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:
- 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)
- 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.
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.
- 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
- 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
- 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;
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.
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.
- El Nino/La Nina events
- 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.
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.
So to summarise so far: the two influences on rainfall we have looked at are
- The movement of the ITCZ
- 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.
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.
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.
- 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
||% normal monsoon rainfall
||La Nina/negative IOD
||La Nina/negative IOD
||El Nino/Positive IOD
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.