Long-term changes in seasonal temperature extremes based on daily data across Saudi Arabia for the period 1981-2010 are analysed by assessing the trends for the four conventional seasons. Surface observations of daily maximum and minimum temperatures from high-quality datasets at 27 stations are used as the input. The trend throughout each season is then derived by employing Sen's slope estimator to four extreme value indices, four relative indices and three mean value indices. Warming trends for extreme value indices are observed for the majority of stations, particularly significant (at 95% level) in spring and summer seasons, however, mixed increase/decrease trends are found for the cold temperature extremes in autumn and winter seasons. Relative indices show significant warming trends for the majority of stations in all seasons; however, strong warming (above 5 days decade −1 ) is witnessed in the spring, summer and autumn seasons. The rapid rise (fall) of the number of warm (cool) days compared to warm (cool) nights is observed in the winter, summer and autumn (winter and spring) seasons. Warming of cool/warm nights is insignificant for the majority of stations in winter. The national average of mean value index diurnal temperature range shows an increasing trend for all seasons; however, its mixed increase/decrease trends are observed for the majority of stations in summer and autumn seasons. Time series analysis reveals that irrespective of seasons, warming is clearly visible in Saudi Arabia after 1997. Variations of warming for different regions across the country are also noticed.

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ABSTRACT:The synoptic characteristics and statistical variability of seasonal dust over southwestern Saudi Arabia are studied for the period from 1979 to 2006 using the aerosol index (AI) from the Total Ozone Mapping Spectrometer (TOMS) satellite, dust observations from surface stations, and meteorological data from the National Center for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) reanalysis data set.The seasonal AI distribution indicates that approximately 80% of each year was dusty and that most of the dust occurred during hot months. In addition, the surface observations of the dust types show that the stations with the largest number of dust observations throughout the year were close to the desert, except during the summer, when the stations near the Red Sea had the largest number of dust observations.The synoptic forces that influenced the dust cases were the relative positions of high-pressure systems (Azores or Siberian) and low-pressure systems (Sudan or Indian), alongside their interactions. The relative positions of the atmospheric systems are highly pronounced at a pressure level of 850 hPa; at this pressure, the systems are oriented from north (anticyclonic system) to south (cyclonic system), turn anticlockwise to become oriented from west (anticyclonic system) to east (cyclonic system) during the summer, and then turn clockwise during the winter. Moreover, the interaction of the atmospheric systems influences the wind pattern of the seasonal composition over the southern Arabian Peninsula, which produces an anticyclonic wind pattern during winter, a cyclonic wind pattern during spring, a northerly/northwesterly wind pattern during summer and an anticyclonic wind pattern during autumn.The dust sources changed because of the relative positions of these atmospheric systems, in which the 'Toker Gap' Sudan was the summer/autumn dust source, and the 'central and eastern' Arabian Peninsula was the winter/spring dust source.

The monthly and seasonal variability and distribution of dust events over northern Saudi Arabia were studied using ground‐based measurements from 11 surface stations for the period of 1978–2010. Additionally, to study the synoptic climatology of the dust variability, the aerosol index (AI) data from the Total Ozone Mapping Spectrometer (TOMS) satellite and meteorological data from the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets were used. The dust types observed at the surface stations are classified into two categories: weak dust and dust storms. A statistical study of the ground measurements demonstrated that weak dust category events are prevalent in the cold months, whereas storm category events are prevalent in the hot months. Additionally, the annual distribution distinguishes two periods for dust observations, before and after 1989, where the number of events in the first period is lower than the annual average but increases during the second period. The synoptic climate study indicated that two main atmospheric wind patterns, anticyclonic and northerly (shamal) patterns, accompany the dust events in the study area. The dust in winter and autumn is mainly affected by the anticyclonic pattern, while that in spring and summer is mainly affected by the shamal pattern. In addition, a synoptic study of selected cases confirmed the climate results and demonstrated the existence of two atmospheric patterns corresponding to winter and summer. Both patterns include troughs over the Red Sea and Arabian Gulf and a ridge or high‐pressure cell over the eastern Mediterranean region during the summer atmospheric pattern and over the mid‐Arabian Peninsula during the winter atmospheric pattern. The characteristic of non‐dust storm composition demonstrated that the storms exhibit pronounced synoptic systems (winter and summer) with the highest pressure/geopotential gradient near the Arabian Peninsula.

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