From UPSC perspective, the following things are important :
Prelims level : PDO
Mains level : Factors affecting Monsoon
Low monsoon rainfall in NE
- Northeast India, one of the wettest places on the Earth has been experiencing rapid drying, especially in the last 30 years.
- Some places which used to get as high as 3,000 mm of rain during the monsoon season have seen a drop of about 25-30%.
- This decreasing monsoon rainfall is associated with natural changes in the subtropical Pacific Ocean.
- The team used observed rainfall and sea surface temperature data for the period 1901-2014 for the study.
- The results show out that the reduction in rainfall during a major part of the last 114 years may be associated with global man-made factors, while the trend during the last 36 years is associated with natural phenomena.
- Only about 7% of the rainfall in this region is associated with local moisture recycling, which means that anthropogenic activities can affect only this small percentage.
- So the rapid drying is a part of inter-decadal variability of monsoonal rainfall which is strongly associated with the PDO.
Pacific decadal oscillation (PDO)
- PDO is a long-lived El Niño-like pattern of Pacific climate variability.
- Both PDO and ENSO have similar spatial climate fingerprints yet the major difference is that PDO persists for 20-30 years while the typical ENSO persists for 6 to 18 months.
- The PDO, like ENSO, consists of a warm and cool phase which alters upper level atmospheric winds.
- During a “warm”, or “positive”, phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a “cool” or “negative” phase, the opposite pattern occurs.
- Shifts in the PDO phase can intensify or diminish the impacts of ENSO according to its phase.
- If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Nina impacts may be magnified.
- This in turn affects the northeast Indian summer monsoon during its negative phase.
- Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing “true” ENSO impacts from occurring.