Date:04/05/2004 URL: http://www.thehindu.com/2004/05/04/stories/2004050401861000.htm
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Opinion - Leader Page Articles

El Nino & the Indian Ocean Dipole

By N. Gopal Raj

Scientists are beginning to suspect the hand of another seesaw ocean-atmosphere system, this time in the equatorial Indian Ocean, in influencing the Indian monsoon.

IN THE early years of the 20th century, Sir Gilbert Walker, Director-General of Observatories in India, was looking at ways to predict the outcome of the Indian monsoon. His interest was more than academic as a poor monsoon in those days could easily precipitate famine. Sir Gilbert noticed that when atmospheric pressure was abnormally high in Darwin in northern Australia, it would be unusually low at the Pacific island of Tahiti, and vice versa. More importantly, he found that the "Southern Oscillation," as he named the seesaw pressure changes, correlated well with the monsoon. The monsoon often failed in years of high pressure at Darwin (and low pressure in Tahiti). Conversely, the monsoon was likely to be good in years of unusually low atmospheric pressure at Darwin.

Many decades later, scientists discovered that the Southern Oscillation was linked to periodic changes in the Pacific Ocean. During an El Nino, the warm surface waters at the Asian end of the Pacific gradually spread eastwards towards South America. As a result, the central and eastern Pacific becomes warm and rainy, while the western Pacific turns drier and cooler. During a La Nina, the opposite happens, with the western Pacific becoming warmer and wetter than usual. As the Southern Oscillation and El Nino (or La Nina) are closely connected, they are often referred to jointly as El Nino-Southern Oscillation (ENSO). Over India, the El Nino has usually been the harbinger of drought and the La Nina of rain.

Since ENSO events have a profound impact on climate all over the world, predicting them became a scientific challenge of considerable importance.

The United States and Japan established an array of ocean buoys in the Pacific to continually monitor ocean and atmospheric parameters. Scientists have developed statistical techniques and computer simulation models that use this data to predict ENSO events with tolerable accuracy some months in advance. This ought to have significantly improved the ability to predict the Indian monsoon. Unfortunately, it appeared that the link between ENSO and the Indian monsoon had weakened in recent decades. So much so, the strongest El Nino of the 20th century occurred in 1997 and yet the monsoon was normal. K. Krishna Kumar of the Indian Institute of Tropical Meteorology (IITM) at Pune and U.S.-based researchers argued in a paper published in Science in 1999 that the weakening link between ENSO and the Indian monsoon could be the result of global warming.

Now, however, scientists are beginning to suspect the hand of another seesaw ocean-atmosphere system, this time in the equatorial Indian Ocean. It is called the Indian Ocean Dipole (IOD). When a dipole is forming, wind patterns change in the equatorial Indian Ocean from April-May and the dipole peaks in October. When a positive dipole is developing, the winds in the equatorial Indian Ocean blow from east to west. These anomalous winds drive changes in the ocean currents, making the Arabian Sea off the Somali coast in Africa unusually warm, enhancing cloud formation, while the eastern Indian Ocean around Indonesia becomes colder than normal and more dry. In a negative dipole year, the reverse happens, and the Arabian Sea near Africa becomes cooler and less cloudy while the Indian Ocean is warmer and more rainy around Indonesia.

"Initially when we looked at the dipole, we did not find a strong correlation with the Indian monsoon, probably because we were not looking at the right thing then," says P.N. Vinayachandran, who worked with the well-known Japanese oceanographer, Toshio Yamagata, studying the Indian Ocean Dipole. Dr. Vinayachandran and Prof. Yamagata are among the authors of a much-cited Nature paper published in 1999 that described the dipole and suggested the index that is currently used to measure it.

In 2001, a paper from Prof. Yamagata's group in the journal Geophysical Research Letters (GRL) suggested that the Indian Ocean Dipole was modulating ENSO's impact on the Indian monsoon. This paper used computer simulations of processes happening in the atmosphere to show that a positive dipole increased monsoon rainfall over India while a negative dipole reduced rain. The authors of the paper therefore argued that "the IOD events affect the Indian summer monsoon on their own and thus apparently weaken or strengthen the influence of the ENSO on the [monsoon]." Despite the Southern Oscillation being adverse, the presence of a positive dipole had facilitated normal or excess rainfall over India in 1983, 1994 and 1997.

This research had been taken further, said Karumuri Ashok, the first author of the GRL paper, at an international conference in Pune earlier this year. More computer simulations had been carried out to study the changes in atmospheric circulation created by positive dipoles and El Ninos, acting separately and together. As a result, it was possible to explain just how a strong positive Indian Ocean Dipole was able to counter the negative impact of an El Nino on the Indian monsoon. Moreover, while the GRL paper had looked primarily at the potential role of eastern pole (around Indonesia) of the dipole, it was now clear that the western pole (off the African coast) was also important in causing surplus rain over western India, according to Dr. Ashok.

Meanwhile, Dr. Vinaychandran and fellow scientists at the Centre for Atmopheric and Oceanic Sciences at the Indian Institute of Science (IISc) are discovering that the atmospheric component of the dipole, which they have called the Equatorial Indian Ocean Oscillation (EQUINOO), could be playing a bigger role than imagined. "Anomalous wind patterns during the monsoon can seriously affect rainfall, without these winds being sustained long enough to create the changes in sea temperature that are characteristic of the Indian Ocean Dipole," points out Dr. Vinaychandran.

In a paper published in Current Science last December, Sulochana

Gadgil, Dr. Vinaychandran and graduate student P.A. Francis looked at the relationship between EQUINOO, ENSO and the monsoon rainfall for the years from 1979 to 2003. With EQUINOO, they said, "we can explain not only the droughts that occurred in the absence of El Nino or in the presence of a weak El Nino, but also excess rainfall seasons in which ENSO did not predominate."

"We have since examined data for the last 46 years and have been able to statistically establish that the combination of EQUINOO and ENSO account for much of the Indian monsoon years with large excess or deficit rainfall," Prof. Gadgil told The Hindu . But the analysis also showed that neither EQINOO and ENSO nor a combination of the two could explain small variations in the monsoon rainfall. This work would be published shortly, she added.

The IISc scientists have been particularly struck by how a negative EQUINOO, acting alone or in conjunction with an El Nino, could reduce monsoon rainfall. The drought of 2002, one of the worst in recent years and which most forecasters missed, happened in the face of only a weak El Nino. But the deficit could be explained when the strong negative EQUINOO was also taken into account. Moreover, the EQUINOO was a better predictor than the Indian Ocean Dipole. Even in 2002, there was no significantly negative dipole. Again, in 1979 and 1986, which too were drought years, there was neither a significantly negative dipole nor an El Nino, but both years saw strong negative EQUINOOs.

How closely the Indian Ocean Dipole and ENSO are linked is a contentious issue. Dr. Ashok takes the view that the Indian Ocean Dipole events are largely independent of ENSO and arise out of the coupled ocean-atmosphere dynamics in the tropical Indian Ocean itself. "Only 35 per cent of the IOD events co-occur with ENSO, which demonstrates the independent nature of IOD," he points out. In 1961, a positive dipole occurred without an El Nino in the Pacific; in 1967, a positive dipole coincided with a La Nina; and the positive dipole of 1997 happened along with a strong El Nino.

On the other hand, Raghu Murtugudde of the University of Maryland points to the high correlation between the Southern Oscillation and winds in the central equatorial Indian Ocean, the region of the EQUINOO. He believes that some unifying factor, such as the `Western Pacific deep convection,' a powerful "heat engine" that leads to tremendous cloud formation, could be driving the tropical climate.

As the IISc paper did not suggest any new mechanisms at work, it "doesn't really enhance the predictive understanding of monsoons," says Dr. Murtugudde. The IISc group noted in its paper that "the signal for unfavourable [wind] anomalies is seen from mid-April" in 2002. "Had we known more about the association of [the Indian monsoon rainfall] with EQUINOO, we could have made an educated guess about the season of 2002 having deficit rainfall," they added. But Prof. Gadgil is cautious about using early wind anomalies for prediction. "We first need to understand the physics behind the evolution of EQUINOO, and how various processes nurture or kill it," she says.

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