1 استادیار، پژوهشکده هواشناسی و پژوهشکده هواشناسی و علوم جو، تهران، ایران
2 کارشناس ارشد، سازمان هواشناسی کشور، تهران، ایران
عنوان مقاله [English]
Iran is located in arid and semiarid areas based on continental divisions; any change in precipitation would have potential effects on agriculture, economic and other related issues in general. Therefore, it is of high importance to know and identify the moisture-related sources needed to study the country’s precipitation data. For this purpose, it is important to correlate monthly precipitation with Sea Surface Temperature (SST) anomalies. The SST variations are responsible for a large portion of the atmospheric circulation and precipitation variability. Many studies have been done for empirical evidence of relationships between SST anomalies (deviation from the long-term mean) and rainfall anomalies (e.g., Tae-Suk and Moon, 2010; Uvo et al., 1998; Mechoso et al., 1990; Moron and Navarra, 2001; Shabbar and Skinner, 2004). This paper is mainly concerned with indentifying the relationships between the abnormalities seen in the Indian Ocean’s water surface temperatures and the ones seen in Arab sea.
As apparently seen, it has not only some potential effects on moisture-based feeding but also on the ways, the climate scales and flows are being formed. To verify the relationship between Sea Surface Temperature Anomalies (SSTA) and rainfall over southern of Iran, by using ARPEG model output and divergence of specific humidity flux in the low levels of troposphere including 1000, 925, 850, 800 and 700 hPa, moisture resources and how the moisture is transfered to the southern parts of iran, were studied. The second step will be concerned with the study of the relationship between SST anomalies and precipitation. The basic data used in this study consists of, 1) mean monthly values of SST from Octebr to March for the years 1956- 2005 over the Arabian Sea and Indian Ocean; 2) mean monthly rainfall for 9 synoptic stations over southern of Iran. Because precipitation and SSTs exhibit strong seasonality, it is of interest to correlate monthly precipitation anomalies with the SST anomalies. To do this, correlation coefficients, between the SST anomaly of the three parts (Arabian sea, western and eastern Indian Ocean) and the precipitation anomaly for each month, were calculated.
The results of case studies, based on wind and streamlines, vector and divergence of specific humidity flux show that the Arabian Sea, the Red Sea and Persian Gulf have to important role in feeding moisture to the southern Iran, and the maximum flux divergence were occurred in the lower troposphere (1000 and 925hPa) over these area. In addition, the maximum convergences were observed in the southern of Iran and the south east of Saudi Arabia.
Monthly correlation coefficients between Arab sea SST anomalies and precipitation anomalies are summarized in Table 1. Correlation coefficients that exceed the weak threshold of relationship (above 0.30 or below -0.30) are highlighted in bold. No strong relationships (coefficients above 0.60 or below -0.60) were found, however some values do approach more than 0.40. The positive relationships have been found in October, November, January, and February at these stations. No stations exhibits significant (either weak, or strong) relationships in December and March (Table 1).
Monthly correlation coefficients between the eastern Indian Ocean SST anomaly and precipitation anomaly are summarized in Table 2. Most of the stations have very little correlation between monthly rainfall and SST anomalies, as evidenced by correlation coefficient between 0.20 and -0.20. However, the coefficients do reveal a few interesting relationships. The positive coefficients (more than 0.30) are found in October at stations Abadan, Boushar, Shiraz and Yazd. There are not any significant relationships for December and March (Table 2). The correlation coefficients for the western Indian Ocean are found positive, in October in Yazd, in November in Ahvaz and in January in Ahvaz, Boushar and Bandar Abbas stations. No stations exhibits significant (either weak, or strong) relationships in December, February and March (Table 3).
The data presented above reveal that precipitation anomalies over southern Iran are related to SST anomalies in both the Arab sea and Indian Ocean. However, this study found indications that positive relationship may exist for southern Iran in some months. The correlation coefficients were not randomly dispersed throughout the seasonal (October-March) precipitation. The most positive coefficients were found in the autumn months of October and November, especially for Arab Sea and eastern Indian Ocean in the winter months of January and February for Arab Sea and western Indian, implying that warmer than normal SSTs, are connected with increased precipitation. Only a few stations have positive coefficients in November and February. Also in December and March not any significant correlation werefound.
Several possible reasons exist to explain these relationships. For the majority of sites in the southern Iran, precipitation has no strong relationship with SST anomalies of Arab and Indian Ocean in the all months (October-March). Precipitation is, in a basic sense, a function of local vertical motion and water vapor quantity. These two variables are influenced by a variety of global-, synoptic-, and meso-scale features, including planetary waves, mid-latitude cyclones, upper-tropospheric subsidence, mid-tropospheric humidity, local topography and other humidity sources. It is interesting to examine these for more sites and months that do exhibit positive correlations. The stations that reported positive correlations are mostly located in the south of Iran. So, positive SST anomalies can be considered as one of the forcing for increasing precipitation over the southern Iran.