On the Relationship between Climatic Variables and Pressure Systems over Saudi Arabia in the Winter Season

H.M.HASANEAN,H.Abdel BASSET,and M.A.A.HUSSEIN

1Department of Meteorology,Faculty of Meteorology,Environment,and Arid Land Agriculture, King Abdulaziz University,Saudi Arabia

2Department of Meteorology and Astronomy,Faculty of Science,Al-Azhar University,Egypt

On the Relationship between Climatic Variables and Pressure Systems over Saudi Arabia in the Winter Season

H.M.HASANEAN?1,H.Abdel BASSET2,and M.A.A.HUSSEIN1

1Department of Meteorology,Faculty of Meteorology,Environment,and Arid Land Agriculture, King Abdulaziz University,Saudi Arabia

2Department of Meteorology and Astronomy,Faculty of Science,Al-Azhar University,Egypt

The synoptic circulation over Saudi Arabia is complicated and frequently governed by the effect of large-scale pressure systems.In this work,we used NCEP–NCAR global data to illustrate the relationship between climatic variables and the main pressure systems that affect the weather and climate of Saudi Arabia,and also to investigate the in fl uence of these pressure systems on surface air temperature(SAT)and rainfall over the region in the winter season.It was found that there are two primary patterns of pressure that in fl uence the weather and climate of Saudi Arabia.The fi rst occurs in cases of a strengthening Subtropical High(SubH),a weakening Siberian High(SibH),a deepening of the Icelandic Low(IceL),or a weakening of the Sudanese Low(SudL).During this pattern,the SubH combines with the SibH and an obvious increase of sea level pressure(SLP)occurs over southern European,the Mediterranean,North Africa,and the Middle East.This belt of high pressure prevents interaction between midlatitude and extratropical systems,which leads to a decrease in the SAT, relative humidity(RH)and rainfall over Saudi Arabia.The second pattern occurs in association with a weakening of the SubH,a strengthening of the SibH,a weakening of the IceL,or a deepening of the SudL.The pattern arising in this case leads to an interaction between two different air masses:the fi rst(cold moist)air mass is associated with the Mediterranean depression travelling from west to east,while the second(warm moist)air mass is associated with the northward oscillation of the SudL and its inverted V-shape trough.The interaction between these two air masses increases the SAT,RH and the probability of rainfall over Saudi Arabia,especially over the northwest and northeast regions.

climatic variables,pressure system,surface air temperature,rainfall,relative humidity,Saudi Arabia

1. Introduction

Few studies have been published relating changes in Saudi Arabian surface air temperature(SAT)regimes to the large-scale atmospheric circulation(Frank et al.,2002; Chakraborty et al.,2006;Sun et al.,2008;Hasanean et al., 2013).The climate of the Arabian Peninsula represents an issue of particular concern within the context of regional climate change and variability.Local and regional climate is infl uenced by both large-scale atmospheric circulation and surface features(Kidson,1994).Furthermore,atmospheric circulation changes and fl uctuations are important aspects of the climate.Namias(1948)suggested that anomalies of monthly air temperature are determined by the monthly geopotential height fi elds at the middle tropospheric level.Consequently, advective processes exerted by the atmospheric circulation are an essential factor controlling regional air temperaturechanges(Trenberth,1990,1995;Xu,1993;Hurrell,1995; Hurrell and van Loon,1997;Slonosky et al.,2001;Xoplaki et al.,2000;Jacobeit et al.,2001;Pozo-Vazquez et al.,2001; Slonosky and Yiou,2002;Xoplaki,2002).

A considerable amount of modeling and observational work has shown that atmospheric teleconnections have a notable in fl uence on regional and global climate.The study of Hurrell(1995),for example,revealed that the North Atlantic Oscillation controls the climate over the North Atlantic region extending from North America to Europe.Also, Thompson and Wallace(1998)and Gong and Wang(1999) explained that the Arctic Oscillation and Antarctic Oscillation are related to the climate over the mid–high latitudes of the two hemispheres.However,Philander(1983)demonstrated that the Southern Oscillation is accountable for the climate over the tropics and some parts of the extratropics. So,atmospheric teleconnection is a useful phenomenon for investigating climate variability(Dickson and Namias,1976; Wallance and Gutzler,1981;Leathers et al.,1991;and recently Sun et al.,2008).

Since the 1970s,obvious changes in the wintertime atmospheric circulation have occurred over the oceans of the Northern Hemisphere,and these changes have had a profound effect on surface air temperatures(Hurrell,1995).Local changes in meteorological variables in the tropics and midlatitudesaremainlycontrolledbyatmosphericcirculation (Hurrell and van Loon,1997).Vorhees(2006)studied the in fl uences of global-scale climate fl uctuations on Southeast Asia.The study focused on fall–winter temperature and precipitation in Southwest Asia and its relationship with upperand lower-level circulation anomalies in the Eastern Hemisphere.Also observed was that the Siberian High plays an important role in the climate over Saudi Arabia in the winter season.Hasanean et al.(2013)revealed that the Siberian High affects the SAT of Saudi Arabia in the winter season. A number of studies have shown that interannual to decadal changes in winter SAT and precipitation in the Middle East are related to the North Atlantic Oscillation(Eshel et al., 2000;Cullen et al.,2002).The studies of Turkes(1996a, 1996b)revealed that the association between the Middle East and Atlantic sector climate variability is to be expected due to the North Atlantic Oscillation working to regulate Atlantic heat and moisture fl uxes into the Mediterranean region.In winter,the dominant source of Middle Eastern precipitation is the cyclones from the west.

The aim of the present reported work was to study the relationship between climatic variables and the main pressure systems that affect the weather and climate of Saudi Arabia in the winter season.In particular,we investigated the in fl uence of these pressure systems on the SAT and rainfall of the region.

2. Data and methods

We used National Centers for Environmental Prediction–National Center for Atmospheric Research(NCEP–NCAR) reanalysis data with a resolution of 2.5°(lat)×2.5°(lon) (Kalnay et al.,1996;Kistler et al.,2001)obtained from the Climate Data Center(CDC)of the National Oceanic and Atmospheric Administration(NOAA)to calculate the objective centers of action indices for the monthly-averaged pressure.Speci fi cally,the data included:monthly-averaged sealevel pressure(SLP),SAT and RH at 700 hPa,and zonal and meridional wind speed at 850 hPa.

For rainfall,the density of the rain gauge network in Saudi Arabia was insuf fi cient for obtaining accurate spatial distribution data.The installation and maintenance of a dense rain gauge network is dif fi cult in hilly and remote desert areas.Therefore,following Schneider et al.(2011),data for 1948–2010 from the Global Precipitation Climatology Centre(GPCC V6)monthly precipitation dataset were used.The GPCC V6 monthly precipitation dataset covers the period 1901–2010 and is calculated from global quality-controlled data from 67 200 meteorological stations with recording histories of 10 years or longer(Schneider et al.,2011).This product contains the monthly totals on a regular grid with a spatial resolution of 0.5°(lat)×0.5°(lon).

Figure 1 shows the climatological mean SLP during winter(December–February)over(0°–80°N,100°W–140°E)for the period 1948–2010(63 years).The most pronounced feature is that the surface circulation is dominated by a huge subtropical high(SubH),the Siberian High(SibH),Icelandic Low(IceL),and Sudanese Low(SudL).Regions of strong anticyclonic circulation are seen around the SubH,centered over the eastern Atlantic(situated around the latitude of 30°N)and the SibH over northern Mongolia;the SibH often spreads over a very large part of Asia in winter.Strong cyclonic circulation is seen around the IceL,centered between Iceland and southern Greenland between 60°N and 65°N(Sahsamanoglou,1990)and the SudL centered near the Abyssinian Plateau(10°N,38°E).

The quantitative indices of the SubH,SibH,IceL and SudL are de fi ned as the regional mean SLP averaged over the areas(25°–40°N,55°W–0°),(40°–60°N,80°–120°E),(50°–65°N,60°–20°W)and(5°–15°N,20°–40°E)in the winter season,respectively,and provide a measure of the strength of these pressure systems.These rectangular areas generally cover the central regions of the anticyclones and cyclones, and are represented in Fig.1.It is clear that the pressures over the centers of action in the winter season of the SubH, SibH,IceL and SudL are＞1021,＞1036,＜992 and＜1006hPa,respectively.Area-averaged indices are usually more reliable and can provide greater insight than single-point indices,such as those used by Sahsamanoglou et al.(1991)and Mokhov and Petukhov(2000).This is because errors and variability at single locations are averaged out and because the area-averaged indices represent the variability of the center of action rather than a single location only(Panagiotopoulos et al.,2005).Teleconnection patterns can be extracted using correlation analysis between the indices of the pressure systems(i.e.the SubH,SibH,IceL and SudL)with the SAT and SLP over Saudi Arabia.

3. The dominant pressure systems

The SubH is a signi fi cant belt of high pressure situated around the latitudes of 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere.It moves equatorward during the winter season and varies in position and strength.The migration of the SubH toward the equator during the winter season is due to increasing north–south temperature differences between the poles and the tropics(Barry and Chorley, 1992).The latitudinal movement of the SubH is strongly correlated with the progression of the monsoon trough or the Intertropical Convergence Zone.The movement and oscillation of the surface SubH are also associated with the movement and oscillation of the SibH.During the winter season the surface SubH moves eastward and the Saudi Arabian region is affected by its extension.The SubH is a dominant atmospheric circulation system in the lower troposphere and controls the whole of the east/west and mid-Atlantic.The SubH exerts a powerful in fl uence on climate over the midlatitudes.

The SibH in winter is centered over northern Mongolia, with maximum pressure of＞1036 hPa over(47°N,70°E).Its southward extension covers the northern and southern parts of the Persian Gulf and Saudi Arabia,and brings cold air to this area.The SibH is the dominant atmospheric circulation system in the lower troposphere,controlling almost the whole of continental Asia.There is evidence that the SibH exerts a powerful in fl uence on climate over the mid–high latitudes(Guo,1996;Zhu et al.,1997;Gong and Wang,1999; Miyazaki et al.,1999;Yin,1999;Hasanean et al.,2013).

The IceL represents one of the six centers of action in the circulation of the Northern Hemisphere,along with the subtropical,Paci fi c,and winter Siberian highs,and the Aleutian and summer Asian lows(Serrez et al.,1997).The IceL has long been regarded as a semi-permanent low pressure cell in the North Atlantic,typically located between Iceland and southern Greenland(60°–65°N)(Sahsamanoglou,1990).In winter,the IceL is part of a broad area of low SLP dominating sub-polar latitudes,maintained in part by low-level thermal effects of the comparatively warm underlying ocean.The IceL and Aleutian Low are also located downstream of the major mid-tropospheric stationary wave troughs,where eddy activity is favored(Blackmon et al.,1984).A trough of low pressure typically extends northeastward from Iceland over the Norwegian and Barents Seas.On occasion,a trough extends northward along the west coast of Greenland and the Davis Strait.The IceL is the northern part of the North AtlanticOscillation(NAO),whiletheSubHmakesupthesouthern portion.

The SudL is an active system both in the warm and cold seasons.In summer it has thermal behavior and brings hot and dry air to the Arabian Peninsula,causing dusty weather. In winter it is an area of dynamic low pressure and brings humid conditions to the Arabian Peninsula and south of the Mediterranean Sea in the cold and rainy seasons(Rasuly et al.,2012).The Red Sea Trough(RST)is considered as an extension of the SudL(El-Fandy,1948).So,we can say that thenorthwardorsouthwardoscillationoftheRSTarisesfrom the northward or southward oscillations of the SudL.Generally,the movements of the SudL can be classi fi ed into two distinct types of oscillations.The fi rst is the above-described displacementofitscenterfromneartotheAbyssinianPlateau and back again,twice during the course of the year.The second includes a series of relatively small oscillations super-posed on the annual track.These small oscillations are most noticeable in the two transitional seasons(El-Fandy,1940). El-Fandy(1948)showed that the so-called small oscillation accompanies the passage of troughs of low pressure or is secondarily associated with depressions traveling farther north and east over the Eastern Mediterranean,Eastern Europe and the eastern part of the Arabian Peninsula.

4. Relationship between the pressure systems and meteorological variables

The Pearson correlation method was used to detect the relationships between the indices of our pressure systems of interest(i.e.the SubH,SibH,IceL and SudL)and the SAT (Fig.2).An inverse relationship between the SubH and SAT is found over most of regions of Saudi Arabia,except the southwest region(Fig.2a).This means that the strengthening and intensi fi cation of the SubH tend to decrease the SAT over Saudi Arabia.A signi fi cant positive correlation is found between the SibH and SAT over Saudi Arabia,with maximum values over the northern region and extending to the central regions(Fig.2b).This is in agreement with the study of Hasanean et al.(2013).Therefore,the strengthening and intensi fi cation of the SibH during the winter season leads to warming of the northern and central regions of Saudi Arabia.The relationship between the IceL and SAT is presented in Fig.2c.High positive correlation with a signi fi cance of 95%is found over northern Saudi Arabia.It is also positive, but not signi fi cant,in the central region.The relationship be-tween the IceL and SAT is negative over the southern part of Saudi Arabia.Therefore,the weakening of the IceL leads to increases in the SAT over northern and central regions of Saudi Arabia.Deepening of the IceL leads to increases in the SAT over the east of Europe and northeast of Africa,but decreases the SAT over Saudi Arabia and other regions of the Middle East.Figure 2d illustrates the correlation between the SudL and SAT during the winter season over the Saudi Arabian region.A strong negative and signi fi cant correlation is found between the SudL and SAT over the north and central regions of Saudi Arabia.Higher values appear over the central region centered at(24°N,45°E).This means that a weakening of the SudL(increase of SLP)leads to decreases in the SAT over northern and central regions of Saudi Arabia,while a deepening of the SudL leads to warming in these regions.

Table 1 shows the relationships between the indices of the four pressure systems of interest.It is clear that the highest signi fi cant correlation occurs between the SubH and IceL (r=-0.5).The second highest correlation occurs jointly between the SibH and SudL,and the IceL and SudL(r=-0.4). The correlation coef fi cients between the SubH and SudL, and the SibH and IceL,are almost equal and signi fi cant at the99%level.The lowest and least signi fi cant correlation is that between the SubH and SibH(r=-0.11).

Table 1.Correlation coef fi cients between the time series of the indices of the dominant pressure systems.Statistical signi fi cance at the(*)95%and(**)99%con fi dence level is indicated.

The exchange of mass between the SubH and IceL centers of activity has been characterized by the NAO index and linked to surface climate variability(Rogers and van Loon, 1979;Rogers,1984).An intensifying and weakening of the SubH and IceL have marked effects on Mediterranean systems.When the SubH and IceL are weak,the NAO index becomes negative and the pressure gradient reduces overthe Atlantic(HurrellandvanLoon,1997).Inthissituation,therelatively warm and moist air moves toward lower latitudes and is associatedwithheavyrainfalloverthe EasternMediterraneanand also the northwest of Iran.Anomalous temperature variations over North Africa and the Middle East(cooling),associated with the stronger clockwise fl ow around the SubH center,are also remarkable during high-NAO-index winters (Hurrell et al.,2000).Drier-than-average conditions prevail over parts of the Middle East during high-NAO-index winters,as well as over much of central and southern Europe, and the Mediterranean.

In the winter season,the precipitation over Saudi Arabia is created from two sources.The fi rst source is the movement of the westerly upper troughs,which are associated with surface depressions.The second source is the penetration of the RST,which advects warm and humid air.This system is associated with the Siberian ridge,which brings cold and dry air to the area.This mechanism is especially active over the western region and southwest highlands of Saudi Arabia(Abdullah and Al-Mazroui,1998).The RST synoptic system develops over the Eastern Mediterranean region during the cool season(October–April),when the SubH retreats southwestward out of the Eastern Mediterranean(Alpert et al.,2005). Moreover,the SubH moves westward and the SibH generates northeasterly winds across the Arabian Peninsula,bringing mild temperatures.On the other hand,the weakening of the SibH means that fewer depressions are moving into the Mediterranean in winter(Makrogiannis et al.,1991;Sahsamanoglou and Makrogiannis,1992;Wilby,1993).In the next section,the in fl uences of the pressure systems on climatic variables over Saudi Arabia are discussed.

5. In fl uences of the pressure systems on meteorological variables

In this section,we explore the dominant pattern of pressure systems arising from a weakening or intensi fi cation of each of our pressure systems of interest(i.e.the SubH,SibH, IceL and SudL),and also their relationships with meteorological variables.To facilitate this,we constructed composites of meteorological variables for the 10 winters during the period from 1948 to 2010 in which each of our pressure systemsof interest was at its weakest and strongest.These years are listed in Table 2.The composite maps were constructed using the NCEP–NCAR monthly-averaged reanalysis data and comprised:SLP,SAT and rainfall at the surface;horizontal wind,geopotential height and air temperature at 850hPa;and RH at 700 hPa.Those fi elds not shown in the composites were consistent with the explanation given below.

5.1.In fl uence of the Subtropical High

Figure 3a is the composite SLP distribution for the 10 winters during which the SubH was at its weakest(i.e.low SLP).The SubH is weakest(note isobar 1018 hPa)over southern Europe,the Mediterranean,northern Africa,and also over Saudi Arabia.The IceL is weak and its associated trough reaches the east of the Mediterranean(with a cutoff low over the northern Mediterranean)to join with the northward oscillation of the RST.Figure 3b illustrates the composite SLP distribution for the 10 winters during which the SubH was at its strongest.Comparing Figs.3a and b,there is a dramatic difference in the circulation of southwestern Europe and the Middle East that accompanies the difference in SubH pressure.When the SubH is at its strongest(Fig.3b),the pressure inside its center reaches more than 1026 hPa(35°N, 20°W)and combines with the SibH(note isobar 1018 hPa). An obvious increase of SLP occurs over southern Europe, the Mediterranean,North Africa and the Middle East regions. The IceL is deeper,with the pressure at the center reaching 992 hPa.

Figure 4a shows the composite difference of SAT between the average of the 10 winters during which the pressure of the SubH was at its lowest and that when the pressure of the SubH was at its highest.It is clear that there is an obvious increase of SAT(warming)during the years of weakest SubH pressure over Saudi Arabia,the east and northeast of the Mediterranean,Egypt and Sudan(Fig.4a).Thiswarming ranges from 0.1°C over the south of Saudi Arabia to 3°C over Turkey.This warming during the years of a strong SubH is due to the SibH dominating over the Saudi Arabian region,providing a very cold and dry air mass;while during the years of a weak SubH,there are two sources of relatively warm and moist air:from the northwest(southern Europe), and from the south and southeast of Saudi Arabia(Fig.4b). Figures 4c d illustrate the composite difference in RH at 700 hPa and rainfall,respectively,between the average of the 10 winters during which the pressure of the SubH was lowest and that when it was highest.There is an obvious increase of RH and rainfall during the years of when the SubH was weakest.The obvious increase of RH occurs from the surface up to 500 hPa(not shown).Thus,we can conclude that, in weak SubH years,a considerable increase in SAT,RH and rainfall can be expected over the Saudi Arabian region.

Table 2.The 10 winters during the period from 1948 to 2010 in which each of our pressure systems of interest were at their weakest and strongest.

5.2.In fl uence of the Siberian High

Figure 5a illustrates the composite SLP distribution for the 10 winters during which the SibH was at its weakest.The pressure in the center of the SibH is about 1032 hPa,and is high over the Middle East region.Interestingly,the SubH combines with the SibH,and the belt of the SubH overlaps with most of the climatic locations of the SibH.Figure 5b displays the SLP averaged for the 10 winters when the SibH was at its strongest.It is clear that the SibH is separated from the SubH,and a pronounced weak region—indicated by following the 1018 hPa isobar—occurs over Saudi Arabia,southern Europe and the Mediterranean.It is also interesting to note that,in this case,the pattern arising from a pressure system leads to an interaction between two different air masses:the fi rst(cold moist)air mass accompanies the Mediterranean depression travelling from west to east,while the second(warm moist)air mass accompanies the northward oscillation of the SudL and its inverted V-shaped trough(the RST).The interaction between these two air masses(pressure systems)increases the probability of rainfall over Saudi Arabia,especially over the northwest and northeast regions.This pattern also leads to an increase in the SAT over Saudi Arabia,southeast Europe,the east of Africa,and the east Mediterranean.

Figure 6a shows the composite difference of SAT between the average of the 10 years during which the pressure of the SibH was highest and that when it was lowest.There is an obvious decrease of SAT(cooling)during the years of weakest SibH pressure over Saudi Arabia,the east and northeast of the Mediterranean,Egypt and Sudan(Fig.6a).This cooling ranges from 0.5°C in the south of Saudi Arabia to 2°C over Turkey and northern parts of Saudi Arabia.This is because,during the years of lowest SibH pressure,the SibH dominatesovertheSaudiArabianregion,providingverycold and dry air;while during the years of highest SibH pressure, there are two sources of relatively warm moist air:from the northwest(southern Europe),and from the south and southeast of Saudi Arabia(Fig.6b).Figures 6c and d illustrate the composite difference of RH at 700 hPa and rainfall,respectively,betweentheaverageofthe10wintersduringwhichthe pressure of the SibH was highest and that when it was lowest.There is an obvious increase of RH and rainfall during the years of highest SibH pressure.The obvious increase of RH occurs from the surface up to 500 hPa(not shown).Thus,wecanconcludethat,duringtheyearsofstrongestSibHpressure,a considerable increase in SAT,RH and rainfall can be expected over the Saudi Arabian region.

5.3.In fl uence of the Icelandic Low

Figures 7a and b show the composite SLP distribution for the 10 winters during which the IceL was weakest(low SLP)and the 10 winters during which it was strongest(high SLP),respectively.The point of lowest pressure is located near(60°N,30°W).Also,the SubH combines with the SibH (note isobar 1018 hPa)to form the climatological belt of the SubH around the globe.

During the strong IceL winters(Fig.7b),the pressure at its center reaches about 1008 hPa,and the SubH is separated from the SibH.An obviously lower pressure appears over Saudi Arabia,most of North Africa,the Mediterranean and Europe.The most interesting feature is the southward extension of the trough associated with the IceL and its accompanying cut off low over Italy,as well as the northward oscillation of the RST.This situation is favorable for interaction between midlatitude and extratropical cyclones.This interaction causes a strong area of convergence between the two air masses associated with the two cyclonic systems.

Figure 8a shows the composite difference of SAT between the average of the 10 winters during which the pressure of the IceL was highest and that when it was lowest(deepening).There is an obvious decrease of the SAT(cooling) during the years of deepening of the IceL over Saudi Arabia, the east and northeast of the Mediterranean,Egypt and Sudan(Fig.8a).This cooling ranges from 0.1°C over the south of Saudi Arabia to 2.5°C over Turkey.This is because,during the years of a deepening IceL,the SibH dominates over the Saudi Arabian region,providing cold and dry air(Fig. 7a);while during the years of highest IceL pressure,there are two sources of relatively warm moist air:from the northwest(southern Europe),and from the south and southeast of Saudi Arabia(Fig.8b).Figures 8c and d illustrate the composite difference of RH at 700 hPa and rainfall,respectively, between the average of the 10 winters during which the pressure of the IceL was highest and that when it was lowest. There is an obvious increase of RH and rainfall during the years of highest IceL pressure.The obvious increase of RH occurs from the surface up to 500 hPa(not shown).Thus,we can conclude that,during the years of highest IceL pressure, a considerable increase in SAT,RH and rainfall can be expected over the Saudi Arabian region.Also,the in fl uence ofthe IceL is approximately similar to that of the SibH.

5.4.In fl uence of the Sudanese Low

Figure 9a shows the composite SLP distribution for the 10 winters during which the SudL was weakest(deepening).The results illustrate low SLP over southern Europe, the Mediterranean,North Africa and Saudi Arabia.These areas of low SLP allow the SudL and its associated inverted V-shape trough(the RST)to oscillate northward to reach the east of the Mediterranean.We also fi nd that the trough associated with the IceL extends southeastward to cover southern Europe and the Mediterranean,with a cut off low over Italy. This situation leads to anomalous westerly fl ow from the Atlantic and Europe to the Middle East because of the decrease in SLP.Another source of high moisture fl ux in the Middle East is the Arabian Sea and Indian Ocean,associated with the northward oscillation of the RST.Figure 9b shows the composite SLP distribution for the 10 winters during which the pressure of the SudL was at its highest(weakening of the SudL).Under such conditions,the pressure inside its center reaches 2012 hPa(7.5°N,30°E),and the SubH is connected with the SibH(note isobar 1018 hPa).Obvious regions of high SLP occur over southern Europe,the Mediterranean, North Africa and the Middle East.Also,the IceL is weak, with a central pressure of 994 hPa.

Figure 10a shows the composite difference of SAT between the average of the 10 winters during which the SLP of the SudL was lowest and that when it was highest.There is an obvious increase of SAT(warming)during the years of strongest SudL pressure over Saudi Arabia,the east and northeast of the Mediterranean,Egypt and Sudan(Fig.10a). This cooling ranges from 0.1°C over the south of Saudi Arabia to 3°C over the southeast of Turkey.This is because, during the years of highest SLP for the SudL,the SibH dominates over the Saudi Arabian region,providing a very cold and dry air mass;while during the years of lowest SLP for the SudL,there are two sources of relatively warm and moist air: fromthenorthwest(southernEurope),andfromthesouthand southeast of Saudi Arabia(Fig.10b).Therefore,during the years of lowest SudL pressure,an increase in SAT and rainfall over Saudi Arabia occurs.This means that the a weakening of the SudL(increase of SLP)leads to decreases in the SAT over the northern and central regions of Saudi Arabia, while a deepening of the SudL leads to warming in these regions.This is because,when the SudL deepens,it oscillates northeastward and brings warm and moist air to central Saudi Arabia.

6. Conclusion

The present reported work aimed to study the relationship between climatic variables and the main pressure systems(i.e.the SubH,SibH,IceL and SudL)that affect the weather and climate of Saudi Arabia in the winter season. We also investigated the in fl uence of these pressure systems on SAT and rainfall over Saudi Arabia in winter.First,we studied the relationships between the indices of our pressure systemsofinterestwithSATandSLP.Aninverserelationship was found between both the SubH and SudH with SAT over most of Saudi Arabia,except the southwest region,which means that the strengthening and intensi fi cation of the SubH tends to decrease SAT over this region.A signi fi cant positive correlation was found between the SibH and SAT over Saudi Arabia,with maximum values over the northern region and extending to central regions,which means that the strengthening and intensi fi cation of the SibH during the winter season leads to warming of northern and central Saudi Arabia.

The in fl uences of our pressure systems of interest on the SAT and rainfall over Saudi Arabia was studied by constructing and comparing composites of meteorological variables for the 10 winters during the period from 1948 to 2010 in which each of the pressure systems was at its weakest and strongest.The most signi fi cant fi nding came in the form of two patterns,the fi rst of which is associated with each one of the following cases:(1)a strengthening of the SubH;(2) a weakening of the SibH;(3)a deepening of the IceL;or(4) a strengthening of the SudH.In this pattern,the SubH combines with the SibH and an obvious increase of SLP occurs over southern Europe,the Mediterranean,North Africa,and the Middle East.This belt of high pressure prevents interaction between midlatitude and extratropical cyclones,which leads to decreases of SAT,RH and rainfall over Saudi Arabia.The second pattern is associated with each one of thefollowing cases:(1)a weakening of the SubH;(2)a strengthening of the SibH;(3)a weakening of the IceL;or(4)a weakening of the SudH.In this case,the pattern arising from a pressure system leads to interaction between two different air masses:the fi rst(cold moist)air mass accompanies the Mediterranean depression travelling from west to east,while the second(warm moist)air mass accompanies the northward oscillation of the SudL and its inverted V-shape trough(the RST).The interaction between these two air masses(pressure systems)increases the probability of rainfall over Saudi Arabia,especially over the northwest and northeast regions.This pattern also leads to increased SAT,RH and rainfall over the region.

Acknowledgements.This work was funded by the Deanship of Scienti fi c Research(DSR),King Abdulaziz University,Jeddah (Grant No.155-003-D1433).The authors therefore acknowledge with thanks the DSR for their technical and fi nancial support.The authors also thank the NCEP/NCAR for providing the SLP,SAT, RH and wind velocity data,and the Global Precipitation Climatology Centre for providing the monthly precipitation dataset.

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10.1007/s00376-014-4149-5.

(Received 8 July 2014;revised 27 September 2014;accepted 9 October 2014)

?Corresponding author:H.M.HASANEAN

Email:hhasanean@kau.edu.sa