Indian Ocean Dipole Phenomenon's Impact on Correlation
between Indian Monsoon and El Niño/Southern Oscillation

November 29, 2001

  Prof. Toshio Yamagata, Program Director, and researchers Dr Ashok Karumuri and Dr Guan Zhaoyong of the Climate Variations Research Program in the Frontier Research System for Global Change (a joint project of NASDA and JAMSTEC) have used the observational data from 1958 to 1997 (Ref.1) and an atmospheric general circulation model to investigate the impact of the recently discovered Indian Ocean Dipole (IOD) phenomenon (published in September 1999 issue of Nature), that has been receiving much attention, on the interannual variability of the Indian Summer Monsoons. Through this research, they have revealed that the influence of the IOD on the Indian Monsoons is opposite to the effect of El Niño/Southern Oscillation (ENSO) (fig. 1), and that the IOD-monsoon rainfall relationship varies complementarily to that between the ENSO and monsoon rainfall.
This discovery will be published in the December 1 issue of Geophysical Research Letters, a journal of the American Geophysical Union.

  At the beginning of the last century, research on the Indian Summer Monsoon (represented by the All-India Rainfall) led to the discovery of the Southern Oscillation, which is the atmospheric phenomenon associated with El Niño. Recently, however, findings have indicated a mysterious weakening in the correlation between the Indian Monsoons and ENSO (El Niño/Southern Oscillation). Consequently, based on the observational data from 1958 to 1997, an investigation was conducted into the influence of the Indian Ocean Dipole (IOD) phenomenon on interannual variability of the Indian Summer Monsoon rainfall.

  Between 1958 - 1997, whenever the correlation of the Indian Summer Monsoon rainfall (ref.2) with ENSO (El Niño/Southern Oscillation) is low, the correlation with the Indian Ocean Dipole (IOD) is high, and vice versa. In the 1990s, intense and frequent Dipole Mode events occurred, resulting in a weak correlation between the Indian Monsoon and ENSO (fig.1). The IOD event not only affects the zonal (east-west) circulation in the troposphere, but also impacts on the meridional (north-south) circulation (fig.2). The simulations using the Frontier atmospheric general circulation model (FrAM1.0) clearly showed that a positive Dipole Mode Event, in particular, produces a positive rainfall anomaly along the monsoon trough prevailing across Northern India, and over the Bay of Bengal (ref. 3). During a typical El Niño period the amount of precipitation accompanying the Indian Monsoons is reduced, but a positive Dipole Mode Event counteracts this effect when they occur together (ref.4). The mechanism that causes the complementary impact of these two main phenomena on Indian Monsoon rainfall has now been clarified for the first time. These findings will considerably improve the short-term prediction of climate change in various countries affected by the Asian Monsoon in addition to India.

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