The effects of NAO on some meteorological parameters over Middle East and South West Asia

This paper is devoted to the study of the differences in distribution of some important meteorological quantities over the South West of Asia (SWA) between the positive and negative phases of the North Atlantic Oscillation (NAO). The data used are winter time gridded fields of wind as well as geopotential height at different levels and mean sea level pressure (MSLP), each at four times a day, taken from the NCEP/NCAR reanalysis data sets for a 55-year period from 1950 to 2005. The spatial resolution of data is 2.5×2.5 in longitudinal and latitudinal directions covering a grid from 90W to 90E in longitude and from 20N to 80N in latitude. The 168 months of this period are divided into two categories of critical months: 29 positive and 33 negative NAO months. For a selected number of meteorological quantities, the ensemble mean of the anomaly fields at critical positive and negative months are calculated. A low-pass filter is applied to the 500hPa geopotential height to filter disturbances with periods of less than 10 days and then the ensemble mean of the anomaly of the filtered 500hPa geopotential height as well as the outgoing long-wave radiation are calculated for the two above groups of months.For the selected quantities, the results show that the differences between the two phases of NAO take their maxima over the North Atlantic Ocean and fall to much lower values over the SWA. In critical positive (negative) months there is a cold (warm) air anomaly over the North Atlantic that is due to the differences of flow anomaly between the two phases of NAO. The mean wind-speed of the subtropical jet at 300hPa level over the SWA in critical positive months is about 8 ms-1 greater than that in critical negative months. The intensification of the subtropical jet at positive months is expected to lead to the formation of stronger than normal cyclones over the SWA. The corresponding differences between the patterns of outgoing long-wave radiation indicate a southward displacement of the Atlantic storm track in negative phase associated with which there is a noticeable transport of warm and humid air to the Mediterranean region and south of Europe. In most of the SWA, the cloudiness is higher over the negative phase than the positive phase. With regard to streamline maps at 700hPa, the sources of the observed changes in cloudiness in the south and north of the SWA seem to be the Indian and Atlantic oceans, respectively.