أحوال الطقس المصاحبة لأجف السنوات وأرطبها
خلال فصل الشتاء (1961-2000) في الأردن
د. علي أحمد غانم
قسم الجغرافيا – الجامعة الأردنية
مجلة جامعة دمشق للآدب و العلوم الإنسانية, المجلد الثانى والعشرون - العدد 1 - 2 - 2006 - ص ص 39 - 59 :
الملخص
تم تحليل بيانات الأمطار ودرجة الحرارة و سرعة الرياح واتجاهها وخرائط الطقس، للتعرف على الأحوال الجوية خلال الفترات الجافة والرطبة في الأردن. وركزت الدراسة على فصول الشتاء للمدة 2000 – 1961. ولقد وجد أن أكثر فصول الشتاء جفافاً كان في عام 1984/1983 وأرطبها في عام 1992/1991.
ولقد تأثرت الأردن بعدد كبير من المنخفضات الجوية(15 منخفضاً) التي سببت هطول الأمطار الغزيرة في شتاء 1992/1991، ورافق ذلك وجود حوض trough في طبقات الجوالعليا فوق شرق البحر المتوسط. أما الفترات الجافة فتميزت بسيطرة مرتفع جوي سبب حدوث الجفاف في معظم شتاء 1984/1983, ورافق ذلك وجود نتوء ridge في طبقات الجو العليا فوق الإقليم, وتعرضت المنطقة لأقل من 7 منخفضات. ومن التغيرات الجوية التي حدثت خلال الفترات الرطبة بالمقارنة مع الفترات الجافة انخفاض في درجات الحرارة، وانخفاض قيم ارتفاع الضغط على مستويات (hpa 700, 850, 500) ، بالإضافة إلى تزايد سرعة الرياح في جميع المستويات. الكلمات المفتاحية: الأردن ، جاف ، رطب ، شتاء، أمطار.
Weather Conditions Associated With Extreme Dry And Wet Episodes In Jordan During Winter (1961-2000)
Dr. Ali Ahmad Ghanem
Geography Department, The University of Jordan
Abstract :
Winter climatic data of rainfall, temperature, wind speed and direction, and synoptic maps were analyzed in order to identify the weather conditions during wet and dry episodes in Jordan. The study focuses on winter seasons from 1961 to 2000. It is found that the wettest winter was in the rainy year 1991/1992, and the driest was in 1983/1984.
Jordan was affected by more cyclones (about 15 cyclones) that caused large amounts of rainfall in 1991/1992 winter. This was associated with upper level trough over the eastern Mediterranean region. During the dry episodes less than 7-cyclones occurred, and high-pressure systems controlled the region that caused dryness, they were associated with upper level ridge over the region. Several changes can be recognized during the wet episodes in comparison with the dry episodes, such as lower temperature, and lower geopotential height of pressures at 850, 700, 500 hpa, in addition to the increase of wind speed at upper levels. Key words: Jordan, Dry, Wet, winter, Rainfall.
Introduction:
Ninety percent of the Jordan is considered arid and semiarid, which has an annual rainfall average less than 200mm, while 6% of the area receives more than 300mm. The annual rainfall average is about 600mm in the northern highlands (Ras Muneef and Salt), and decreases east and southward to less than 50 mm (Touchan and Hughes, 1999; Sahawaneh, 1973). The coefficients of variation were more than 70% in the southern and eastern parts of the country, and less than 30% in the north and the highlands (Kutiel et al., 2000; Shehadeh, 1986 and 1990).
In Jordan, the most important climate element is rainfall. Rainfall determines the economics in the country, especially agriculture. The prediction of wet and dry seasons is useful for agriculture and water resources planning. The rainy season in Jordan extends from Oct to May.
About 70% of the annual rainfall is concentrated in winter months of Dec, Jan, and Feb. The rest of the rainfall amount occurred in the autumn months of Oct and Nov, and the spring months of Mar, Apr and May. The rainy periods in winter alternate with dry periods, each has its distinct pattern of weather conditions and length (Matarira and Jury, 1992).
The average rainy day varies from station to another. It ranges from about 45 days in the northern and highland stations (Salt, Irbed and Ras Muneef) to about 15 days in the eastern and southern stations (Aqaba, Ma’an and Jurf El-Darawish). The major cause of the large variability in the annual rainfall totals is the cyclone track. Most cyclones developed in the Mediterranean Sea tend to move northeast or northward, in which Jordan becomes far from their influence in some years. This explains also the southward decrease of rainfall in the country (Shehadeh, 1990 and 1976).
Climatologists and Meteorologists have been concerned with extreme climate and weather events such as droughts and floods, in addition to the weather conditions associated with these events (Dawood, 1992; Ghanem, 1993 and 1994; Fritsch and Rodgers, 1981; Changnon and Kunkel, 1999; Lindgren and Newmawn, 1981). Climate elements, mainly rainfall and temperature, were used to calculate climatic indices in order to identify wet and dry periods (monthly, seasonal and annually), and then to explain their causes and the associated synoptic conditions, which differ from region to another (Harouna and Carlson, 1994; Ghanem, 1995; and LeComte, 1985).
The aim of this paper is to study and to analyze the conditions associated with the wet and dry episodes that occurred in Jordan during winter months in the period 1961-2000. That was carried out through the analysis of weather elements of rainfall, temperature, winds, and the circulation patterns over Jordan and the surrounding areas. Which is a step toward establishing a successful wet and dry season forecasting.
Data
The data were used in this study include:
1. The monthly rainfall totals for the rainy season during the period 1961- 2000.
2. The daily rainfall, temperature, winds speed and direction during winter for the driest season of 1983/1984 and the wettest season of 1991/1992.
3. The daily surface and upper levels (850 hpa, 700 hpa and 500 hpa) synoptic maps during the 1983/1984 and 1991/1992 winters.
Monthly totals of rainfall were used to generate a time series for the rainy seasons from 1961 to 2000, from which the wet and dry years were delineated. The daily rainfall data for the wettest and driest years were used to generate pentad records (5 days total) that were used to delineate the wet and dry pentads within winter months of December, January and February. Synoptic charts, winds, and temperature data were used to identify weather conditions associated with the wettest and the driest pentads.
The data were obtained from the Jordan Meteorological Department for 15 stations distributed over Jordan, which have complete records for the period 1961 – 2000. Table 1 and figure 1 show the location and the elevation of the stations used in the study.
Table 1: Location of rainfall stat ions and their elevations above mean sea level in Jordan.
Deir Alla 31° 13′ 35° 37′ -224
Ghor Safi 31° 02′ 35° 28′ -350
Irbed 32 ° 33′ 35 ° 51′ 616
Er-Rabbah 31° 16′ 35° 45′ 920
Shoubak 30° 31′ 35° 32′ 1365
Jordan univ. 32° 01′ 35° 53′ 980
Salt 32° 02′ 35° 44′ 915
Aqaba A.P 29° 33′ 35° 00′ 51
Ras Muneef 32° 22′ 35° 45′ 1150
Amman A.P 31° 59′ 35° 59′ 779
Mafraq 32° 22′ 36° 15′ 683
Safawi 32° 12′ 37° 08′ 674
Q.A.I.Airpot 31° 43′ 35° 59′ 772
Ma’an 30° 10′ 35° 47′ 1069
Jafer 30° 17′ 36° 09′ 853
Methodology:
The major wet and dry years within the study period (1961-2000) were delineated by using the rainfall anomaly index (Okoola, 1999), which is
m
Xt = 1/m ∑ 100Xij/⎯Xj
j=1
Where Xt is the time dependent rainfall index (percent) , m is the number of stations studied, Xij is the annual rainfall total in a station, and ⎯Xj is the annual rainfall average in a station.
The results of this index were classified into three categories:
a) Wet years for the values larger than 125%.
b) Dry years for the values less than 75%.
c) Normal years for the values between 75% and 125%.
Principal Component Analysis (PCA) was used to confirm the results of the previous index. PCA can be used to cluster together seasons or periods with similar patterns. Then PCA analysis was also used to delineate wet and dry pentads within winter seasons of 1983/1984 and 1991/1992, which were analyzed to delineate wet and dry rainfall spells.
These pentads were used to study the detailed weather characteristics of the dry and wet spells, through the analysis of surface and upper levels synoptic maps, temperatures, and wind speed and direction, in order to identify circulation patterns and conditions associated with the dry and wet episodes. The correlation coefficients between the variables temperature, geopotantial height, and wind were calculated Findings and Discussion
1. The dry and wet episodes:
The results of the time dependent rainfall index are presented in figure
2. The dry and wet winters were associated with low and high values of the index, respectively. The resulted indices showed that there were seven dry years, 19 normal years, and 14 wet years. PCA was further used to provide area-average rainfall estimates, the PCA isolated the years with similar spatial patterns. There are large loadings during the years 1973/1974, 1979/1980, 1987/1988, and 1991/1992; and low loadings during the years 1976/1977, 1983/1984, 1998/1999 and 1999/2000. Through these methods, the winters of 1983/1984 and 1991/1992 were selected as the driest and wettest years occurred in Jordan during the study period 1961-2000.
Figure 2: Time series of area-average seasonal rainfall indices.
The 1983/1984 and 1991/1992 winters were divided into pentads starting from 22-December to 21-March. There were 18 pentads in each year; the rainfall totals were calculated for all pentads. The wet and dry pentads during each of the selected years (1983/1984 and 1991/1992) were delineated by using the PCA analysis of the rainfall series, running from pentad one (22-26 December) to pentad 18 (17-21 March). These pentads were also divided into three groups: wet, dry, and normal. There were three wet pentads (6,8 and 17) occurred during the 1983/1984 winter, and eight wet pentads (2,3,6,9,10,11,13 and 14) were occurred in 1991/1992 winter. Eleven dry pentads (2,4,5,7,9 and 11-16) were occurred in 1983/1984 winter, and three dry pentads (4,16,17) were occurred in 1991/1992 winter. The other pentads were normal.
It is clear that more wet pentads were occurred during the wet 1991/1992-year than the dry 1983/1984-year; and more dry pentads were occurred during the dry year than in the wet year. All days of the dry pentads were rainless; therefore, their standard deviation (SD) was zero.
Rainfall amounts during the wet pentads were more than 2xSD above the average. The number of rainy days (≥ 1mm) during 1991/1992 winter was higher, more than twice that in 1983/1984 winter in all studied stations. For example, there were 18,13 and 5 rainy days in Ras Muneef, Amman and Ghor Safi in 1983/1984 winter, which increased to 41, 30 and 14 rainy days in 1991/1992 winter for the same stations, respectively (Table 2).
Table 2: Number of rainy days (≥1mm) in 1984 and 1992 winters for the stations under consideration
Station 1984 1992
Deir Alla 13 32
Aqaba 2 5
Ghor Safi 5 14
Ras Muneef 18 41
Irbed 14 44
Amman 13 30
Er-Rabbah 15 34
Mafraq 11 26
Shoubak 14 28
Safawi 5 12
Jordan Univ. 14 38
Q.A.I.Airport 11 26
Salt 12 38
Ma’an 2 9
Jafer 4 9
2- Circulation patterns:
From synoptic point of view, the weather in the eastern
Mediterranean region was controlled by the occurrence of cyclones in winter and the subtropical anticyclones in summer.
But in autumn as in spring the number of active cyclones in the eastern Mediterranean area was low, 15% of the cyclones were affected Jordan during autumn, 35% in spring, and about 50% in winter (Shehadeh, 1990 and Barry and Chorley, 1998).
In winter, the Mediterranean basin with its warm water becomes a center of the development of cyclones that separate between the two major anticyclones, the Azores and the Siberian high pressures. Figure 3 shows the distribution of cyclones and anticyclones during the wet periods. The low-pressure systems developed in the Mediterranean Sea with deep trough aloft. Two high pressures located over Europe and Asia surrounded these lows. Figure 4 shows the existence of high-pressure systems over the area during the dry periods, which were associated with ridges aloft.
Most precipitation in Jordan and the surrounding area is caused by the cold fronts associated with the midlatitude cyclone systems. These fronts are usually associated with west dominant winds, which gain moisture from the warm water of the Mediterranean Sea (Peixoto et al., 1982 and Rosen et al., 1979). It should be mentioned that the southward displacement of the polar jet stream has a significant role bringing Jordan in the path of more low pressure systems, which cause precipitation mainly in the northern parts of the country (Shehadeh, 1986; Al-Khateeb, 2001).
The weather conditions associated with the dry and wet pentads were defined through the analysis of the daily synoptic maps, temperature, wind direction and speed at the surface and aloft (850, 700, and 500 hpa) during the winters of 1983/1984 and 1991/1992. The longest dry and wet spells were also identified as examples for the wet and dry weather conditions.
a) Circulation patterns of the dry pentads:
During the dry pentads of winter, Jordan was under the influence of high-pressure system, the Siberian or the Azores anticyclones. When one of these anticyclones became strong, it expanded its influence to the eastern Mediterranean region causing dryness. Figure 3 and 4 show the distribution of cyclones and anticyclones at the surface and 500 hpa level in dry rainless days. It can be seen that the high pressure (1025 hpa) controlled the Mediterranean region. This pattern happened frequently, less than 7 cyclones occurred in 1983/1984 as well as the other dry years.
High-pressure systems were associated with relatively weak dry winds (southeasterly to easterly winds), but mostly with calm dry conditions caused by the subsiding air associated with the high-pressure systems.
The dominance of these anticyclones over the area prevented the Mediterranean and the European cyclones from reaching the eastern Mediterranean region that in these situations they followed tracks to the north of the region. Therefore, the effects of these low-pressure systems were limited to central and Eastern Europe. While Jordan, as a part of the eastern Mediterranean region was far away from the influence of most cyclones in 1983/1984 and stayed rainless most of the days (figure 5).Then the 1983/1984- winter was dry because the area was under the control of the high-pressure systems with rare occurrence of low-pressure systems.
The location of the upper trough and ridge is a very important factor affecting the development of the Mediterranean cyclones in winter.
During the dry pentads, the upper ridge was over the eastern Mediterranean region, while the upper trough was over the western parts of the Mediterranean and Europe. Figure 6 and 7 are examples showing the location of the upper-level troughs and ridges that have occurred during the dry pentads mainly in the dry 1983/1984-winter. This pattern caused the low-pressure systems that developed in the Mediterranean to move northeast or northward into Europe. So that the eastern Mediterranean region stayed under the influence of the dry winds or the subsiding air associated with the high pressure.
The longest dry spell occurred during the period (10/2 to 10/3/1984), which included six pentads (11 to 16); the weather during this spell was dry and rainless for 31 days. The 1983/1984 winter was a season with continuous dryness, which interrupted by scattered rainy days. Figure 6 and 7 show examples of the weather patterns controlled the region during
this dry spell. They show that the eastern Mediterranean region was under the influence of the subsiding air associated with high-pressure (1025- hpa) system. The dry spell was associated mostly with calm conditions at the surface, with an average temperature about 6.1° C.
During this dry spell there was a ridge at 500-hpa level over the eastern Mediterranean region and a trough over Western Europe (figure 6 and 7). Table 3 shows the daily temperatures, geopotential heights, and wind speed during the 31-day dry spell. In this dry spell, the average geopotential heights were 1503 GPM, 3071 GPM and 5668 GPM at 850 hpa, 700 hpa and 500 hpa, and the average temperatures were 7.6° C, 0.8° C and –17.8° C at the same levels, respectively. There were significant
correlations (88-95%) at the 0.01 level between the geopotantial heights at the 850, 700, and 500-hpa. While the correlations (55-71%) was significant at 0.05 between the temperatures at those levels. The southwesterly winds were the dominant during this dry spell, with an average speed of 9.5, 18 and 35 Knots at 850, 700, 500 hpa, respectively.
There were correlations of 66%-69% (significant at 0.01 level) between the wind speed at 850 and 700 hpa, and between the 700 and 500 hpa, while the correlation was negative and weak between 850 and 500 hpa, because the effect of low level (at the surface and 850hpa) conditions during the dry spells did not extend to the high levels (at 500 to 100hpa)(figure4).
Table 3:Geopotential height (GPM), temperature (T°c), wind speed (Knots) at 850, 700 and 500 hpa during the dry spell (9/2 to 10/3 /1984) in Jordan (Mafraq Radiosonde station).
b) Circulation patterns of wet pentads: In the 1991/1992 winter, the eastward retreat of the Siberian Anticyclone, and the southward retreat of the Azores Anticyclone, allowed the development of low-pressure systems between them, in the eastern part of the Mediterranean Sea. These low-pressure systems tended to follow a northeast to eastward tracks, putting Jordan and the surrounding area under the influence of most of these systems (Figure 8). These systems were associated with cold and warm fronts. The cold front, usually, extends southward from the cyclone center, which causes precipitation when it passes through the eastern Mediterranean. Figure 9 shows a pattern of those low-pressure systems that were developed frequently during the wet 1991/1992-winter. This was in association with the jet stream location. The jet stream in the wet year 1991/1992 advanced more southward than in the dry year 1983/1984, which influenced the path of low pressures. The path of the most (about 15) low-pressure systems that have developed in 1991/1992 winter was through the eastern Mediterranean region (Figure 8), while it was away from the region in 1983/1984 winter (Figure 5).
Therefore, the 1991/1992 winter was wet, with large amounts of precipitation (more than twice the annual average) occurred in most stations in Jordan. Figure 9 and 10 are examples showing the location of the cold front associated with the low pressure, which influenced the weather of the country causing the occurrence of rainfall in most of the days.
At upper level, a trough was over eastern Mediterranean and a ridge was over western Europe (figure 9 and 10). This pattern allowed the lowpressure systems that developed in the Mediterranean to move eastward putting Jordan and the surrounding areas under the influence of moist westerly winds, which caused precipitation over the area. Al-Khateeb (2001) shows that the waves over Western Europe in the Atlantic during the wet year were wider and deeper than in the dry year.
The longest wet spell occurred during the 1983/1984 and 1991/1992 winters, was in the period (30/1 to 13/2 /1992), which included three pentads (9,10 and 11), and all the 15 days of this period were rainy in Jordan. Figure 9 and 10 show examples of the weather patterns occurred in this period as well as in the other pentads. A low-pressure system developed between two high-pressure systems, the Siberian to the east and the Azores to the west. These low-pressure systems were accompanied with clod fronts and moist westerly winds with an average speed of 8 Knots, and average temperature of 3.1° C.
At 500-hpa level, there was a trough over the eastern Mediterranean region (figure 9 and 10) that was responsible for the development of lowpressure systems, and their movements through the eastern Mediterranean Sea caused precipitation in Jordan. Table 4 shows the daily temperature, geopotential heights, and wind speed at upper levels during this wet spell. The average geopotential heights were 1445 GPM 2970 GPM, and 5491 GPM, the temperature averages were (-0.5° C), (-9° C) and (-25.2° C) at 850,700 and 500 hpa, respectively. The temperature at (850 hpa) had a significant strong correlation (66%) with that at (700 hpa), and a weak correlation with that at (500 hpa). The upper level winds were mostly southwesterly, with average wind speed of 20.7 Knots, 32.5 Knots and 43.9 Knots at 850, 700 and 500 hpa, respectively. There were significant correlations between the wind speeds at all levels, the strongest was 82% between wind speed at 700 and 500 hpa.
Table 4: Geopotential height (GMP), temperature (T°c) and wind speed (Knots) at 850, 700 and 500 hpa during the wet spell (30/1 to 13/1 /1992) in Jordan (Mafraq Rodiosonde station).
C) Comparison:
From the weather conditions of the dry and wet pentads occurred in Jordan, the following changes can be recognized:
1) During the wet episodes Jordan was under the influence, and in the path of most low-pressure systems that developed in the eastern Mediterranean Sea, while a high pressure controlled the region during the dry episodes.
2) An upper level trough was over the eastern Mediterranean region during the wet episodes, which was replaced by a ridge during the dry episodes.
3) The temperature was lower in the wet episodes than in the dry spell, the average temperature was lower at all levels by 7-8° C. They decreased, for example, from –17.8° C to –25.2° C at 500 hpa, and from 7.6° C to –0.5° C at 700 hpa. The temperature at (850 hpa) has a correlation of 71% with the temperature at (700 hpa) and 74%with the geopotantial height at (500 hpa) at 0.01 significant level.
4) The geopotential heights were lower during the wet than in the dry episodes. The average geopotential heights dropped from 5668 GPM in the dry episodes to 5491 GPM in the wet episodes. The same trends witnessed for 500 hpa can also be identified at 700 hpa and 850 hpa levels (table 2 and 3). There was a significant positive correlation between the temperature and the geopotantial height at upper levels. The standard deviations of the geopotantial heights were higher in the 1992-wet spell than the 1984-dry spell, this were due to the larger changes occurred in the wet than the dry spells.
5) At the surface, strong wet westerly winds prevailed in the wet episodes, while calm conditions occurred in most days of the dry episodes. Winds were southerly and easterly over eastern Mediterranean during the dry spell, and were westerly and northwesterly in the wet spell, which were associated with the transport of moisture from the Mediterranean Sea in the region. This was similar to the results of Peixota et al. (1982) And Rosen et. al. (1974). Wind speeds at upper levels had positive correlations (50%- 82%) and significant at 0.01 level.
6) At upper levels, the southwesterlies were stronger during the wet than in the dry episodes. At 850 hpa level the average wind speed increased from 9.5 Knots in the dry spell to 21 Knots in the wet spell. The same trends can be identified at 700 hpa and 500 hpa levels.
Conclusion:
The weather conditions were associated with the wet pentads in the 1991/1992 winter were also happened frequently during the wet years such as in the rainy seasons of 1964/1965, 1974/1975, 1979/1980, 1980/1981, and 1987/1988. Similarly, the weather conditions were associated with the dry pentads during the 1983/1984 winter were also happened frequently during the dry years such as in the rainy seasons of 1962/1963, 1967/1968, 1969/1970, 1976/1977, 1994/1995, 1998/1999, and 1999/2000.
There is a negative relationship between dryness and the crop yield. The productivity of the winter crops decreased in the dry years such as in the 1970s and increased in the wet years such as in the 1960s. The productivity of winter crops in 1992 was higher than that in 1984. For example, the productivity of wheat increased from 64.3 kg/dunum in 1984 to 142.7kg/dunum in 1992, and the productivity of barely increased from 27kg/dunum in 1984 to 123.2 kg/ dunum in 1992.
Acknowledgments: This work was supported by grants from the Deanship of Scientific Research at the University of Jordan. I would like to thank Mr. Y. Aabaid for drawing the maps.
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