The Indian Ocean Dipole (IOD)
IOD, must have seen in most of weather enthusiasts, analysts posts as they keep IOD and ENSO conditions as one of the factors for predicting the performace of monsoon. It's time that one should know what is IOD and what are its basic characteristics, types of IOD and how they influence monsoon and other parts of the world.
The Indian Ocean Dipole (IOD) is basically defined as the difference in sea surface temperature anomalies between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia. The IOD affects the climate of Australia and other countries that surround the Indian Ocean Basin, and is a significant contributor to rainfall variability in this region. Remember the catastrophic 2019 Australian forest fires, well IOD was one of the factors which lead to this situation. IOD more of like a very important factor one must catalog while researching Australian weather.
Credits Bom.Au |
In scientific terms, the IOD is a coupled ocean and atmosphere phenomenon, similar to ENSO but in the equatorial Indian Ocean. It is thought that the IOD has a link with ENSO events through an extension of the Walker Circulation to the west and associated Indonesian through flow (the flow of warm tropical ocean water from the Pacific into the Indian Ocean). Hence, positive IOD events are often associated with El Niño and negative events with La Niña. When the IOD and ENSO are in phase the impacts of El Niño and La Niña events are often most extreme over Australia, while when they are out of phase the impacts of El Niño and La Niña events can be diminished. However there have been certain exceptions to these cases as well.
Mechanism of IOD:
The Indian Ocean is bounded on three sides by landmasses, it is considered to be one of the warmest of the five oceans on the planet. Water from the Pacific flows into the Indonesian Ocean from the islands of Indonesia and Australia on the eastern side, influenced by tropical easterlies. Quite confused what are tropical easterlies?
Well, tropical Easterlies, which are found between 0 and 30 degrees latitude, generally travel from the northern hemisphere's northeastern side to the southern hemisphere's southeastern side.
The chilly arctic waters do not mingle with the water of the Indian Ocean since it is landlocked from the north by the Eurasian Landmass. The temperature of the Indian Ocean fluctuates as water from the Pacific Ocean combines with water from the Indian Ocean.
The three various phases of IOD are determined by variations in the Indian Ocean. The surface water temperature is as follows:
1.Neutral Indian Ocean Dipole (IOD)
2.Positive Indian Ocean Dipole (PIOD)
3.Negative Indian Ocean Dipole (NIOD)
We will have a gist of what are these phases of IOD below.
1. Neutral IOD:
The first of the three, the IOD's neutral phase begins in the spring when the sun's rays are evenly dispersed throughout the northern and southern hemispheres.
*Easterlies flow between 0-30 degrees North and 0-30 degrees South of the equator as a result of this.
*The surface temperature of oceanic waters is affected by these trade winds.
*When warm water from the Pacific Ocean interacts with water from the Indian Ocean, the temperature is evenly distributed in both the eastern and western Indian Oceans.
*The Neutral Period of the Indian Ocean Dipole (IOD) is the name given to this phase when it is either positive IOD or negative IOD.
Credits Bom.Au |
2. Positive IOD (PIOD):
First let's see classification of our Indian Ocean, eastern Indian Ocean (closer to Australia), western Indian Ocean which is near (Africa)now. Let's see what is positive iod, postive iod occurs when the eastern Indian Ocean experiences strong easterly trade winds which may cool the waters and exactly opposite will happen in western Indian Ocean which stays warm during positive IOD.
Though this impacts Tamilnadu and Indian rainfall, majorly this affects Australia monsoon the best example is 2019 which a postive iod year where south and central Australia experienced below average rainfall the reason behind this is simple, cool waters thus less convection, warm water more convection. ENSO combination also matters a lot another interesting fact is very late withdrawal of monsoon in the year 2019 happened almost on Oct 9th southwest monsoon started to withdraw most delayed record in history.
Some of the positive IOD years in last 10 years are:
2012
2015
2019 (STRONGEST)
Credits Bom.Au |
3. Negative IOD (NIOD):
Now that we have seen about what is IOD, positive and neutral IOD we will next look upon negative IOD which prevailed for almost last 2 years.
So what exactly is negative IOD?
To answer this, let's take positive IOD example of how Arabian Sea is generally much more active than its counterpart Bay of Bengal. Well negative IOD is just the reverse of positive IOD in which Bay of Bengal is more active (produces cyclogenesis) than Arabian Sea.
Last year was a classic example of NIOD where systems were forming one by one during North- East monsoon during late October and early November and of course Cyclone Mandous which gave good rains to Tamilnadu coast in December.
Some of the negative IOD years in last 10 years are:
2013
2014
2016 (STRONGER)
2018
2021
2022
Having able to know what is IOD, let's check how South- West monsoon last 10 years have fared well with variance in IOD and ENSO conditions.
Year- Performance- ENSO- IOD
2012- Normal monsoon ( La Nina & PIOD)
2013- Above normal monsoon (Neutral & NIOD)
2014- Below normal monsoon (El Nino & NIOD)
2015- Below normal monsoon (El Nino & PIOD)
2016- Normal monsoon (La Nina & NIOD)
2017- Below normal monsoon (La Nina & Neutral)
2018- Near normal monsoon (El Nino & NIOD)
2019- Excess monsoon (Neutral & PIOD)
2020- Above normal monsoon (La Nina & Neutral)
2021- Normal monsoon (La Nina & NIOD)
2022- Normal monsoon (La Nina & NIOD)
Credits Bom.Au |
As you can see from the last 10 years, monsoon when ENSO conditions were La Nina have been normal to above normal . Though we might not able to distinguish the performance of monsoon with IOD, PIOD is generally good for surplus monsoon performance.
Coming to the end of this article, we have seen what is IOD, its characteristics, phases of IOD and how combination of IOD and ENSO conditions have variability with South- West monsoon last decade. With another interesting topic, see you next week.
IOD its introduction & Neutral IOD- Vignesh
Positive IOD- Siva Sai Ram
Neutral IOD & SWM variance with IOD and ENSO conditions + modifications in others parts wherever needed- Saran KTC Weatherman