it is announced if the sea surface temp anomaly in the nino 3.4 region dips below – 0.5 we call it as Lanina.
Lanina is good for SW monsoon historically and bad for North East Monsoon (note 2016 was an Lanina year and Tamil Nadu got worst North East Monsoon Rainfall in history). The opposite Elnino year are statistically good for Tamil Nadu North East Monsoon rainfall and eg.2015 was an Elnino year and we got massive rains and traditionally SWM rainfall are poor across the country during a Elnino year.
As you can see we are Lanina zone during this NEM 2020. This alone does not decide NEM the correlation to Elnino/Lanina is 0.4. We have seen very good rains in Lanina year too.
The forecasts for ENSO region indicate that for rest of the North East Monsoon, the ENSO will be in Lanina state.
Positive IOD – Warm water to shift towards Africa and this sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east near Indonesia and warmer than normal water in the west near Africa.
Negative IOD – Warmer waters to concentrate near Indonesia -Australia. This sets up warmer than normal water in the east (Indonesia) and cooler than normal water in the west (Africa). The images will explain itself.
Lets see the model forecast for IOD, as you can see IOD is going to be in borderline Neutral to Negative zone and this will help in pick up lot of convection in Bay of Bengal. Coupled with Lanina, we are going to see lots of Low Pressure this NEM even in December.
Lets see some of the comparable years with Lanina in Pacific and Neutral to Weak Negative IOD in Indian Ocean and the rainfall in NEM. Years from transition from Nino to Lanina
2008 – TN strong excess with 560 mm and Chennai above normal with 950 mm rainfall
2005 – TN rainfall massive excess 780 mm and Chennai Massive excess with 2100 mm rainfall
1983 – TN rainfall excess with 600 mm and Chennai above normal with 900 mm rainfall
1970 – TN normal rains with 420 mm and Chennai below normal with 630 mm rainfall
1964 – TN normal rains with 480 mm and Chennai above normal with 900 mm rainfall
This combo is not a bad combo, infact with little luck we can even get very good rainfall. Lets hope this combo clicks and we are in for a treat.
3. Madden Julian Oscillation (MJO) – the NEM driver
MJO is an eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the along the tropics and returns to its initial starting point on a average of 30 to 60 days. There can be multiple MJO events within a season, and so the MJO is best described as intraseasonal tropical climate variability.
The MJO consists of two parts, or phases: one is the enhanced rainfall (convective) phase and the other is the suppressed rainfall phase. Strong MJO activity often dissects into halves: one half within the enhanced convective phase and the other half in the suppressed convective phase. These two phases produce opposite changes in clouds and rainfall. The location of the convective phases are often grouped into geographically based on stages from number 1-8 (refer image)
In the suppressed phase, winds converge at the top of the atmosphere, forcing air to sink and, later, to diverge at the surface and s air sinks from high altitudes, it warms and dries, which suppresses rainfall. So MJO suppressed phase can bring dry weather.
In the convective phase, winds at the surface converge, and air is pushed up throughout the atmosphere. At the top of the atmosphere, the winds reverse and Such rising air motion in the atmosphere tends to increase condensation and rainfall. MJO also helps in tropical cyclonegenesis preferentially during the convective phases.
Example of how MJO is everything for NEM rainfall
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2015 saw prolonged MJO presence in our basin and Tamil Nadu / NEM ended in massive positive side.
In 2016, MJO was completely absent in our Basin and NEM went for a toss with worst ever rainfall.
Right now MJO is in Maritime Continent and sending pulse after pulse via Indo-China into Andaman Sea and not allowing the SWM to withdraw from remaining areas of Andhra /Odisha /East India. This is not allowing the monsoon axis /trough to dip down and westerlies continue to persist thereby delaying the onset of North East Monsoon.
It is been a while since we had prolonged MJO presence in November in our Basin, though weak in amplitude, MJO is coming into our Basin in 2020 by 1/2nd Week of November. The presence of MJO is enough for enhanced rains and not be in higher amplitude.
What is that we can conclude from the above, there is one key point to be noted be it any years, if November month sees good rains very rarely that North Monsoon has failed. “A good November always ended the monsoon in normal to excess category”. And with MJO seen active in November, this year we can expect Normal to above normal rains for Tamil Nadu (450-470 mm of rainfall) + or – 10% around these figure.
4. Will delayed NEM onset give less rains ?
There is always a opinion that onset on time would result in good rains and delayed onset means less rains. I have taken the last 30 year analysis with Tamil Nadu and Chennai City Co-relation with respect to onset. Lets see the results
You can see from the above table, on time onset has been a curse for Chennai resulting in 8 years in failure. While for both Tamil Nadu and Chennai if the onset has happened before October 15, most of the time it has ended in excess. Even on 3 years on delayed monsoon onset, we have ended in excess with the last great flood on 2015 was also a delayed onset. There is no fool proof correlation and one cant be sure to predict monsoon with onset dates.
We already saw 1st week October Low Pressure giving rains to Andhra/Odisha, this came from from Pacific too. The present Depression came as pulse from Pacific and expected to cross India peninsula at Andhra Coast and move into Arabian Sea. Next week another pulse is moving into Andaman sea, while entering Andaman Sea itself the low is above the latitude of Chennai and so this too wont give much rains to Tamil Nadu. We are yet to even see the pseudo easterlies and so monsoon onset is going to be delayed till month end.
5. Why North East Monsoon Prediction is difficult and 50% is based on guess ?
As you all are well aware that North East Monsoon has one small corelation with Elnino, most of the Elnino years Tamil Nadu has got above normal rains. Almost in all the recent Elnino years both Tamil Nadu and Chennai has seen above normal rains. Other than that there is very little corelation. In positive IOD years we see very active Arabian Sea while in negative IOD years we see very active Bay of Bengal. The NEM rains are dependent on low pressures and cyclonic systems. The time period is short if one cyclone misses and moves away from Tamil Nadu, there will be break in rains for 4-5 days to week since next system should develop. Eg 2013 we missed 3-4 cyclones and all went to Andhra and our NEM went for a toss. At the same time, if one good Depression or low pressure gets stuck in Gulf of Mannar and gives 4-5 days of rains we nearly can get 30-40% of the total NEM rains. Eg, in 2008 we all lost hope of monsoon, almost first 20 days of November was dry, it was peak monsoon days and then in the last week came Cyclone Nisha and it dumped so much rains we jumped from deficit to excess in that 5-6 days of rains. Orathanadu got 1200 mm rains in 4 days which is more than 500 mm of its total NEM quota. One good system is all what required for monsoon to swing. And no one predict these extremes months in advance. So North East Monsoon be it the official agency or the weather bloggers incl me, its always 50% based on some intuitions / guesses. The 2008 Nisha is one dream rain filled system which bloggers dream till now.
Hope some of the basic questions of NEM are answered. Lets hope November rocks this monsoon and active pacific is seen in December too and many low pulses will be sent into Bay of Bengal in December too and in December systems move in low latitude, though models show bleak picture, my 50% of the guess says this monsoon will be Good for Tamil Nadu particularly for coastal Tamil Nadu. Further with Lanina mode, we can see monsoon extend well into late December.
Just waiting eagerly for the 1st easterly wave to hit Tamil Nadu, that sight itself is special.
