The worst drought in four decades hits Sri Lanka hard

Sri Lanka has just experienced its worst drought for more than forty years. Reservoir levels fell to around 33% of capacity and many tanks dried up or are at low levels. Agriculture, particularly rice padi production has been decimated. One estimate suggested that by November 2016 only 35% of the  nation’s rice padi had been cultivated and that over 1.2 million people have been directly affected by food shortages and loss of income. H.E.P. production was also affected and led to power cuts being imposed during the latter part of the year.

This was the picture for southern India and Sri lanka as of May this year:

source: IWMI

You can find this map for yourself on the IWMI site (they are based in Colombo) and also on the Relief Web site; When it is amplified you will see that a large area of Sri lanka and Tamil Nadu are speckled brown (severe or extreme drought). This was a situation that had been developing for some time but it has come to a head in 2017.

Droughts can be categorised in a number of ways;

  1. When rainfall is well below normal/average levels
  2. When soil moisture content falls well below normal levels
  3. Where lack of rainfall leads to significant decline in agricultural production
  4. Where there is insufficient water to sustain the population

So what we now understand is that rainfall levels fell way short of normal; i.e. there was a rainfall deficit which affected pretty much everywhere apart from the south west of the island, as this map re-printed from Global Risk Insights shows:

 

With the exception of the North West around Puttalam it is the DRY zone which has been affected most. This is the zone which tends to rely on the North-East monsoon for its rainfall. So that means November / December. For the rest of the year high temperatures prevail. High temperatures lead to high evapo-transpiration rates (water loss from the soil and plants) and the soil dries out, storage tanks and reservoirs shrink.

source roar media network

So long as the monsoon rains return in November then all is well and the cycle continues supporting padi production, animal grazing and so on. However, what if the rains are much less than usual? Then soil moisture isn’t replenished and farmers run out of water. That is drought on 3 levels!

What is happening to rainfall?

So what happened to the monsoon in 2015/6/7 especially the North-East Monsoon? Without rainfall data for 2016/7, we can only speculate but anecdotally at least all the evidence points to a failure in the 2016 North East monsoon especially in the Dry Zone. This is borne out by the pattern of rainfall deficit shown on the  rainfall data map where there is a high rainfall deficit pretty much everywhere, South-West excluded.

Possibly this is part of a long term trend. Looking at rainfall data  for the period 2000- 2015 there is a suggestion that in the dry zone rainfall may be declining:

  1. The amount of rain falling between January and August is declining; for example: Batticaloa; Since 2009 7 out of the 9 years have experienced below the average rainfall  for that period. The figures for Jaffna do not show the same trend BUT rainfall in that period only averages 371mm (2000-2015) anyway and given the high evaporation rates that is effectively a very dry season.
  2. For Jaffna and the North the NEM was lower than average 2005/6/7 and then again 2013/4

(Even then the paradox is that whilst rainfall levels may have not dropped that much in some areas the view of senior meteorologists is that  the rains are coming in more intense bursts, that is shorter periods of more intense rain with longer hotter periods between them (attributed to a general rise in temperatures across the South Asia region giving rise to  enhanced convective activity or storm clouds) So the significance of this is that the more intense the rainfall the more of it will run away(surface run-off) rather than soaking into the ground and recharging water tables.)

Causes

The other information we have to factor in is that 2015/6 were both strong El Nino years. El Nino is associated with suppressed convective uplift (the sort that stops rain clouds forming)  2016 also saw the Indian Ocean Dipole in negative phase so the Eastern Indian Ocean was warmer than normal which brought heavy rains to Western Australia but suppressed rainfall over southern India and Sri Lanka.

 

So the dark brown areas are areas of negative rainfall anomaly; i.e. much less raeinfall that normal. You may just be able to make out a big smear of brown across southern India and Sri Lanka. It means that rainfall was well down on the average for June and August.

The El Nino has weakened and is now in neutral as is the IOD but clearly the climate system has taken time to revert. What we won’yt know for a few months yet it how the October/November inter-monsoon period is going to react or what will happen to the North east Monsoon.

Impacts

a.  As of 20 August, more than 1.2 million people across 19 out of 25 districts remained affected by drought. Northern, North-Central and Eastern Provinces were reporting low levels of water for agricultural, drinking and household use. The failure of two harvests in 2017 has raised concerns for the food security and livelihoods of affected communities. (relief web).

b.  Reservoirs  fell to dangerously low levels, many at only 30% of normal levels

c.  Plus increasing numbers of people are not getting either enough food or the income to buy food. The country’s rice harvest is likely to be down about 17 percent from the 4 million tons recorded in 2013, which would make it the lowest in six years. (IRIN). This has led to Sri Lanka having to import rice

d. There is a definite geographical pattern to the impact of this drought. It is the (mainly) poorer and more vulnerable communities of the North and East, still weakened by the effects of the Civil War,  who are suffering most as this graphic taken from a local newspaper indicates;

What this drought has shown above all is that water insecurity has become a major issue for Sri Lanka.

Feeling the impacts
  1. Agriculture; without going into too much detail agriculture in Sri Lanka is not well advanced. 40% of cropland is down to irrigated rice production much of it at subsistence level and is characterised by:
  • low levels of mechanization
  • predominantly rain fed
  • a significant % is at subsistence level
  • costs are high and profitability is low
  • land holdings are small
  • there is too much reliance on traditional practices that determine the type of seed, water levels and harvesting patterns. Harvesting patterns based on scientific research are taking time to gain acceptance.
  • water conservation is not high on the list of priorities

Plus there is not enough water transfer capacity to mover water from the wetter mountain zones into the dry zone to irrigate crops and support the population.

All of which makes Sri Lanka vulnerable to climate shocks such as the current drought.

2.  Hydro Electric Power generation

During normal years when reservoirs are at capacity Sri Lanka can generate around 50% of its needs from HEP. Currently the country is supplying just 34% of its power supply from HEP meaning an increasing reliance on imported fossil fuels which pollute the environments and impact on an already delicate balance of payments situation for the country.

Plus there are other factors which impact on water demand to take into account:

  • The population is now much higher than in the 1970s greatly increasing demand for water.
  • Sri Lanka’s per capita water usage has picked up sharply over the past decades with rising living standards.
  • Piped water, bathrooms with showers and flush toilets, industrialization, tourism, vehicle usage have all driven up water use.
Water Security: Practical Solutions

All the indications are that climatic hazard events will become more, not less frequent in the coming decades. So, what to do?

  1.  The commonsense answer ( but not the most practicable in all probability) is to use less water. That could include:
  • changing charging policies for water use; water tariffs are generally thought to be too low so this means that effectively raising the cost of water to the domestic consumer is needed to curb inefficient use and wastage. However, in Sri Lanka where significant numbers of people are already close to the poverty line (however that may be drawn) such a move will hit the poor hardest
  • having a more effective metering system in place particularly where crop irrigation is concerned to ensure a more efficient use of irrigation water
  • exert greater control over use of water by industrial companies and in the tourist sector

2. Conserve water

  • develop efficient water recycling facilities; for example Colombo does not have a proper waste water treatment plant resulting in partially treated water discharged to the ocean. (source Water Sector of Sri Lanka report 2014). This is a crazy situation. Greater Colombo is bound to be a major water user with a high water demand. Why is the waste water simply flushed away when it could be recycled as it is in many other countries
  • industrial pollution of water resources needs to be dealt with; inland waters in urban areas are polluted heavily with domestic sewage and industrial effluents It seems that in many cases domestic waste finds its way directly into rivers; people often use rivers as a latrine and all sorts of waste is dumped in surface streams rendering them unusable as these two images  found in the Sunday Times show.

So here you can see untreated effluent running into the Kelani River from a local canal

and here is a fairly typical scene in a watercourse by a low income settlement

In rural areas with agricultural runoff pollutes rivers and streams. In urban over-crowded cities, there is biological contamination of ground water.  Except for pipe-borne water supply, irrigation and hydro-power schemes, in general water resources in Sri Lanka are managed very poorly. Regulations are available to control most water related problems but enforcement of these regulations is lacking.

… and this is the point. Water is becoming an increasingly scarce resource. There is a real need to conserve existing supplies and re-cycle water effectively.

3.  Develop exiting inter – basin water transfer schemes. In other words mover water by pipeline and channel transfers from the wetter areas to the dry zone. The Mahaweli River basin project initiated in the 1970’s  was intended to take water from the mountains to the dry zone; and it does. But is that enough? Question rather than answer and obviously any addition to existing arrangements would be expensive. The question is; does the government even consider whether adding to existing water transfer schemes is worth investigating?

4. Innovative methods; harvesting rain water. I found this extract printed in the Daily Mirror 2016;

Water that falls on a roof of 1,000 sq m in Colombo (average rainfall is 2,000 mm) during a period of one year would be around 2,000 cubic meters (i.e 2 million litres or app. 400,000 gallons). The actual cost of this amount of water would be around Rs. 90,000. The rainwater that falls on the roofs of extensive buildings such as hospitals, schools, housing complexes etc. could be collected in tanks in the premises itself. Water thus collected could be used for numerous domestic purposes. Currently we use chlorinated water suitable for drinking to wash cars, water plants, clean toilets etc. Using rainwater for these activities would reduce water bills, save purified water, which could be used for drinking purposes. Once the collection system is installed there is no additional cost involved except on pumping of collected water to the main water supply system. (Dr CS Weerearatna Daily Mirror October 2016)

source pinterest

This is one simple idea which involves collecting rainfall from roofs and storing it in large tanks either fully or partially underground. The only costs involved are the installation costs plus the cost of pumping the water from the tank. Is this being promoted by the government? It doesn’t seem to be. But these are simple low-tech solutions, so it is surprising that so little is being done.

This article began by charting the development of the latest drought to hit Sri Lanka. Monsoons will fail from time to time, that is a given. Although we understand more now about why droughts  occur  we are powerless to stop them happening. All of which means that when they do occur it is important to have strategies in place to help people cope; to reduce their vulnerability to drought. Sri Lanka is not alone , in facing the dilemma of what to do and how to do it. Water security is an issue throughout South Asia. What this drought has done is bring into sharp focus the need to be planning now for the next drought or Sri Lanka will simply have to go through this crisis all over again.

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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.

So

  • 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 independantly, 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!

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.

                                                                                                                                                                       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