Influence of the Arctic Oscillation on the variability of winter mean temperatures in Turkey

Summary

Observed changes and variability in winter (DJF) mean temperature series at the 70 stations of Turkey and the circulation types at 500-hPa geopotential height level were investigated to explain atmospheric controls of the temperature variations during the extreme and normal phases of the Arctic Oscillation Index (AOI). The analysis was performed with respect to relationships between winter (monthly and seasonal) mean temperatures and the AOI and the composite winter (monthly and seasonal) temperature changes corresponding to the various phases of the AOI. Main conclusions and evaluations of the study are as follows:

• (1) Negative relationships are found between year-to-year variability of normalized winter (seasonal and monthly) temperatures and the AOI winter (seasonal and monthly) indices at all stations. Correlation coefficients are statistically significant at the 1% level for the majority of stations.

• (2) The height of the 500-hPa level circulation is anomalously high over middle of the North Africa, the Middle East and the Caspian Sea during the negative phase of the AOI, and anomalously low over the northeast Atlantic-Europe regions. This 500-hPa level AO pattern forms the spatially coherent warm signals in Turkey by increasing the westerly and southwesterly circulation carrying Atlantic and northernmost African warm air into the eastern Mediterranean and Turkey.

• (3) During the positive phase of the AOI the 500-hPa level is anomalously high over the northeast Atlantic and Europe including Turkey, while it is anomalously low over the region of the Icelandic Low. This pattern leads to intrusions of cold polar air southward into the Balkans, Turkey and the eastern Mediterranean by increasing northerly and northeasterly circulation and therefore, spatially coherent cold signals over Turkey.