Influence of Natural Forcing Phenomena on Precipitation of Pakistan

M Adnan, Nadia Rehman, Aftab Ahmad Khan, Kaleem Anwar Mir, M Ahson Khan


According to IPCC TAR (2001), some natural climate variations such as ENSO (El-Nino Southern Oscillation), PDO (Pacific Decadal Oscillation), IOD (Indian Ocean Dipole) and NAO (Northern Atlantic Oscillation) / NAM (Northern Hemisphere Annular Mode), can significantly alter the behavior of extreme events, including floods, droughts, hurricanes and cold waves. This paper examines ENSO, NAO and IOD (DMI) in the context of their influence on the precipitation of Pakistan, by analyzing the historical data of rainfall for the period 1951-2012 supplemented by the re-analysis NCEP pressure data and the CRU precipitation data. These phenomena have been analyzed in relation to rainfall departures (monsoon rains in the case of ENSO, winter rains in the case of NAO and both monsoon & winter rains in the case of IOD) and the corresponding correlations are developed. Decadal correlations have also been worked out. The impact of IOD on El-Nino and La-Nina events in the monsoon dominated region and over the whole of Pakistan, particularly during the years of ENSO episodes, has also been studied. In most of the cases, the monsoon rainfall was found to drop drastically during the strong El-Nino years and positive IOD values, whereas it generally increased during the La-Nina years. During most of the strong El-Nino years, the monsoon depressions emerging from the Bay of Bengal and moving towards Pakistan dissipated over either Bangladesh or India, and did not reach Pakistan. The NAO forcing is seen positively correlated with the winter rains.  Some NAO decadal correlations are found to be significantly high.

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