2019 to Be One of Strongest Indian Ocean Dipole Years On Record

The Weather Channel, by TWC India, 15 September 2019

After an insipid start in June, Monsoon 2019 sprang back in late July and August, putting the country on track to end the season on a ‘normal’ note. Experts have credited the role of the Indian Ocean Dipole (IOD) for the revival of the rains. Called the ‘El Niño of the Indian Ocean’, IOD is a sea surface warming phenomenon whose positive phase is associated with higher monsoon rainfall in India.

Now, one of the scientists who discovered the IOD says that 2019 is set to be one of the strongest IOD years on record—a development that could be good for India, but have grave consequences for other parts of the world.

Dr Saji N Hameed, Professor at Japan’s University of Aizu, is one of the world’s leading experts on IOD. He was part of the team that first identified the phenomenon, presented to the world through a seminal report published in 1999. The Weather Channel caught up with the professor to discuss what this latest development means for India and other regions.

1. It seems at this point that Monsoon 2019 will be a normal one for India. Has IOD ‘saved’ the monsoon for us, or did the rapid weakening of El Niño play a role as well?

The IOD’s influence on the Indian region has a distinct regional signature. There are significant positive correlations between IOD and rainfall over central India and northwestern India This influence extends towards regions of Pakistan and Iran as well. The monsoon rainfall pattern of 2019 reflects the influence of IOD, with excess cumulative rainfall observed over western Madhya Pradesh, Saurashtra and Kutch, and northern parts of the Western Ghats (Goa, Maharashtra, and northern Kerala).

However, the monsoonal rainfall as a whole has significant decadal variations, and many regions of India are not affected by IOD. Therefore, it is not expected that IOD will influence All India (averaged) rainfall as a whole.

As to El Niño, there was none to start with despite some of the climate forecast community talking about one. A more precise statement would be that there was warm SST (sea surface temperature) over certain portions of the tropical (and extra tropical) Pacific, which led to an expectation that an El Niño would develop. In my personal opinion, there are decadal signals over the Pacific which has persisted for several years now, and which is sometimes mistaken for El Niño. In other words, warm SST anomaly on its own is not El Niño; it must be accompanied by certain atmospheric variations.

2. As someone who has been following IOD closely for many years, what are the multi-decadal trends you have noted? Is the phenomenon changing over time?

Strong IOD events tend to occur roughly every 10 years. The reasons for this are only being investigated. The countries that are strongly affected by IOD are those in the eastern Indian Ocean (Indonesia to Australia) and equatorial Eastern Africa, where IOD leads to severe lack of rainfall. Moderate impacts are found over portions of India up to southern Iran. We have also found that IOD can lead to excessively warm (southern hemisphere) spring seasons over central portions of South America (e.g. Bolivia).

We don’t expect these impacts to vary by decade.

3. Coming to 2019, how have IOD values moved since the beginning of the monsoon season, and where do you think they are headed from here?

IOD usually manifests first around early summer and strengthens to a peak in late fall. The current IOD is also following this pattern and has significantly strengthened during August. In fact, it is now very similar in its strength to one of the strongest IOD events in recent decades, that of 2006. [That year, too, a resurgence of rainfall in August across central and western India helped overcome an overall monsoon deficit, but left other parts dry.]

From available research, IOD impacts on Indian rainfall are expected to continue over Northwestern parts of India and may extend till October/November, when IOD reaches peak activity. We expect that the current IOD will develop to be one of the strongest on record.

4. So what exactly can we expect if 2019 becomes one of the strongest IOD years of all time?

The dry spells over Indonesia, Australia and Singapore are strongly tied to the ongoing IOD. In fact, the Australian weather agency has been alerting Australians to adverse climate associated with IOD since early spring of this year. The weather agency of Singapore has, similarly, associated the dry spells over Singapore with IOD.

I mostly fear about the situation in East Africa, which is vulnerable even without an adverse climate event. We are talking about millions of people being displaced, emergence of Rift Valley fever and other flood related issues as experienced during the strong IOD of 2006. On the eastern Indian Ocean, forest fires over the Indonesian and Malaysian islands may be a severe problem in terms of increased CO2 emissions, threat to shipping in the region from visibility issues, and air-quality issues.

In the southern latitudes of the Indian peninsula, we may expect heavy rainfall over Sri Lanka during their Maha rains (September to December).

5. Since a strong IOD is a ten-year phenomenon, does it mean that during next year’s monsoon, India will not get assistance from IOD (especially if an El Niño develops later this year)?

IOD is an interannual phenomenon, like El Niño. Since we had a positive IOD event this year, it is unlikely that another positive IOD event will develop next year. The tendency is for a negative IOD event to develop. If this happens, the Indian monsoon will be adversely affected. El Niños tend to occur every 4 years. If indeed there was an El Niño event the past winter (2018/19), we don’t expect another one for the next few years.

6. What are the gaps in scientists’ current understanding of IOD at this point?

Although many studies have tried to understand the impacts associated with IOD, there are not many looking at the physical mechanisms through which IOD may drive these impacts. Unless we understand these, it is difficult to judge whether a particular impact is really attributable to IOD, and not a statistical artefact of the analysis. While the mechanisms by which an IOD intensifies is reasonably understood, it is not very clear what mechanisms or conditions leads to the initial development of an IOD; the same may be said about El Niño as well. Further, it is not known why we have intense IOD events roughly every 10 years. These are questions I want to find answers to.

7. Is global warming likely to have an effect on IOD?

It is difficult to say. Climate models used in global warming projections have certain deficiencies in simulating the structure of IOD and its relation to ENSO. Thus the future changes of IOD in global warming projections are not yet very reliable, but models are continuously improving and we hope to get a better idea of how global warming will affect IOD in the near future.

Read More at https://weather.com/en-IN/india/monsoon/news/2019-09-16-2019-to-be-one-of-strongest-iod-years-on-record-expert