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Fig. 9. Correlation of Iranian winter precipitation with the 500-hPa gph. the North of Europe. An insignificant positive correla- tion center is also found over the Indian Peninsula. The locations of the correlation centers are quite consistent with the locations of the 500-hPa anomaly pressure centers in Fig. 5(b). The maximum correlation coeffi- cient value (—0.6 to —0.7) is found over Kazakhstan center and the second best correlation center (—0.5 to —0.6) is found over Syria. Similar values are obtained also at higher levels up to 250-hPa for all of the centers. These results are consistent with the large difference found between the GPH anomalies for the two precipitation groups in Iran, as explained previously. (not shown). A significant positive correlation is founc over most parts of Iran for both meridional and zona’ winds, consistent with previous findings. The correla- tion map between the winter precipitation and OLR alsc shows a significant negative center, varying from —0.4 to —0.6, over Iran (not shown).

Figure 9 Correlation of Iranian winter precipitation with the 500-hPa gph. the North of Europe. An insignificant positive correla- tion center is also found over the Indian Peninsula. The locations of the correlation centers are quite consistent with the locations of the 500-hPa anomaly pressure centers in Fig. 5(b). The maximum correlation coeffi- cient value (—0.6 to —0.7) is found over Kazakhstan center and the second best correlation center (—0.5 to —0.6) is found over Syria. Similar values are obtained also at higher levels up to 250-hPa for all of the centers. These results are consistent with the large difference found between the GPH anomalies for the two precipitation groups in Iran, as explained previously. (not shown). A significant positive correlation is founc over most parts of Iran for both meridional and zona’ winds, consistent with previous findings. The correla- tion map between the winter precipitation and OLR alsc shows a significant negative center, varying from —0.4 to —0.6, over Iran (not shown).

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The total monthly precipitation time series for the period 1959-2003 corresponding to 35 stations across Iran were obtained from the Iran Meteorological Organization (Fig. la). These stations have a good temporal coverage (only 0.3% of the monthly observa- tions are missing). This analysis focuses on seasonal wintertime (January, February and March) precipitation. A Principal Component Analysis (PCA) was used to regionalize the winter precipitation characteristics in Iran. The PCs of precipitations were then correlated with climate variables and indices. The running correlation technique was used to examine the strength and temporal stability of the relationship over time. The Fig. 1. a) The map of Iran representing the Alborz and Zagroz mountains, the Dasht-e-Kavir and the Dasht-e-Lut and geographical location of climate stations used in this study, b) spatial distribution of the Iranian winter mean precipitation for the period 1959-2003, c) the coefficient of variation of the winter precipitation, d) the homogenic regions of winter precipitation in Iran.
The total monthly precipitation time series for the period 1959-2003 corresponding to 35 stations across Iran were obtained from the Iran Meteorological Organization (Fig. la). These stations have a good temporal coverage (only 0.3% of the monthly observa- tions are missing). This analysis focuses on seasonal wintertime (January, February and March) precipitation. A Principal Component Analysis (PCA) was used to regionalize the winter precipitation characteristics in Iran. The PCs of precipitations were then correlated with climate variables and indices. The running correlation technique was used to examine the strength and temporal stability of the relationship over time. The Fig. 1. a) The map of Iran representing the Alborz and Zagroz mountains, the Dasht-e-Kavir and the Dasht-e-Lut and geographical location of climate stations used in this study, b) spatial distribution of the Iranian winter mean precipitation for the period 1959-2003, c) the coefficient of variation of the winter precipitation, d) the homogenic regions of winter precipitation in Iran.
Fig. 2. The composite maps of the winter mean SLP (mb) for dry (a) and wet (b) conditions.
Fig. 2. The composite maps of the winter mean SLP (mb) for dry (a) and wet (b) conditions.
Fig. 3. The composite maps of the winter anomalous SLP (mb) for dry (a) and wet (b) conditions. weakening (strengthening) of the anomalous westerly winds along about 35° to 40° latitude, which will be discussed later. The spatial distribution of SLP anomalies is comple- tely different for dry and wet conditions (Fig. 3a and b, respectively). Wet conditions are associated with
Fig. 3. The composite maps of the winter anomalous SLP (mb) for dry (a) and wet (b) conditions. weakening (strengthening) of the anomalous westerly winds along about 35° to 40° latitude, which will be discussed later. The spatial distribution of SLP anomalies is comple- tely different for dry and wet conditions (Fig. 3a and b, respectively). Wet conditions are associated with
Fig. 4. The composite maps of the mean 850-hPa gph (m) for dry (a) and wet (c) and at 500-hPa for dry (b) and wet (d) conditions.
Fig. 4. The composite maps of the mean 850-hPa gph (m) for dry (a) and wet (c) and at 500-hPa for dry (b) and wet (d) conditions.
Fig. 5. The composite maps of the winter anomalous 500-hPa gph (m) for dry (a) and wet (b) conditions. considerable above-normal SLP over a region between 40 N to 70 N over Europe and Russia with a distinct anomalous center over the North Sea and a maximum value above 3 mb. Another distinct anomaly center, but negative (<-2 mb), is formed over the northwestern African coast (Fig. 3b). The locations of these two centers are exactly consistent with the locations of the upper air centers (Fig. 5b). Dry conditions are characterized by below normal SLP patterns extending over most parts of Europe and Russia (Fig. 3a). At the same time, the positive anomaly center over the North Sea moves westward to the North Atlantic Ocean and
Fig. 5. The composite maps of the winter anomalous 500-hPa gph (m) for dry (a) and wet (b) conditions. considerable above-normal SLP over a region between 40 N to 70 N over Europe and Russia with a distinct anomalous center over the North Sea and a maximum value above 3 mb. Another distinct anomaly center, but negative (<-2 mb), is formed over the northwestern African coast (Fig. 3b). The locations of these two centers are exactly consistent with the locations of the upper air centers (Fig. 5b). Dry conditions are characterized by below normal SLP patterns extending over most parts of Europe and Russia (Fig. 3a). At the same time, the positive anomaly center over the North Sea moves westward to the North Atlantic Ocean and
Fig. 6. The composite maps of the winter anomalous SST (°C) over the Indian Ocean for dry (a) and wet (b) conditions in Iran centers have a strong effect on wind circulation over Iran. Kahana et al. (2002, 2004) found that the Syria low is one of the most dominant synoptic types used for prediction of major floods in the Negev Desert. The anomalous circulation patterns over these two centers imply that the average flow over Iran is thus southwesterly (Fig. 7b). These circulation anomalies are completely reversed during the dry period in Iran (Fig. 7a). The low-pressure center over the northwestern African coast disappears during the dry period (Fig. 5a), resulting in a high reduction of the anomalous westerly flow over North Africa and the Arabian Peninsula. On the other hand, two low-pressure
Fig. 6. The composite maps of the winter anomalous SST (°C) over the Indian Ocean for dry (a) and wet (b) conditions in Iran centers have a strong effect on wind circulation over Iran. Kahana et al. (2002, 2004) found that the Syria low is one of the most dominant synoptic types used for prediction of major floods in the Negev Desert. The anomalous circulation patterns over these two centers imply that the average flow over Iran is thus southwesterly (Fig. 7b). These circulation anomalies are completely reversed during the dry period in Iran (Fig. 7a). The low-pressure center over the northwestern African coast disappears during the dry period (Fig. 5a), resulting in a high reduction of the anomalous westerly flow over North Africa and the Arabian Peninsula. On the other hand, two low-pressure
Fig. 7. The composite maps of the winter anomalous vector wind at 500-hPa gph for dry (a) and wet (b) conditions. The shading shows the wind speed (ms _').
Fig. 7. The composite maps of the winter anomalous vector wind at 500-hPa gph for dry (a) and wet (b) conditions. The shading shows the wind speed (ms _').
The zonal and meridional wind anomalies at 500-hPa are in good agreement with the above findings. The zonal wind anomalies during dry conditions are stronger than the wet conditions over western and northwestern areas (not shown), suggesting that the western currents are stronger during dry periods than the wet periods. Furthermore during wet conditions, the meridional wind velocity anomalies strongly increase over Iran (not shown), implying a prevalence of the southern currents over Iran. However, during the dry conditions these anomalies are strongly reduced over most parts of Iran POLED RAAT TRON NORE RN ESATA ET ITS See ERRATIC SAROSE? YEN IS atone REBECA EMIT OTE nna SOREN SPAS TRS tRNA The connection between the winter precipitation (global average winter precipitation over Iran) in Iran and 500-hPa geopotential height fields, is presented in Fig. 9. Three negative significant centers are observed over the northwestern African coast, Syria and Kazakh- stan and one significant positive center is observed over Fig. 8. The composite maps of the winter anomalous outgoing long-wave radiation (v m ”) for dry (a) and wet (b) conditions
The zonal and meridional wind anomalies at 500-hPa are in good agreement with the above findings. The zonal wind anomalies during dry conditions are stronger than the wet conditions over western and northwestern areas (not shown), suggesting that the western currents are stronger during dry periods than the wet periods. Furthermore during wet conditions, the meridional wind velocity anomalies strongly increase over Iran (not shown), implying a prevalence of the southern currents over Iran. However, during the dry conditions these anomalies are strongly reduced over most parts of Iran POLED RAAT TRON NORE RN ESATA ET ITS See ERRATIC SAROSE? YEN IS atone REBECA EMIT OTE nna SOREN SPAS TRS tRNA The connection between the winter precipitation (global average winter precipitation over Iran) in Iran and 500-hPa geopotential height fields, is presented in Fig. 9. Three negative significant centers are observed over the northwestern African coast, Syria and Kazakh- stan and one significant positive center is observed over Fig. 8. The composite maps of the winter anomalous outgoing long-wave radiation (v m ”) for dry (a) and wet (b) conditions
Fig. 10. The running correlation coefficients (with 35-year windows width) between the Mediterranean SLP grid points and (a) PC1, (b) PC4, (c) PC2 (underlined) and PCS.
Fig. 10. The running correlation coefficients (with 35-year windows width) between the Mediterranean SLP grid points and (a) PC1, (b) PC4, (c) PC2 (underlined) and PCS.
The running and simple (in parenthesis) correlation coefficients between the studied atmospheric indices and the principal components of the winte: precipitation in Iran (significance level for running correlation with a 35-year window width is 0.28 and for the simple correlation (45 years) is 0.25)
The running and simple (in parenthesis) correlation coefficients between the studied atmospheric indices and the principal components of the winte: precipitation in Iran (significance level for running correlation with a 35-year window width is 0.28 and for the simple correlation (45 years) is 0.25)
The running partial correlation coefficients (with 35-year windows width) between the studied atmospheric indices and the principal components of the winter precipitation in Iran (significance level for running correlation with a 35-year window width is 0.28)
The running partial correlation coefficients (with 35-year windows width) between the studied atmospheric indices and the principal components of the winter precipitation in Iran (significance level for running correlation with a 35-year window width is 0.28)
Fig. 11. The composite maps of the winter anomalous vector wind at 500-hPa gph during the negative NAO phase. The shading shows the wind speed (ms _'). region. This region is jagged, and is surrounded by a chain of rather high mountains (the Zagros and Alborz mountains). These mountains provide barriers against air currents bringing different air masses, which interact over the region. These conditions lead to a special precipitation regime over northwest Iran in comparison to the other regions. A partial correlation analysis was also used to check whether these indices have direct influence on the PCs or only reflect the effects of other indices. The results are summarized in Table 2. The results of the partial correlation for the WeMO index, SCAND pattern and the first rainfall component indicated that when the
Fig. 11. The composite maps of the winter anomalous vector wind at 500-hPa gph during the negative NAO phase. The shading shows the wind speed (ms _'). region. This region is jagged, and is surrounded by a chain of rather high mountains (the Zagros and Alborz mountains). These mountains provide barriers against air currents bringing different air masses, which interact over the region. These conditions lead to a special precipitation regime over northwest Iran in comparison to the other regions. A partial correlation analysis was also used to check whether these indices have direct influence on the PCs or only reflect the effects of other indices. The results are summarized in Table 2. The results of the partial correlation for the WeMO index, SCAND pattern and the first rainfall component indicated that when the
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