The effect of the North Sea-Caspian pattern (NCP) on winter temperatures in Iran

Summary This study attempts to find possible linkages between the NCP index and the winter temperature variability over Iran. The investigation is based on statistical analysis of simple, partial and multiple correlations and also evaluation of composite maps of the extreme NCP index and maps of correlation between atmospheric variables and the temperature time series. Our results show that the NCP has a strong negative correlation with the winter temperature in Iran. Furthermore, combination of both the NCP and the AO (Arctic Oscillation) indices improve the correlations in all stations, implying both NCP and AO can be considered as major patterns for explaining the Iranian winter temperature variability. The results show that the positive NCP is associated with enhanced precipitation and cloudy conditions, consequently causing below normal temperature over Iran. The anomalies of OLR in this phase are also negative, implying a cloudy sky. For the negative NCP phase these results are completely reversed. The correlation maps indicate that the NCP is negatively/positively correlated with winter Outgoing Long-wave Radiation/precipitation over Iran. The results also show that the SLP and GPH patterns are quite different for the positive and negative NCP phases over Iran. During the negative NCP a small cyclone is formed over the Arabian Sea causing a strong easterly towards Iran. During the positive NCP this cyclone is removed. Our results show that for the positive NCP years an upper-level trough is formed over northern Iran and the eastern Mediterranean. For the negative NCP years this trough becomes weak and is located over central European regions. This trough is closely linked with the winter temperature over Iran. This is expressed by a high correlation between 500-hPa geopotential height at this region and Iranian winter temperature. Authors’ addresses: A. R. Ghasemi, Climate Research Center, Water Engineering Department, Agricultural Faculty, Shiraz University, Shiraz, Iran; D. Khalili, Water Engineering Department, Agricultural Faculty, Shiraz University, Shiraz, Iran.