ENSO Impact on the Surface Pressure Distribution over the Middle East for the Period 1971 to 2000

In recent years, many researchers have studied the effect of ENSO’s teleconnection on the precipitation over various parts of Iran; however, few studies have been done to investigate the effect of ENSO on meteorological parameters. This paper aims to study the effect of ENSO on the surface pressure over Iran and the Middle East. In order for this to be done, a 30-year era (1971-2000) has been selected as the base of research and the sea surface pressure data, as well as the Oceanic Nino Index (ONI) data, has been obtained from the NOAA website. ONI is the difference of the 30 years average of the Sea Surface Temperature (SST) over the NINO3/4 region from its three months running average. Nine ENSO years including three warm phases (El-Nino), three cold phases (La-Nina) and three neutral phases were collected. El-Nino years are 1972, 1982 and 1997, La-Nina years are 1973, 1975 and 1988 and neutral years are 1979, 1980 and 1981. At first, the stationary part of surface pressure data has been separated from the transient part. Since the stationary part is independent from oscillations like ENSO, then only the transient part of pressure has been studied for three El-Nino and three La-Nina years, which are the strongest in the era. In the next part of the research, the partial (net) effect of ENSO has been separated from the effects of other oscillations (i.e. MJO and NAO). The main theory is based on this fact that there are three years (1979, 1980 and 1981) that ENSO were not active and six years with the strongest presence of ENSO. If it is supposed that in all nine selected years all of the oscillations were active, then by subtracting the neutral phase anomalies from warm and cold phase anomalies, the roughly net effect of ENSO will be gotten. In order to do this, at first the mean of anomalies for any phase is computed and then subtracting is done. The most important effect of this averaging is modification of the effect of other oscillation and maintaining the effect of ENSO. Because in these six years ENSO had been very active (SST anomaly more than 1.5 ºc) and the averaging has no any modifier effect on its impact. In continue, the zonal averaging of the net anomalies over the Middle East (from 30 degree east to 70 degree east) for any latitude has been done and the curves of meridianal distribution of it have been investigated. Finally, the correlation coefficient between sea surface pressure and SST of Nino3/4 area is computed by the Pearson product-moment method. The results show that the distributions of anomalies in the same phase (warm or cold) of ENSO years are different. For example, in winter of 1972 all of the Middle East were beholder of positive anomaly whereas in 1982 and 1997, positive anomalies are only seen in the west part of the region (Figures are not shown). In other words, the distribution of the transient part of surface pressure over the Middle East does not follow the ENSO phases. The existences of weak correlation coefficient amounts between ONI and surface pressure data, which are computed by the Pearson method, emphasize these results, especially over northern Middle East and Iran. In next part of the study which is related to the net effect of ENSO, the results are very interesting. In the warm phase of ENSO, there is a wide and strong positive anomaly of pressure over the Black Sea and Eastern Europe during the cold seasons, which made the condition suitable for forming blocking high in these areas. The formation of blocking over Eastern Europe causes the mid latitude and Mediterranean cyclones to pass over Iran and to rain, frequently; whereas, in the cold phase of ENSO, only in autumn the same condition has been seen but with less intensity. These results are very close to the results of other Iranian researchers that say wet year is accompanied by El-Nino and dry year is accompanied by La-Nina. But it must be attended that this effect is not dominant at the presence of other oscillations (former discussion). It seems that the effect of ENSO is weakened or annihilated by the effect of other oscillation during a complicated non liner interaction. This study indicates also, in comparison to El-Nino, La-Nina keeps the Iranian heat low in higher latitudes in summer time. After this displacement the easterly waves may pass over southeast and south of Iran and forming the summery rainfalls of the mentioned areas. In other words, the probability of summery rainfalls over southeast and south of Iran in La-Nina condition is much more than it in El-Ninos. The curves of the meridional distribution of the zonal mean of net anomalies of sea surface pressure are shown that the net anomalies in the El-Nino condition are more positive than those in the La-Nina condition for all latitudes (Figure 2). In winter season the slope of the El-Nino curve is more than La-Nina's at higher latitudes. It confirms the blocking high forming on the north part of the Middle East in wintertime of El-Nino. The results show similar variations for both El-Nino and La-Nina at lower latitudes. It is also seen that in summer season the anomalies of La-Nina at all latitudes are negative whereas the anomalies of El-Nino are positive for latitudes higher than 40 north degree. It means that Iranian heat low is kept at lower latitudes in El-Nino condition comparing to La-Ninas. Since there are different patterns in transient part of sea surface pressure over the Middle East for the same phases of ENSO, then it is concluded that ENSO has no predominant effect on the pressure distribution over the region. But the net effect of it shows that in the warm phase, the condition is suitable for passing the mid latitude and Mediterranean cyclones over Iran and to rain during the cold seasons, frequently; whereas, in the cold phase of ENSO, only in autumn the same condition has been seen but with less intensity. The slope of meridional distribution of net anomalies of the El- Nino which is more than La-Nina's at higher latitudes, confirms this subject. This study indicates also, in comparison to El-Nino, La-Nina keeps the Iranian heat low in higher latitudes in summer time. After this displacement the easterly waves may pass over southeast and south of Iran and forming the summery rainfalls of the mentioned areas.