Indonesian Throughflow: River in the ocean that affects Indian Monsoon rainfall
This is the story of a river that can affect rainfall in India during monsoon season. Unlike the rivers on the land, this river flows in the ocean
- Dr. Vivek Shilimkar
- 6 min read
Climate is one of the most discussed and followed topics around the world. There are many climatic phenomena such as El Nino – La Nina, and their effects are well known worldwide. El Nino – La Nina has been significant for India since it affects the rainfall during monsoon season and, eventually, the water resources in the river and agricultural yield. It is common sense that rainfall affects the water resources in the river. However, this is the story of a river that can affect rainfall in India during monsoon season. Unlike the rivers on the land, this river flows in the ocean, and it is thousands of times bigger than any river that you know.
Hello Layman! I am Indonesian Throughflow; you can call me ITF.
Layman: Indonesian Throughflow?! What kind of name is that?
ITF: Haha! I was given this name because I flow through the Indonesian islands due to the sea level difference between the Pacific and Indian Oceans. Similar to the rivers that follow the slope of the ground. The Pacific Ocean is on my northeast, where sea level is higher than the Indian Ocean on my southwest due to the tradewinds in the Pacific Ocean. This situation results in a significant sea level difference between the two oceans, which gives birth to me, one of the most massive transfers of the water on the Earth, the Indonesian Throughflow.
A river in the ocean
Layman: How massive are you? How many cusec or gallons of water do you transfer from the Pacific to the Indian Ocean?
ITF: I transfer water at such a vast volume that your traditional measurements, such as cusec (cubic foot per second), and gallons are tiny. If you use these measurements, you won’t understand my immensity easily. Hence, the scientist Harald Sverdrup invented a unit after his name, Sverdrup, equivalent to – “One million cubic meters per second.” I know you are lost; let me explain “Sverdrup” to you.
Imagine a river that is 100 meters wide, 10 meters deep, and flowing at a speed of 2 meters per second. Now, combine 500 such rivers – that is one Sverdrup, and I am worth 18-22 Sverdrup. I transport a whopping 10,000 of such rivers mentioned above from the Pacific Ocean into the Indian Ocean. However, I am just a fraction of all the oceans combined. I hope you understand how insignificant you are on the Earth; don’t even think about the universe.
Fig 2: Mutha River in Pune. Yellow colour line shows the 100m distance.
Layman: OMG! That’s a massive water transport between the two oceans. I’ve seen the ocean many times, but I never looked at it from this perspective. I don’t know if I will ever be able to understand the ocean’s vastness.
Indian Monsoon rainfall and Me
ITF: Anyways, while I transport around 20 Sv of water into the Indian Ocean, I make the Indian Ocean less saline and warmer, which can affect Indian monsoon rainfall. Although the mean volume transport is around 20 Sv, sometimes my flow is stronger and sometimes weaker than average. While my flow is stronger than average, the southern Indian Ocean becomes less saline and warmer than average, resulting in more evaporation and lower atmospheric pressure in the southern Indian Ocean. The Opposite is true in the case of weaker flow.
Layman: Okay, I think I get it, but how does it affect the Indian Monsoon rainfall?
ITF: Good question. Winds that give rainfall over India originate over the southern Indian Ocean. They carry moisture from the southern Indian Ocean and that moisture precipitates over the western part of India in the form of rainfall. So, when my flow is stronger, more water evaporates, resulting in above-average moisture in the air and above-average rainfall over the western part of India. Conversely, weaker flow results in below-average rainfall over the western part of India. Let me explain further.
The stronger and weaker flow depends on the sea level difference between the Pacific and the Indian Ocean. Larger the sea level difference, the stronger the flow. So far, humans have not understood what drives the inter-annual sea-level difference significantly and, eventually, my inter-annual variation. However, my decadal variation is much easier to understand. Sea level in the Indian Ocean has an insignificant variation on the decadal timescale, so the sea level difference is controlled by the sea level variation of the Pacific Ocean. Hence, the sea level in the Pacific Ocean drives my variation almost exclusively.
Fig 2. Schematic showing the observed summer monsoon rainfall and net upward water flux (Evaporation) anomalies during the 2005-2015 decade. Vectors are mean winds speed during the summer monsoon season.
My flow was above average during the 2005-2015 decade due to significantly higher sea-level in the Pacific Ocean; the same period shows above-average rainfall over the western part of India, supporting my explanation. In the current decade, the Pacific Ocean will have a below-average sea level resulting in a weaker flow. Eventually, that will result in below-average rainfall over the western part of India.
Future Projections
Layman: Well, it is good to know that at least decadal variation is well understood, and it can be utilized to predict the Indian monsoon rainfall.
ITF: Whoa! Hold that thought there. Although humans understand the past relationship between Indian monsoon rainfall and me, this relationship will change in the future. Scientists have been studying me for quite a time, and future projections of climate (Shilimkar et.al. 2022 - https://doi.org/10.1007/s10872-022-00641-w) show that decadal variation of the Indian Ocean sea level will amplify, which will increase the contribution of sea level in the Indian Ocean to my variation. Making my variation complex and challenging to predict since the decadal variation of Indian Ocean sea level would be new to the scientific community.
Layman: Just when I thought Indian monsoon rainfall variation is getting more precise, you disappointed me.
ITF: That’s because climate interactions are highly complex and non-linear. Amplification of the Indian Ocean sea level in future climate is just one aspect of it for which humans are responsible to some extent.
Layman: I understand where this is going! Now, you will say global warming is responsible for all this.
ITF: I understand you sarcastically said this, but indeed changes will be driven by global warming. Global warming makes the tropospheric air warmer, resulting in slowing down of Walker Circulation. The slowdown of walker circulation in the Indian Ocean will result in the shallower thermocline and, eventually, a higher amplitude of SSHAs. The good thing is scientists have begun to understand my importance in the climate system, so even if my variation will become more complicated, eventually, they will be able to forecast it well in advance and impact on Indian Monsoon rainfall.
Layman: I was not sarcastic; I am skeptical about climate science since we hear it only in the news or social media. I or any other layman cannot have such a discussion with a real scientist or knowledgeable person. I wish there could be some way to know more about climate science and other streams of science.
Well, ITF, I am delighted that we had a discussion, and I learned something about Indian Monsoon rainfall and you. I wish to have many more such discussions with others as well.