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There are a number of synoptic circulation systems over the UAE atmosphere. These systems influence the atmospheric conditions of the UAE. This research defines clearly the climatology of UAE and identifies the major circulation patterns that dominate at the surface and also dominate in the upper atmosphere.
The synoptic weather patterns have been identified throughout my twenty years of experience in the meteorological field. Meteorological data was collected over long period from the National Center of Meteorology and Seismology, UAE. Weather charts were analyzed to determine the synoptic weather patterns and their influence on the atmosphere. The research also shows that some of these synoptic weather patterns were significant in the development of weather conditions. One of the significant weather patterns over UAE is the westerly troughs that trigger rainfall during the winters.
The synoptic weather systems over the UAE are categorized regarding their circulation patterns and characteristics. These patterns are influenced by global energy imbalances which lead to different temperatures in different regions, causing pressure variations (Perrone, 1979).The UAE is situated on the Tropic of Cancer just below the sinking edge of the Hadley cell; (Hadley cell is a pattern of circulations which produces winds named as trade wind and tropical easterlies. The air near the equator is warmed by the sun and this warm air rises in the atmosphere.) The region dominated by subtropical anticyclones (Subtropical anticyclones are the reason of desert regions. In these anticyclones air becomes warm and dry which migrates with the change in seasons and cause dryness) hence making the region a subtropical desert (Ahrens, 2007).
Throughout the year the weather shows a high pressure extending southward into the central Saudi Arabia with lower pressure over the eastern Arabian Gulf.
ITCZ: Intertropical Convergence Zone is the zone where the winds coming from the north and the south part of the globe are converged. This can be seemed as a thunderstorm near the equator.
Despite the Intertropical Convergence Zone (ITCZ) existed near by the same latitudes of the UAE during the summer months June to September, it almost never passes across UAE except twice during my twenty years of experience in the meteorological field. The ITCZ causes heavy thundershower activities wherever passes especially during the summer months.
Along the west coastal belt, land breezes dominate light south to south easterlies at night while the moderate north westerly sea breezes dominate during the day. Along the east coast where the vicinity of the mountains results in gusty and unpredictable wind shifts (Hamid, 1992). The strong southerly winds are followed by a strong gust of the northerly Shamal which uplifts desert sand into the atmosphere reducing visibility for some distance (Perrone, 1981). This consequently result after a passage of a trough.
The north Westerly winds that are dominant in the central and northern region of the Arabian Gulf may not extend to the south of UAE. However, when they reach the south, sea weather around Abu Dhabi becomes rough, while the other coastal waters of the UAE experience minimal swells (Perrone, 1981).
General Circulation of the Atmosphere
General circulation of the atmosphere is characterized by winds on a global scale which consists of air flows throughout the world; it is the averaged winds around the globe over a long period. The highs and lows in the atmosphere are existed along with the general circulation of the winds. This general circulation is mainly caused by the different levels of heating of the earth's surface (Barry and Chorley, 1982). The incoming solar radiation (insolation) is approximately equal to the outgoing earth radiation (terrestrial radiation). This energy balance is not the same at different latitudes; this is due to the tropics experiencing a net gain in energy, while the Polar Regions experience a net loss (Ahrens, 2007).
The Polar Regions are at high pressure zones with cold winds while the equator regions are at low pressure zones, so the atmosphere transports warm air towards the pole and the cool air towards the equator. The difference in temperatures cause pressure gradient force which makes air moves from the high to low pressure. (De La Mare, 1978)
The Synoptic Weather Patterns over the UAE
The UAE is located in a subtropical hot desert region with high temperatures and low precipitation. Despite the weak semi permanent thermal low pressure during summer which exists over the Arabian Gulf, the anticyclone cell above it, in the middle and upper troposphere, obstructs the formation and growth of clouds (Evans, 2004). This leads to the low precipitation in the Arabian Peninsula. There are two separate seasons in UAE: a long dry summer season (April to November) with very high temperatures and short winter season (December to March) with temperatures ranging from mild to warm with little precipitation (De La Mare, 1978). The mean annual rainfall since 1980 for Abu Dhabi: 66.3mm, Dubai; 96.7mm, Ras AlKhaima: 124.4mm, AlFujairah: 72.3mm, AlSharjah: 110.5mm, and AlAin:89.3mm (NCMS, 2011).
The synoptic weather patterns of the UAE are closely related with its climatic patterns. They are characterized by different pressure systems. The main pressure systems are: i) Siberian High; it occurs during the winter in the UAE from November till March. This system is characterized by cold and dry northerly to northwesterly winds. It sometimes causes extreme minimum temperatures of 0°C above mountains and further inland over the united Arabian emirates. ii) Easterly Trough; it occurs during the summer in the UAE as a result of the weakening of the Siberian high at the end of the winter season. The easterly trough is characterized by the warm moist air and may cause precipitation over the eastern part of the UAE. iii) Red Sea Trough; it is a surface low pressure trough associated with upper trough of about 500hpa which extends northward from the southern Red Sea. It is an extension of the tropical Sudan low pressure over the Red Sea, which affects the Arabian Gulf regions through its passage. It is experienced during the winter season with unstable weather conditions. It causes rain in the UAE especially when it is being fed by warm moist air from the Arabian Sea. iv) Thermal Low; the thermal low develops as a result of the high insulation over the Empty Quarter. It occurs during the summer months from June to the end of August. It is characterized by hot and dry southerly wind that lead to very high temperature in the region. V) Westerly Trough; it occurs during the autumn, winter, and spring months. It is associated with unstable condition, which results in heavy rainfall.vi) Westerly Cut-Off Low; it is an upper low pressure that occurs during the winter time. This system leads to unstable weather conditions which are associated with very cold air and precipitations over the UAE. Also, it rarely leads to freezing rain and snowfall above the northern mountains over the UAE. (D, 2001)
The Earth's Orbit, the Synoptic Weather Patterns in the UAE, and the Global Climate Change.
The synoptic weather patterns will change over time; Milankovitch Cycle Theory explains how the earth's orbit and rotation has influenced climate change over a long period of time (Milankovitch Cycle Theory is developed for explaining the long term effect of climate changes. Due to the long term changes in climate three elements of the earth gets cyclic variations: precession, eccentricity, and obliquity). The sun-earth interaction also known as orbital variation changes and has been changing positions for the past several years. The eccentricity and obliquity shows the changes in seasons where we have warmer summers and cooler winters. The earth's orbit also affects the distribution of the incoming solar radiation; this as a result leads to changes in atmospheric pressure systems. Research has also shown that thousands of years from now the Northern hemisphere will experience summer in December and winter in June due the change in the tilting angle of the earth's axis (Barry and Chorley, 1982).
To find out the effect of global climate change on the weather synoptic pattern over UAE, I focus only on the effect of the change of the Earth's spin axis. The change of the Earth's spin axis leads to alter on the "Ecliptic" in terms of the Sun's declination angle changes. If the earth was not leant on the rotation axis than the Sun's declination angle would be 0 degrees. This explains that the Earth's spin axis is twisted with respect to the level of the Earth's solar orbit by 23.5° (Carl Nave, 2011). The Earth's axis position is thought to be shifted in the future hence the declination angle would change. As a result, virtual latitudes may shift; this causes change on the general circulation of the atmosphere. The actual path of the synoptic weather patterns would change accordingly. The Intertropical Convergence Zone (ITCZ) for instance fluctuates with respect to the declination of the Sun; it moves towards the south in winter and moves towards the north in summer. As discussed previously, if the declination angle changes, the ITCZ will shift further more towards north in summer, which means climate will change over UAE; precipitation will increase due to the ITCZ activities. On other word, the expected future change of the declination angle influences the location of the ITCZ which can vary up to 45 of latitude north or south of the equator. As the ITCZ shifts to the northern hemisphere, the UAE weather patterns are likely to change. The ITCZ brings convectional precipitation (Thundershower clouds) hence it will help the UAE rainfall increase over time. (Ahrens, 2007)
This project has identified the synoptic weather patterns over the UAE and can be categorized into six synoptic patterns which are: red sea trough, easterly trough, Siberian high, thermal low, westerly trough, and westerly cut-off low. The highest rainfall occurs during the winter which results from clouds formed over the UAE under the influence of the westerly troughs in the upper atmosphere. A freezing rain and/or snow fall above the northern mountains only when the westerly cut-off low reaches UAE,
Global climate change which is influenced by the change of the Earth's spin axis alters the synoptic circulation systems over the world in general. The alterations lead to change in synoptic weather patterns. The effect will be greatly on the subtropical regions thus causing increased in precipitation over the UAE.