Precipitation in west and northwestern Iran associated to PDO and ENSO signals and relevance to drought

Abstract

The temporal relationship between El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and precipitation in the west and northwestern regions of Iran is explored through the use of multichannel singular spectrum analysis (MSSA), using monthly and seasonal precipitation data (in the form of Standard Precipitation Index, SPI), and the bivariate ENSO time series (BEST) and PDO from 1957 to 2008. Three SPI time aggregations: annual (from October to September), cold (from October to April) and warm (from May to September) seasons were chosen. To assess spatial and temporal patterns of droughts, a principal component analysis (PCA) and MSSA were applied to the SPI series. MSSA shows significant oscillations (with variance explained between 20 and 50%) around four and seven years among the SPI-PDO and SPI-BEST channels. The results show an in-phase influence of ENSO and PDO on precipitation during the cold season, contrasting the off-phase relationship during the warm season. An increased variability in precipitation regime, noticeable after 1990, was also detected in the reconstructions. These results are relevant in determining the likelihood of droughts in this part of Iran (mainly due to critically dry cold seasons during negative PDO and ENSO phases), which may be helpful for water management and agriculture planning in the region.