Making sense of the Sri Lankan Monsoon

Rainfall in Sri Lanka is not predictable and monthly averages mean very little. Although the 3 main rainy seasons start pretty much on time (give or take a fortnight) the amount of rain that falls during those seasons is variable from year to year,and in the North and East the dry season may be getting drier. So why is the monsoon so variable both from one year to the next and over longer periods?

First a little bit of simplified theory.

Surface temperature, air pressure and surface winds

things you need to know if you don’t already:

  1. rising air = low pressure: it is caused by one of three mechanisms:

a.  heating from below – convection

b.  warm ( less dense ) air rising over cooler (more dense air ) – frontal rainfall

c.  where two air masses meet or converge – convergence.

Rising air is associated with cloud formation and rainfall. (air cools, condenses – cloud and rain)

  1. descending air = high pressure, the result of:

a.  cooling from below which causes air to become heavier at the base and sink towards the surface.. or

b.  upper atmosphere convergence below the tropopause which forces the air downwards

Descending air is associated with dry conditions ( descending air heats up)

The diagram below gives a general idea of how that works. Air in a low pressure cell rises into the upper atmosphere until it reaches the boundary with the troposphere ( the tropopause) where it is prevented from rising and moves sideways. Being much colder and/ or where there is upper atmosphere convergence, the air sinks back to the surface creating a circuit if you like.


Fig 1

3.  Surface winds move from high pressure to low pressure

What controls the monsoon

 There are three processes at work and because they operate semi-independently of one another it makes the understanding of how the monsoon operates tricky.

  1. The Inter – Tropical Convergence Zone ( ITCZ ) and the way it moves explains the seasonal reversal of winds over the Indian Ocean basin; the change from the South West Monsoon to the North east Monsoon
  2. The ENSO Pacific Ocean events (EL Nino and La Nina) impact on the Indian Ocean by causing winds and rainfall to shift around in response to what happens in the Pacific Ocean
  3. The Indian Ocean Dipole where changes to  Sea Surface temperatures (SST’s)  also re-organise circulation patterns in the Indian Ocean

The Inter – Tropical Convergence Zone ( ITCZ )

The ITCZ is a zone of rising air (Low Pressure) located around the equator where the water (SST’s) is warmest. This is convective uplift. Rising air is associated with cloud formation and rain. At around 30 degrees of latitude either side of the ITCZ are regions of descending air. The sub-tropical high pressure belts. Surface winds blow from the high pressure belts inwards towards the ITCZ.

Fig 2. Global Circulation Patterns

Notice the don’t flow at right angles to the equator. The actually move as curved lines; north -east to south – west in the northern hemisphere, ( The North East Trade winds) and south – east to north west in the southern hemisphere (the South East Trade winds). This is due to the coriolis force; check it out here.

So why do winds migrate?

  • Remember, the SST’s control the location of the ITCZ. As the sun (which heats the ocean) moves north in the northern summer, it follows the highest SST’s will migrate north and that drags the ITCZ north.It also follows that the reverse will happen in the southern summer and the ITCZ will migrate southward.

Fig 3 The migration of the ITCZ

  • Now that is going to have an impact on the pattern of surface winds, as this simple diagram shows;

Fig. 4

and what this shows is that:

  • in January the N.E. Trades are pulled south of the Equator, deflecting to the left of their path. This is the North east Monsoon.In July the opposite occurs.
  • The S.E. Trades are pulled across the equator, and as the coriolis forces deflects the wind to the right of its path  instead of being S.E. trades they become S.W monsoon winds.

Figure 5 is a more detailed version of the process.

Fig 5

Notice the region of high pressure in the southern Indian Ocean. This is called the Mascarene High; for more try out this link. I will get back to this later in the blog.

So the migration of the ITCZ explains the seasonal reversal of wind patterns and broadly when that happens. However, it doesn’t explain why both the North East and the South West monsoons are so variable in terms of how much rain falls. That is because there are other forces at play which have a direct impact on the pattern of SST’s which in turn control surface air pressure and winds.

They are

  1. El Nino/La Nina events
  2. The Indian Ocean Dipole which has three phases; positive, negative and neutral
The influence of El Nino / La Nina on the monsoon

El Nino is a Pacific Ocean event, right? Well yes it is, but what happens in the Pacific Ocean has a knock on effect on the circulation patterns in the Indian Ocean; and it is complicated.

A.  More backgound: The Walker Circulation Pattern

The Walker circulation is an ocean-based system of air circulation that influences weather and is the result of the difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean. Normally, the air over the tropical western Pacific is warm and wet with a low pressure system, (rising air) and the cool and dry eastern Pacific lie under a high pressure system (descending air).

This creates a pressure gradient and causes surface air to move east to west, from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up in the atmosphere,west-east winds move in the reverse direction to complete the circulation.

Fig 6 ENSO neutral

What you need to notice is that there is a major zone of uplift over south east Asia. Also note the area of descending air over the Middle East and the weaker uplift zone over East Africa. That helps to maintain a predominant west to east surface air flow over the Indian Ocean Basin which reinforces the monsoon.

So if you are ok with that then let’s look at how the El Nino upsets everything;

B.  ENSO events; El Nino and La Nina

El Nino is an ocean sea surface temperature event that is now pretty well understood. During El Nino events the normal Walker circulation pattern weakens, allowing warmer water to migrate eastwards towards the coast of South America. At the same time the main zone of uplift (low pressure) moves towards the central Pacific and in the western Pacific the surface airflow reverses to become west to east.

Notice now that there is descending air (high pressure) over South East Asia, and a strengthened zone of upfift over East Africa. The net effect is to establish an easterly airflow over the Indian Ocean, working against the South West Monsoon in particular.

Fig 7

So what you might expect is that in El Nino years the South West Monsoon is weaker over South Asia. This in turn can lead to reduced rainfall, and possibly, drought conditions.

La Nina is the reverse of El Nino so far as the Pacific Ocean circulation is concerned. Here the warmer water moves into the western pacific intensifying the zone of uplift over South East Asia ; the cell moves westwards effectively. Notice the zone of uplift over East Africa has gone.. bad news for those areas.. but the west to east airflow over the Indian Ocean pattern strengthens intensifying the South West Monsoon.

Fig 8

So to summarise so far: the two influences on rainfall we have looked at are

  1. The movement of the ITCZ
  2. El Nino/La Nina

It is worth noting that these events act independently of one another..

But now we have to add the third element; The Indian Ocean Dipole

C.  The Indian Ocean Dipole

 First identified in 1999, the Indian Ocean Dipole refers to spatial differences in sea surface temperature over the tropical Indian Ocean. There are three phases:

  • A neutral phase; when the SST is broadly the same across the tropical ocean basin.
  • The positive phase; this is where there is cooler than normal water in the tropical eastern Indian Ocean and warmer than normal water in the tropical western Indian Ocean.

Fig 9

Increased convection over the western Indian Ocean (warmer air rise = low pressure = rain) has a knock on effect for the monsoon; why?

ok so remember that the SW Monsoon rules in May – August; the ITCZ migrates northward and the winds blowing from the SE become south westerlies when they are dragged across the equator into the northern hemisphere. ( check out figs 3&4 ) Plus being warmer the relative humidity of the air is increased. This should mean the monsoon intensifies

  • The negative phase; this is where there is warmer than normal water in the tropical eastern Indian Ocean and cooler than normal water in the tropical western Indian Ocean.

Fig 10

Here the pattern reverses. To the west of India there is a zone of descending air which surpresses the moisture content of the surface winds and leads to lower rainfall.


D. Putting it all together

These three influences don’t necessarily synchronise with one another and are pretty much independent of one another as well.

Generally ENSO impacts the Indian Ocean by re-organising the atmospheric circulation but so does the Indian Ocean Dipole.


  • El Nino = drought
  • La Nina = enhanced monsoon
  • Positive Dipole = enhanced monsoon
  • Negative Dipole = surpressed monsoon

But as I wrote earlier.. it now gets tricky.

  1.  Don’t forget the system can also be in neutral!

2.  Not only that but the ENSO and Dipole events vary in intensity and impact.

Last point; various phases of both ENSO and the IOD can occur concurrently but at different relative strengths. Confused yet?

One example; a moderate El Nino such as occurred in 1997 should have lead to a poor monsoon over India but it didnt. this was because it was outweighed in influence by a stronger positive IOD event and in 1997/8 India received above average rainfall. This puzzled many meteorologists and led to the discovery of the IOD in 1999.

So what doe the evidence show? The following table illustrates how the different events have come together to affect the monsoon in recent years.

An IOD event can offset the impact of El Nino or La Nina although in 2004 it was El Nino that “won”.

Impact of ENSO events


year occurrence Impact % normal monsoon rainfall
2004 El Nino Drought 88
2005 Neutral Normal 101
2006 Neutral/positive IOD Normal 103
2007 La Nina Excess 110
2008 La Nina/negative IOD Above normal 105
2009 El Nino Severe drought 79
2010 La Nina/negative IOD Normal 100
2011 La Nina Normal 104
2012 El Nino/Positive IOD below normal 92
2013 Neutral above normal 106


So that’s what it comes down to.. a dynamic system driven by variations in sea surface temperature which drive atmospheric circulation patterns.

The complicating factors are that:

  • The time spans between ENSO events are not even.
  • The ENSO events vary in strength.
  • Occasionally the IOD intervenes

Looking then at all of this: It does shed some light, however, on why monsoon rainfall is highly variable and, therefore, so difficult to forecast. It also may help us to understand why South Asia is prone to periodic drought; the subject of the next article.

Footnote: Don’t forget Global Warming!!

According to Dr. Evan Weller abased at Monsah University in Australia global warming is set to complicate matters even more.

ASs climate changes, the warming up of the ocean will not be the same across the globe. Some regions will warm more than other regions. Over the eastern Indian Ocean, the waters to the north are predicted to warm faster than those in the south. This will have the effect of  pushing the ITCZ further north over the eastern Indian Ocean. It will also affect the SST  gradient north to south and that impacts on pressure differences and ultimately circulation patterns. The question is how will this interact with ENSO and IOD events and what effect will that have on the climate of South Asia.

The South Asian Monsoon; same as it ever was?

What is happening to the Indian Ocean monsoon? Has it become less predictable? Is it becoming affected by global warming? and finally, are droughts in Sri Lanka getting worse as a result?

The monsoon rains are important not only for agriculture in the region but also power generation.  Sri Lanka generates around 40% of its electricity from H.E.P. for example. So getting an understanding of how the monsoon seasons work is really quite important for a whole range of reasons.

a.  In the first of three linked articles I am going to be analysing rainfall and drought data to find out what is actually going on.

b.  The second article will look at what drives the monsoon, in particular the interplay of 3 factors and how they lead to changing patterns of sea surface temperature ( SST ), pressure and wind patterns, and how this affects rainfall. The three phenomena are:

  • The migration of the Inter Tropical Convergence Zone ( ITCZ )
  • The El Nino/La Nina events which occur in the Pacific Ocean ( ENSO )
  • The Indian Ocean Dipole ( IOD )

c.  The third article will focus on patterns of drought in Sri Lanka.

Analysing Rainfall: getting the data

For this article I am using rainfall data for 4 stations; Batticaloa, Jaffna, Colombo and Galle. I had data for the period 2000 – 2015 and added to that data for the same locations for the period 1985-90. (I would have liked more ie; 1980 – 2000 but I couldn’t access the data).

So my sample size is  small and skewed towards the later period but it does give some indication of trends.

I chose some simple statistical methods to analyse the data;  I looked at each month in turn  over the 15 year period and calculated for each month and for each station:

  • the mean rainfall
  • standard deviation
  • coefficient of variation; it was this measure that I was really looking for; see below:

The coefficient of variation  ( Cv )is a measure of the spread of data that describes the amount of variability relative to the mean. It is calculated by dividing the standard deviation by the mean.

values close to zero indicate that the the data set shows a lower degree of variability and vice versa; In the results section  I will give just the Cv (not the mean or SD )

So if the data shows a Cv of say 0.50 what that suggests is that for any one year  the actual rainfall received will be in a wide range: 50% above and below the mean. Let’s say average rainfall is 500mm for a month then with a Cv of 0.5  the actual rainfall could be expected to fall within a wide band 250 mm to 750 mm.

Not very predictable.

I carried out the same calculation for the 1985-90 periods so that I could compare the two. I also looked at the pattern of rainfall through the year to see if it changes at all. I wanted to know the following:

  1. How variable is the annual rainfall total from year to year for each station
  2. For any given month how variable is the rainfall total over the 15 year period, and in comparison with the shorter 1985-90 period.
  3. Whether the amount of rainfall during the monsoon periods changing and if so how?
  4. Whether the distribution of monthly rainfall changed significantly over the period; is the monsoon coming earlier or later?


The location of the 4 weather stations


The monsoon seasons in Sri Lanka;

There are four monsoon seasons in Sri Lanka:

Period Name Comment Regions Affected
March – Mid May First Inter-monsoon (FIM) limited impact
May-July South West Monsoon (SWM) S.W. Winds Heavy rain Southern Coast, South West, West
October- November Second Inter-monsoon (SIM) S.W. winds Heavy rain and tropical cyclones possible South and South West, East coast
November – December North East Monsoon (NEM) North east Winds North and East

That has an impact on the rainfall distribution for different parts of the country;

  1. Colombo and Galle  in the south west of Sri Lanka both show two rainfall peaks during the year coinciding with the SWM and SIM periods.  (not one as I guess many studnets living in Europe ans Notice that the rainfall for SIM is higher on average than for the SWM; not what you would expect from the text books?

                                                                                                           SWM                                            SIM

note: what becomes apparent is that where the monsoon seasons are concerned you cannot generalise; Sri Lanka is different from the sub continent of India. Even within India there are significant departures from the generalised “norm”; so the lesson is not to accept broad generalisations from text books where climate is concerned.


  1. Batticaloa is on the east coast and has a different rainfall pattern; one that is dominated by the NEM.


note the vertical scales on the two graphs are not the same; the graphs are there for illustrative purposes only

Of the two, Batticaloa looks like it is the most vulnerable to drought for two reasons;

  1. there is a long dry period from March through to November when temperatures and evapotranspiration rates are high
  2. The east coast is heavily dependent therefore on the NEM; if it fails to produce enough rain in November and December, or fails altogether then there is much less groundwater available for crops following on. Reservoirs (called tanks locally) and rivers dry up.


when the rains fail

What the data shows

Looking at the data there are a couple of general points to begin with:

  1. The onset of each monsoon period is pretty much fixed give or take a week or so although there is a suggestion that the SWM is arriving slightly earlier ie late April / early May rather than later in May.
  2. Actual rainfall for any given month varies quite substantially from average values for that month. The coefficient of variation is high.  That is true for both the 1985 period and also the 2000-2015 period. So the monthly averages don’t mean a great deal. Rainfall is variable for any given month and from year to year. From the data I have, I suggest it always has been.
  3. The NEM is a changeable event; some years wetter some years drier., but not predictable
  4. The SWM rainfall is on the increase since 2007
  5. In the North and East the dry season seems to be getting drier, if you add that to a significantly lower monsoon rainfall  total as in 2005 – 8, it can spell big problems for farmers: in particular, drought.


Which bring me on to the last point in part one; why is the rainfall so unpredictable? The reason is that there are several factors at play.

  • Rainfall in the Indian Ocean basin is determined by wind direction; which in turn is heavily influenced by the migration of the ITCZ.
  • However it is also influenced by two other phenomena; which are at least partially dependent on one another..
  1. The ENSO or El Nino event
  2. The 3 phases of the Indian Ocean Dipole;

all of which affect sea surface temperatures, and therefore pressure and wind systems.

Simple it isn’t?

As a taster then here is something to be thinking about.

ENSO events:

weak            2004/5, 2006/7

moderate      2002/3, 2009/10

very strong    2015/6

IOD dipole:

positive:         2006, 2012

negative:        2010

You could have a look at the summary of rainfall data above and see where there may be potential match – ups.

Part 2 looks at how it all works

Appendix: Summary  of Results

( for those who are interested in the detail; I have the raw data available on request )

  1. Batticaloa; main rainfall season is the North East Monsoon (NEM)

The average Cv for 1985-90 is 0.67; the average Cv for 2000-2015 is 1.01

However the Cv for the NEM is marginally lower for the period 2000-15

Concentrating on the period 2000-2015:

  1. Cv is higher during March to October (dry season); generally >1.0
  2. Cv falls slightly during the NEM season; between 0.38 and 0.56
  3. overall drier years in 2001, and 2005/6/7:  ? drought ?
  4. July is the driest month and there is a suggestion that July is becoming drier over the period: (2000-07 av. 35.75; 08-15 av 24.7)
  5. The NEM generally starts in November although in 2004,2011,and 2015 it arrived in September
  6. December is the wettest month
  7. NEM rainfall was less for the period 2005-2008 and also 2013
  8. 2011 was the wettest year during the period at 3581mm (80% above average)
  1. Jaffna: main rainfall season is the North East Monsoon  (NEM) plus possibly the Second Inter-monsoon (SIM)

The average Cv for 1985-90 is high; 0.86 and is even higher in 00-15; 0.97

Looking at the period 2000-15

  1. Cv is much higher during drier months; range 0.72 – 1.46 and lower during the NEM at around 0.47
  2. Cv is also lower at 0.44 during October (SIM)
  3. drier years were; 2002/3, 2005/6, 2009, 2012/3
  4. June/July are the driest months and are becoming drier; 2002-7 av 48.5mm, 08-15 av 18.4mm
  5. NEM arrives in November in 12 of 15 years
  6. November is wettest month
  7. NEM rainfall was much lower 2006-8 and 2013/4; 2009/11 NEM rainfall was above average
  8. 2015 was the wettest year in the period at 1839 mm but was only 10% above the average for the NEM

A possible question to investigate is the degree to which Jaffna may be affected by the SIM given its location.

Common to both

  • high variability from year to year especially in the dry season
  • decreasing rainfall in June/July
  • drier 2005-8 and 2013
  1. Colombo; affected by 3 seasons. First Inter Monsoon (FIM), South West Monsoon (SWM), Second Inter Monsoon (SIM); although FIM impact is negligible

at around 0.06 the Cv for both periods (85-90 and 00-15) is high

Looking at the period 2000-15

  1. Cv is lower during both the SWM and the SIM (0.32 and 0.43)
  2. drier years overall in 2004 and 2011
  3. No significantly drier years apart from 2011 which had a lower SIM
  4. January and February showing decreasing rainfall; example Jan av. 00-07 av 108.4, 08-15 av 75.1
  5. July is the driest month October/November (SIM) the wettest
  6.  September marks SIM arrival except 2005 and 2011
  7. April/May consistently marks start of SWM
  8. signs that onset of SWM  shows higher average: 00-07 av 195mm, 08-15 av 371mm not quite so marked for SIM
  9. 2010 was the wettest year at 3370mm (43% above average)
  1. Galle: affected by 3 seasons. First Inter Monsoon (FIM), South West Monsoon (SWM), Second Inter Monsoon (SIM); although FIM impact is negligible

In comparison with Colombo the Cv’s are slightly lower than for Colombo but still >0.5 for both periods but there is no significant difference between the Cv values for the two time periods.

Looking at the period 2000-15

  1. Cv is does not drop during SWM although it does so for the SIM
  2. 2001/2 and 2013 were drier years,
  3. SWM shows increasing average 00-07 av 162.3 mm; 08-15 av 268.1 with similar increase for May; The SIM data does not show a trend betond a slightly drier October and a slightly wetter November
  4. January shows a decreasing average from 00-07 av 116mm to  08-15 av 82mm
  5. January is the driest month, October is the wettest month
  6. October marks the arrival of the SIM
  7. April/ May marks the arrival of the SWM
  8.  As with Colombo signs are that the onset of the SWM is bring heavier rainfall; 00-07 av for April was 162mm for May was 241mm and for 08-15 av for April increased to 268mm and 298mm respectively.
  9. 2007 and 2010 were the wettest years (32% above average)

Common to Colombo and Galle

  • rainfall is decreasing in January
  • rainfall is increasing in April; onset of SWM is bringing heavier rainfall
  • suggestion that monsoon is getting earlier