No abstract

Daily values of precipitation in southwestern parts of Iran along with the corresponding Madden-Julian oscillation (MJO) phase are analysed to introduce some auxiliary indicators that can identify the wet and dry spells within a particular MJO phase. As a case study, the identification procedure focuses on phase 1 during the rainy season (Nov-Apr). The study shows that this phase contains two opposite spells, referred to as spells with pervasive precipitation (SWPP) and spells with widespread dryness (SWWD). Although the precipitation characteristics are essentially different between SWPP and SWWD, their associated MJO indices are found statistically identical. Therefore, the MJOprecipitation relationship fails to identify the wet/dry spells within an MJO phase. To resolve this deficiency, other climate indicators, which behave differently between these two spells, are introduced. The westward moisture flux over equatorial parts of the Indian Ocean and the northeastward moisture flux over the North African tropics are up to seven times stronger in the SWPP compared to SWWD. The dominance of a strong cyclonic (anticyclonic) circulation over the eastern side of the Mediterranean Sea and the negative (positive) west-east temperature gradient over southern parts of Iran are introduced as SWPP (SWWD) indicators. The study reveals that the SWPP (SWWD) coincides with a strong (weak) convection dipole that resembles the positive Indian Ocean dipole. The intensified (weakened) wind speed in the subtropical jet stream and persistence of the jet-exit-regions above Iran (Egypt and Saudi Arabia) are other SWPP (SWWD) indicators. Negative (positive) anomalies of sea surface temperature over the North Atlantic Ocean, the Black, Mediterranean, and Caspian Seas, as well as the Persian Gulf, signify the prevalence of the SWPP (SWWD). To the best of our knowledge, this is the first time that auxiliary indices are utilized to improve the MJO-precipitation relationship in southwestern Iran.

No abstract

No abstract

In a changing climate, drought indices as well as drought definitions need to be revisited, because some statistical properties, such as long-term mean, of climate series may change over time. The study aims to develop a Non-stationary Standardized Precipitation Evapotranspiration Index (NSPEI) for reliable and robust quantification of drought characteristics in a changing environment. The proposed indicator is based on a non-stationary log-logistic probability distribution, assuming the location parameter of the distribution is a multivariable function of time and climate indices, as covariates. The optimal non-stationary model was obtained using a forward selection method in the framework of Generalized Additive Models in Location, Scale and Shape (GAMLSS) algorithm. The Non-stationary and Stationary forms of SPEI (i.e. NSPEI and SSPEI) were calculated using the monthly precipitation and temperature data of 32 weather stations in Iran for the common period of 1964–2014. The results showed that almost at all the stations studied, the non-stationary log-logistic distributions outperformed the stationary one. Both drought indicators SSPEI and NSPEI significantly differed in terms of spatial and temporal variations of drought characteristics. While SSPEI identified the long-term and continuous drought/wet events, NSPEI revealed the short-term and frequent drought/wet periods at almost all the stations of interest. Finally, it was revealed that NSPEI, compared to SSPEI, was a more reliable and robust indicator of drought duration and drought termination in vegetation cover during the severest drought period (the 2008 drought), and therefore, was suggested as a suitable drought index to quantify drought impact on vegetation cover in Iran.