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.

As climate changes, so sea surface temperatures will rise, but the increase won’t be even. 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. It may well serve to intensify the south west monsoon but there is no agreement on that at the moment. It is also possible that the location of the warmer water pool in the Indian Ocean (both positive and negative phases) may shift in location and this could also affect the local surface wind patterns. There is still much that is not understood!

Waldringfield UK; a community led flood protection project

In 2013 a storm surge in the North Sea threatened communities up and down the East coast of the UK. Whilst many coastal settlements had benefited from flood protection schemes others like Waldringfield, a small village on the Deben estuary in Suffolk were left out of the loop and vulnerable. So when the surge came the inevitable result was flooding to the village.

Despite the flood there were no plans to put flood protection in place. Instead of taking a fatalistic view, however, the local community came together to raise the funds to create, not only an effective flood protection scheme (one of the first of its kind in the UK),  but at the same time create a nature reserve to enhance the local area. This case study is the story of how this project developed.

What is a storm surge?

Storms along the coast can cause sea levels to rise way above their normal level which leads to coastal flooding. So what causes a storm surge? The two diagrams taken from the UK Meteorological  Office site explain how this can happen.

  • A deep low pressure cell (depression) moves eastwards into the North Sea basin.
  • The low pressure at the centre of the storm “pulls” the water level up, by about 1 cm for every 1 millibar change in pressure.

europe - detailed

source: UK meteorological office

  • As the depression moves down the North Sea basin it generates high winds from a northerly direction. The winds push the sea water southwards and towards the coast, causing it to “pile up” along the coast, raising the sea level and creating a “surge”. This is a predictable event. The residents in Waldringfield knew 24 hours in advance that the surge was on its way, for example.
  • The strong winds in the storm generate large waves on top of the surge which can cause damage to sea defences, or spill over the top of sea walls adding to the flood risk.

europe - detailed

source: UK meteorological office

Introducing Waldringfield

Waldringfield is a small village on the west bank of the river Deben in Suffolk, on the East coast of the United Kingdom. The village comprises 225 houses with a population of 464 (2012). The village has a village hall, pub, boatyard and is home to Waldringfield Yacht Club.

The maps below show the location of Waldringfield in Suffolk and in the UK

                                                                                      waldringfieldlocation east coast

The 2013 Flood

On the 5 December, 2013 a large storm surge hit the east coast of the UK causing widespread flooding along the coast. This was a prime example of low pressure, high winds and high tidal conditions combining to create surge conditions;

  • It was the largest tidal surge since 1953 and water levels were actually higher than in 1953
  • Many East coast estuaries were flooded; The Stour, Deben and Orwell rivers all reported  flood damage.

Waldringfield is on the Deben estuary and suffered significant flooding on the river frontage. The following is the list of damage:

  • The river wall to the north of the village was overtopped, causing flooding of the meadow behind it.
  • The boatyard and about 18 residences on the Quay were also flooded to a depth of about 5 feet,  as well as several beach huts and land to the south of the village.
  • There was one casualty, who was taken to hospital by ambulance.
  • The fire service attended to a fire around 1.30am, which was caused by the flood water shorting the electric gates of a property on the Quay.
  • A heating oil tank and a gas tank floated off it stand, but, fortunately, remained attached to it feed pipes
  • The river wall was badly damaged in places, but it wasn’t breached.

The total cost of the damage and repairs was estimated to be: £10 million overall

The Waldringfield Flood Defence Group (WFDG)

The group got together informally 6 months prior to the flood. They were aware of the flood risk plus they were also aware that Waldringfield was not included in existing flood protection schemes for the Deben estuary. This was possibly because only 18 properties and the boat yard were at risk of flood at that time. (even though the real estate value of those properties is possibly above £20 million

Note: The village of Waldringfield  stretches about 1km inland from the river Deben and the majority of the village sits well above flood level. The lane known as The Quay is the  area liable most likely to flood in the village, see map below:

Waldringfield map cropped Page 1

When the flood occurred, and with no direct help from the government forthcoming, the 18 affected  households formed The Waldringfield Flood Defence Group (WFDG). It had significant support from a number of sources including: The Environment Agency, Suffolk Coastal District Council. The Deben Estuary Partnership, Waldringfield Parish Council, Lord Deben, Chairman of the UK’s Independent Committee on Climate Change, and the local MP, Dr Therese Coffey. Very quickly the group came up with a plan to create a two stage project for the immediate area area: this involved:

  1.  an outline plan for the design of new flood defences; including a raised sea wall 1km in length, flood gates, and movable flood barriers to protect the boatyard (completed within 3 months)
  2. the protection and preservation of a freshwater meadow and marshland habitat north of the village through the repair and strengthening of the river wall.
  3. restoration of  salt marsh which would add to the protection the sea wall.

Funding for the  £1million project was achieved mainly through grants,  including £633,000 from the Government’s Coastal Community Fund. The initial work on the salt marsh  was partly funded by the Suffolk Coast & Heaths Area of Outstanding Beauty unit.), and is visible at low tide.  A further 1,000m of protection is being funded by the Coastal Communities fund at a cost of £100,000

How do salt marshes protect flood defences?

Salt marshes play a major role in flood defence.  Salt marshes are effective buffers to wave action, by creating shallow water which reduces the power of waves. The wider the salt marsh the the more effective it is in protecting the sea wall. One study carried out at Cambridge University found that “salt marshes can reduce the height of damaging waves in storm surge conditions by close to 20%”

What has been achieved?

Stage 1: Raising the flood defences: protecting homes, businesses and jobs.

The live link will take you to the project page which details each step in the process. Stage 1 was completed in February 2015. The main points are as follows:

  • A new reinforced wall was built in front of the properties on The Quay at 3.5m OD. Each of the riverside properties now has its own steel reinforced gate to allow access to the footpath


  • A  steel flood barrier was erected next to the boatyard. This can be closed by the Environment Agency in the event of a flood warning.
  • Removable steel barriers have been erected in the boatyard; they will be removed to allow boats  access to the river for launching but can be put in place in the event of a flood warning. (see below)

closable flood barrier removable flood barrier

Stage 2: Raising the river wall: repairing a footpath and creating a freshwater wildlife reserve

The live link will take you to the project page which details each step in the process. Stage 2 was completed in October 2015.

The main object of the scheme has been to create a wildlife reserve on a freshwater marsh area, which was formerly owned by the local vicar. When he died his executors agreed to allow the conversion of part of the farmland to a lagoon and nature reserve.

The lagoon area at present is bare.. it looks like a building site.. but it has been planted with sedges and within a few years it will naturalise to form an attractive habitat for birds and mammals, such as the water vole and a family of otters.


see small scale map for location

After the flood there was concern over state of sea wall to north of village and the footpath which was also damaged when flood water overtopped the wall. So a decision was made to flatten the top of the wall and to widen the footpath.

The footpath is now much wider and more level as the photo below shows. Some locals probably feel that it is unnatural but access has been improved, and the path is accessible and usable all year. Access to the nature reserve will also be improved for locals and visitors alike.


the footpath looking north; now wider and flatter

Salt marsh restoration

The saltmarsh in front of the sea wall to the north of the village varies in width. Immediately north of the village it is quite degraded; see below but widens out . The WFDG scheme allows for the installation of brushwood  fences which have been installed in the marsh in front of the sea wall. The hope is that these fences will trap sediment on the outgoing tide and help to build up the marsh in front of the sea wall to add some degree of additional protection to the sea wall and the footpath. There remains another 1000 metres of fencing to install to complete the job.

degraded marsh

the degraded salt marsh

brushwood fences

brushwood fences in place to protect the sea wall

Local community action: is this the way forward?

This scheme is the first of its kind so far as I can tell and The Environment Agency is keen to use this project as a pilot to demonstrate how local communities projects such as this one can be the forerunner for other schemes which fall outside of government support.

  1.  The WFDG were successful because:
  • they were already organized
  • they had the necessary skills to produce a fully drawn up and costed project plan
  • they had the skills to lobby for financial support
  • they acted quickly
  • they worked together and without internal wrangling/disagreement

2.  The value of the community based approach was that they designed it themselves and so it was fit for their purpose; basically they got what they wanted but also created a scheme with significant utility and value to the village as a whole.

3.  At the same time they turned it into a multi purpose project by:

  • protecting the sea wall
  • creating a much more accessible and usable footpath alongside the river, an improved amenity for all
  • working with the estate of the recently deceased vicar to create the wildlife reserve which will be of broad ecological value but also will provide a real amenity to be enjoyed by locals and visitors and will add to the attractiveness of the riverside

I recently interviewed Janette Brown, the secretary of the Waldringfield Flood Defence Group and started by asking her to take us back to the night of the flood. You can listen to the full interview here: