عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Similar to the Southern Oscillation which is the most evident teleconnection pattern in the Southern Hemisphere, the North Atlantic Oscillation (NAO) is the most significant teleconnection pattern in the Northern Hemisphere. The NAO is known as one of the most important factors effective on the seasonal and annual variability of the atmospheric general circulation in the Northern Hemisphere. The NAO is a meridional large-scale oscillation in the sea level pressure of the subtropical Atlantic high pressure (Azores high pressure) and subarctic Icelandic low.
In this study, Empirical Orthogonal Functions (EOFs) technique is used to investigate variability of the geopotential heights at 1000-hPa and 500-hPa levels. The data used covers the winter months (December to February) between 1948 and 2005 in the Mediterranean region and the south-west of Asia obtained from the NCEP/NCAR Reanalysis data set. EOF is a statistical method to calculate the variability of meteorological quantities. Meteorology and climatology scientists are faced with large amount of data (time series) which are obtained from meteorological stations or a regular network. Recently, EOF techniques have been used to reduce all known and unknown factors effective on the atmospheric variability to several new factors. These new factors (patterns of variation) are independent statistically and explain the major part of the total variance in the primary data. The advantage of this method is to describe the spatial and temporal variability of the meteorological quantity using just a few modes. Usually, most of the variance of a time series is distributed in the first few EOFs which can tell us about the undelying dynamical mechanisms working in the atmosphere. This method is described as an experimental method because the EOF patterns are computed using the covariance analysis of a specific data set.
Results show that 32.8 and 16.2 percent of the total variance of geopotential height at 1000-hPa level are due to the first and second EOF modes, respectively. Therefore, the first mode indicates an Arctic Oscillation (AO) pattern. The AO is a simultaneous anomaly in the Sea Level Pressure (SLP) between polar and mid-latitude regions in the Northern Hemisphere. This anomaly has been indicated as positive and negative phases. The first two modes of the geopotential height variability at 500-hPa level explain 24 and 21.4 percent of the total variance, respectively. At this level, the first mode indicates the East-Atlantic/West-Russia Oscillation.
In the next stage, all winter months are classified to critical positive and negative months based on their NAO indices, and EOF modes are separately extracted for the two groups. The critical negative months included 33 months. Their first two extracted modes for the 1000-hPa geopotential height explain 35 and 20 percent of the total variance, respectively in the whole Northern Hemisphere. The first two modes for the Mediterranean region show 31.5 and 17 percent of the total variance, respectively. The critical positive group included 29 months and the first two extracted modes for the 1000- and 500-hPa geopotential height show 31.4, 23.5 and 29, 17 percent of the total variance, respectively. It is also found that modes of the critical positive phase of the NAO correspond to the EOF modes of all the data used. However, they show more variability. In the critical positive phase of the NAO, the geopotential height variability at the 500-hPa level is noticeably greater than the critical negative phase in the Eastern Mediterranean and in Iran.