Met officials monitoring IOD factor
the likely absence of El Nino and La Nina this year, met officials are closely watching another factor — the Indian Ocean Dipole (IOD) — to see how it impacts the crucial June-September rains in the country. IOD is the measure of the temperature difference in surface waters in the east and west equatorial Indian Ocean. IOD is in the positive phase when waters in the west are warmer, and vice-versa.
A positive Indian Ocean Dipole is seen to aid the monsoon while a negative one could adversely impact monsoon. This year a weak negative IOD is expected to develop during the second half of the season.
Although Indian Ocean Dipole’s connection with the Indian monsoon isn’t seen to be as strong as that of El Nino/La Nina, it is known to have affected rains in the past. In 1997, a strong positive IOD overrode the negative influence of an El Nino, resulting in normal rains. year data shows, there are years when this relationship doesn’t hold.
However, the monsoon’s performance varies significantly during the neutral years as well. These variations are a result of many other local and large-scale factors. Among the large scale factors is the Indian Ocean Dipole (IOD), which is expected to be weakly negative during the second half of this year’s monsoon. A positive Indian Ocean Dipole phase is seen to generally aid the monsoon while a negative phase could depress rains.
Another highly unpredictable condition with sharp, although short, impacts on rainfall is the Madden Julian Oscillation (MJO), a periodic eastward moving weather disturbance close to the equator. MJOs can de- press or enhance rainfall for a week or two, depending on their position and strength. Slow-moving or stationary MJOs can have longer impacts. A well-positioned MJO can invigorate the monsoon while its absence tends to prolong breaks in monsoon rains. MJOs, however, are very hard to predict.
Finally, the distribution and intensity of monsoon rains comes down to the number of low-pressure systems and depressions coming inland from the Bay of Bengal. During active monsoon periods, the frequency of these systems are usually high. On some occasions, even winds from the northwest (western disturbances) affect rainfall.
The interplay of all these factors make monsoon forecasting a highly hazardous profession.