عنوان مقاله [English]
Simultaneous variations in weather and climate over widely separated regions have long been noted in the meteorological literature. Such variations are commonly referred to as "teleconnections". In the extra-tropics, teleconnections link neighboring regions mainly through the transient behavior of atmospheric planetary-scale waves. One of the most well-known teleconnections is the Arctic Oscillation. Arctic Oscillation is the leading mode of extratropical circulation from the surface to the lower-level stratosphere in the northern hemisphere. Fluctuations in the Arctic Oscillation create a seesaw pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. For instance, a positive Arctic Oscillation phase is accompanied by low pressure over the north Polar Regions and high pressure at the mid-latitudes. These features are reversed in a negative Arctic Oscillation phase. The Arctic Oscillation trends are highly correlated with atmospheric phenomena such as variability in sea level pressure, storm tracks and precipitation throughout northern hemisphere.
Materials and methods
The purpose of this study is to examine the impact of the Arctic Oscillation on the frequency of days with rainfall event in Iran. For doing so, we used three dataset. 1) The gridded daily precipitation of GPCC in a 1˚ latitude × 1˚ longitude resolution. These data have been extracted for 154 grid points within the political boundary of Iran. Therefore, our initial matrix of daily precipitation have been consisted of 9125 rows, one for each day from March 21, 1988 (Farvardin 1, 1367), to March 20, 2013 (Esfand 29, 1391), and 154 columns, one for each grid point in Iran. 2) The daily Arctic Oscillation index from the Climate Prediction Center of the National weather service, NOAA. These data have formed a matrix in 9125×1. 3) The mean daily geopotential height data of 700 hPa level at 2.5˚ × 2.5˚ grid resolution from National Center Environmental/ Department of Energy (NCEP-DOE). This matrix is also consisted of 9125 rows, one for each day from March 21, 1988, to March 20, 2013, and 5328 columns, one for each grid point in northern hemisphere. In this study, was used to investigate the impact of the Arctic Oscillation on the frequency of rainfall evens. Then, the lag correlation was used to find the highest correlation between the Arctic Oscillation and the frequency of the days with rainfall event. Based on this, the frequency of rainfall event was investigated. Finally, the long term mean geopotential height of the 700 hPa level in association with the highest correlation was analyzed. MATLAB software was employed to analyze the data.
Results and discussion
The statistic and its significant test showed that the relationship between the Arctic Oscillation and the frequency of days with rainfall event is significant from October 23- November 21 (Aban) to April 21 – May 21 (Ordibehesht). Then, the obtained results of lag correlation showed simultaneous correlation in the two months of October 23- December 21 (Aban and Azar) and lag time for December 22 – March 20 (winter) and March 21- May 21 (Farvardin and Ordibehesht). Based on the obtained correlation results, the frequency of the days with rainfall event from November to May was investigated during the positive and the negative phases of the Arctic Oscillation. The results have indicated that the probability of rainfall events during the positive phase of the Arctic Oscillation is the highest. A survey on mean daily geopotential height of 700 hPa level, when the Arctic oscillation is positive, reveals that 700-hPa level is anomalously low over the polar caps and over the region of the Icelandic Low while it is anomalously high over the western half of Africa to southwest Europe. This pattern leads to enhanced pressure gradient over the eastern half of Atlantic and northwest Europe. This 700 hPa level pattern forms a trough over the eastern Mediterranean. Positive vorticity and northerly flow in this area create dynamic conditions to develop low pressure system. When the system is accompanied with other weather conditions can cause rainfall in Iran. In addition to the eastern Mediterranean trough, the sub-tropical high pressure also plays an important role in the rainfall events. Reduction in the zonal range of high pressure at the time of the occurrence of a positive phase of the Arctic Oscillation and its retreat from the southern half of Iran and even in the formation a divergent core over north Arab Sea as the most important source of humidity can increase the probability of rainfall event in Iran.
The results showed that the impact of the Arctic Oscillation on the frequency of the days with rainfall event starts from October 23- November 21 (Aban) and continues to April 21 – May 21 (Ordibehsht). The probability of rainfall event during the positive phase of the Arctic Oscillation is the highest as well. Synoptic pattern of 700 hPa showed that the positive phase of the Arctic Oscillation increase pressure gradient over the eastern half of Atlantic. This pattern provides conditions to develop eastern Mediterranean trough in mid troposphere and low pressure system in low troposphere over the eastern Mediterranean. Decrease of pressure due to the positive phase of Arctic Oscillation in mid-latitude affects subtropical high pressure and retreat from southern half of Iran. Its retreat and even formation of a divergent core over north Arab Sea can increase the probability of rainfall events in Iran.