Document Type : Original Article
Department of Natural Geography, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
Sea Surface Temperature (SST) anomalies can greatly affect climate fluctuations of arid regions. The impact of SST on the amount of rainfall is not limited to coastal areas, but distant areas may also be affected by changes in patterns of SST areas. Understanding the correlation between SST and rainfall pattern has an effective role in prediction of drought and rain in areas that are influenced by water temperature fluctuations.
Materials and methods:
In this research, in order to investigate the relationship between anomalies of Mediterranean Sea SST (as one of the sources of rainfall in Iran) and rainfall in western regions of Iran, after statistical analysis and identification of hot and cold periods of water, which had a significant correlation with rainfall in the study area, two samples were selected and the way water temperature affects rainfall was analyzed using a Synoptic-Dynamic condition analysis. For this purpose, in selected regions, the following factors were depicted and examined in a GRADS environment: daily rainfall data; mean sea level pressure field maps; heights of 500, 700, and 850 hPa; temperature field at 850 and 1000 hPa; specific humidity field at 850 and 1000 hPa; wind and orbital components at 500-1000 hPa; vertical velocity field at 500 and 700 hPa; relative vorticity field and horizontal divergent of relative vorticity field at 500 hPa; and specific humidity field and horizontal divergent of specific humidity field at 800 and 1000 hPa.
Results and discussion:
The results showed that, although a statistically significant correlations exist in the whole study region between high SST and rising rainfall in April (95%) as well as low SST of the Eastern area decreasing rainfall in March (99%), according to a Synoptic-Dynamic analysis the system that brought rain to the region in April was the Sudanese system and the main source of humidity was the southern area. These synoptic systems, which are associated with atmospheric fronts, are formed or strengthened under the influence of a deepened trough at mid-level height, creation of a positive vorticity in the eastern part of a trough, existence of an upward movement, convergence at lower heights, and reduced pressure. By providing humidity at lower heights and upward movement, suitable conditions are provided for convective rainfall in the study area. Also, under the influence of the relatively stable presence of the Azores anticyclone over the Mediterranean Sea on the Earth's surface and the study area, reduced rainfall in May prevents the influence and expansion of the Sudanese system and the formation of Mediterranean cyclones at higher levels, deepening of a low height trough, and approaching western Iran. Although water temperature decreased in humidity advection from the Mediterranean Sea to the studied area, no change occurred and only humidity advection from southern area reached zero. According to Omega and vorticity maps, it is also observed that the presence of stable atmospheric conditions, vorticity advection, and downward movements in the region all reduced rainfall in this month. Conclusion: In general, the dynamic and thermodynamic complexity of weather from the Mediterranean Sea to western and north-western Iran prevents recovery of the effect of SST fluctuations caused by decreasing and increasing precipitation. Therefore, it seems that an increase and/or decrease in water temperature of the Mediterranean Sea has no impact on the amount of humidity advection and increase and/or decrease of rainfall in the study area. What is more, different atmospheric patterns accompanied by positive or negative SST abnormalities in different months, play a controlling role in humidity feeding through various sources and occurrence of rainfall.
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