Synoptic aspects of winter rainfall variability in Oman

In this study, the observed relation between winter rainfall in Oman and the large-scale circulation and synoptic activity was examined on a monthly basis. The investigation focuses on wet and dry spells that occurred during 1984-2007 based on composite analysis. As the investigations have been carried out for a number of meteorological values, a rather complete picture of the large-scale conditions that are favorable for winter precipitation generation are achieved. The major factor that modulates the winter rainfall over Northern Oman is the East Mediterranean upper trough. The mobile upper trough coexists with a ridge covering Western Europe. It is found that the wet seasons over Nothern Oman are coupled with distinct sea level pressure (SLP) and geopotential height (GPH) anomaly over Europe and adjacent areas. While during the wet spells there are mainly negative SLP and GPH over the center of Asia to the northeast of the study area, positive SLP/ GPH anomalies are found over Western Europe. This pressure pattern triggers the moist westerlies towards the Arabian Peninsula. During the dry spells large-scale positive SLP/GPH anomaly areas prevail over Russia, negative SLP anomaly is normally found over southwestern and Western Europe. This pattern advects dry and cold northeasterly air to the study area. Our results also indicate that, the Red Sea Trough under wet winter spells in the northern of Oman is accompanied by a deep 850 and 500-hPa trough, extending from the Black Sea to the Red Sea. This trough is strongly weakened during dry spells. We also found that the wet winter conditions are controlled by the negative phase of the North Atlantic Oscillation. The Negative NAO phase corresponds to enhanced westerly flow (850-hpa up to 200-hpa) which brings moisture from the Atlantic ocean across the Mediterranean area towards Oman.