Keywords

1 The UAE Climate in the Global Context

The UAE, as in most of the Arabian peninsula, is characterized by a hot desert climate: “BWh” in the Köppen climate classification system (Peel et al. 2007). BWh regions are the most common types of land area on earth, covering 14.2% of the global land surface. They straddle either the northern or the southern tropics (~30° N or S), and are characterized by extreme aridity and annual average temperatures above 18 °C. The whimsical name “lands of the empty bucket” gives an intuitive characterization of these arid climates: a bucket left outdoors is virtually never found to be full of water, because year-round evaporation exceeds precipitation (Warner 2009, ch. 2).

The hot desert climate origin must be traced back to the features of the large-scale circulation of the atmosphere. In the tropical belt the circulation is dominated by two enormous convection cells, one in each hemisphere, named after the eighteenth century meteorologist George Hadley (Fig. 3.1). Warm, moist air rises in a relatively narrow belt called “intertropical convergence zone” (ITCZ) close to the equator, rising to the bottom of the stratosphere (ca. 15 km high). As it travels upward, its moisture condenses into water droplets, forming abundant clouds and storms that sustain the equatorial rainforests. Once aloft, the now dry air masses bisect and travel either northward or southward to about 30° of latitude, where they descend back to the surface, becoming warmer in the process. The descent of this dry air mass that had lost much of its moisture in the ascending phase is called “subsidence” and it is the main cause for the existence of hot desert regions. In fact, any moisture rising from the surface becomes diluted into the warm, dry, subsiding air masses, in a process that severely hampers cloud formation, and nearly eliminates it when it happens over land, where ground-level moisture is not particularly abundant. As a result, under an area of subsidence there is little to no precipitation, and, lacking the shade of clouds, the near totality of the incoming solar radiation reaches the soil, promoting evaporation (Warner 2009, ch. 2). The presence of such conditions result in vegetation and animal assemblages that are low in diversity, often highly constrained to particular microenvironments, and characterized by unique adaptations to support their continued survival under extreme aridity and temperatures (Chaps. 5 and 13).

Fig. 3.1
An illustration of the Hadley circulation in spring and autumn. The equator is at the bottom, a roughly symmetric cell is formed above it by warm, moist air moving upwards and cold dry air moving downwards. The Hadley cell forms between 30 degree North and 30 degree South.

Schematic diagram depicting the Hadley circulation in spring and autumn, when the northern and southern cells are roughly symmetric. The intertropical convergence zone (ITCZ) is the stormy area where warm, moist air uplifts. Hot desert areas occur below the descending branch of each cell. NOAA image, from scijinks.gov/trade-winds/ (Public Domain)

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The shape and strength of the Hadley convection cells follows a seasonal cycle: in winter the northern hemisphere cell is weaker and the narrow band where warm, moist air is uplifted is located to the south of the Equator. The opposite occurs in the summer. This seasonal cycle is highly distorted and enhanced where there is a North-South asymmetry in the distribution of land masses, most notably in the case of Asia and the Indian Ocean. Because land warms and cools more readily than the ocean, during the summer the ITCZ migrates much further north, over the Indian subcontinent and the Indochinese Peninsula, and regresses south of the Equator over the Indian Ocean during the winter. This extreme seasonal shift of the ITCZ is the Indian Monsoon, and the associated summer rainfall maintains Southern Asia lush and humid, even though it lies at the same latitudes as the Sahara desert.

The seasonal shift of the ITCZ is the main determinant of the seasonal climate cycle of the Arabian peninsula. During the winter, the subsidence of air produces a persistent region of high pressure in the middle of the peninsula that organizes the dominant winds in an anticyclonic (that is, rotating clockwise) pattern (Fig. 3.2a). Mid-latitude winter storms forming on the Mediterranean are capable of bringing some clouds and rain to the northernmost part of that region and the Mediterranean Levant (Kushnir et al. 2017). Sometimes, the high pressure area in Arabia weakens and the occasional Mediterranean storm is able to penetrate further into the Arabian peninsula. These are the main sources of winter rainfall for the inner part of the Arabian peninsula. Very rarely, storms of this sort are able to reach the UAE. Those rare storms, which are associated with the strongest Kaus-Shamal winds (see below), are powerful ones, and when they occur, they bring bursts of very heavy rainfall, resulting in widespread flooding (Membery 1997). Such rainfall events can be important for enhancing germination of annual plant seedbanks, leading to short-lived but widespread blooms of vegetation in the wake of storms (Chap. 5). Most of the (little) rain that the UAE receives is triggered by the presence of the Hajar mountains. Winds may occasionally push and uplift moist air over the mountain slopes, which then condenses, causing storms and rain. Such rainfall differences, in part, support the unique floral and faunal communities that characterize the Hajar mountain range (Chaps. 6 and 13), as well as the vegetation of the alluvial plains that specialize on tapping into the underground waters flowing subsurface from the mountains following rains (Chap. 5).

Fig. 3.2
2 maps of Arabian peninsula mark and highlight the average surface wind vector. January. The wind speed is between 0 and 8 meters per second. The wind in the south moves to south and southwest. July. The wind speed is between 0 and 14 meters per second. The wind in the south moves to northeast.

Climatological average (1979–2021) of the surface wind vector over the Middle East region. Panel a, left: month of January. Panel b, right: month of July. Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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During the summer, the ITCZ brushes the southern part of the Arabian Peninsula, and it can be recognized as a narrow belt of very low surface wind (associated with upward air motion) running from the Strait of Bab el Mandeb in the south of the Red Sea to the Strait of Hormuz at the entrance to the Arabian Gulf (Fig. 3.2b). This is connected with a low pressure area sitting over the vast expanse of the Rub’ al Khali (Fonseca et al. 2022), known in English as the “Empty Quarter”: a hyper-arid sand desert that occupies most of the eastern part of Saudi Arabia and of the southern UAE. The low pressure draws-in dry air from all of the northern Arabian peninsula, but moist air coming from the Indian Ocean is deviated by Earth’s rotation in north-easterly direction (due to Coriolis forces), flowing towards India in a fast stream of air known as the “Somali Jet” (a portion of which is clearly visible in the lower-right corner of Fig. 3.2b). Only the south-western tip of the Arabian Peninsula, roughly coinciding with modern-day Yemen, is systematically reached by monsoonal humid air that, thanks also to the favorable topography, every summer brings rain well in excess of 200 mm/year, peaking above 400 mm/year in some places. These rain levels far exceed those found anywhere else in the Arabian Peninsula (Almazroui et al. 2012) and justify the name that the ancient Romans gave to this southern region: Arabia Felix: ‘Fertile Arabia’. Most of Oman and the UAE only occasionally experience summer rain of monsoonal origin. Other than in Yemen and in some portions of southern Oman, the total amount of summer rain in the Arabian peninsula is much lower than that of winter rain (Jing et al. 2020).

2 Climate of the Past

The changing breadth of the seasonal shifts in the position of the ITCZ have been the main factor shaping the climate of North Africa and of the Arabian Peninsula during the Quaternary Period (ca. the past 2.6 million years). It is well known that changes in insolation (that is, sunlight intensity) are determined by changes in Earth’s orbital parameters (ellipticity of the orbit, inclination of the Earth’s rotation axis) which drive the expansion and retreat of ice sheets at high latitudes. Those same changes also determine the strength and latitudinal extent of the monsoonal rains in Africa, Arabia and India. Stronger insolation during the summer leads to larger seasonal shifts in the position of the ITCZ.

After the end of the last glacial phase, when summer insolation was at its maximum, the ITCZ occurred much further north than in the present day (Kutzbach 1981). Therefore, during summer, abundant rainfall would reach areas that today are arid. While North Africa was very dry during the last glacial maximum, between approximately 11,500 YBP and 5500 YBP, the Sahara was not a desert, but rather a fertile, humid savannah, which sustained numerous neolithic civilizations (Bard 2013).

During that period, the sea was slowly rising to the current-day level as the polar ice sheets melted, flooding into the riverine basin which today is the Arabian Gulf (Chap. 4), and the Somali jet blew much further north over the Arabian peninsula, rather than offshore of the Oman coastline (Fleitmann et al. 2007). Abundant summer monsoonal rainfall would have been common in the southern half of the peninsula, including all of modern-day UAE, while Mediterranean, wet-winter conditions would prevail in the northern half (Lüning and Vahrenholt 2019). This was a period of time known as the ‘climatic optimum’ when early civilization was established in Mesopotamia, and when early maritime trade networks were established throughout the Arabian Gulf region, including at many early villages along the UAE’s Gulf coast (Kennett and Kennett 2006; Uerpmann and Uerpmann 1996; Charpentier and Mery 2008; Beech et al. 2000). The UAE at this time had savanna-like grasslands and dense shrubbery across much of its extent (Kallweit 2003; Rose 2022), and it has been suggested that today’s ghaf stands are remnants of much more widespread systems that occurred during this humid period (Chap. 5).

Both the onset and the end of the African Humid Period (i.e. the ‘climatic optimum’ that ended approximately 5500 YBP) appear to have been abrupt, even though its astronomical causes undergo a very gradual change (deMenocal et al. 2000). Unraveling the causes of such an abrupt climate shift is an ongoing puzzle that has not yet been completely resolved. It has long been known that changes in the vegetation cover feed-back on the strength and the extent of the Hadley circulation by affecting the fraction of sunlight that is reflected back into space rather than absorbed by the ground (Charney 1975). It has also been suggested that the subsidence of air over western Sahara and the Mediterranean may be linked to the strength of the Asian monsoon, in a counterintuitive desert-monsoon feedback (Rodwell and Hoskins 1996). Other feed-back mechanisms include the the mutual influence of vegetation and of the hydrological cycle, and of the atmospheric and oceanic circulations on each other (Claussen et al. 1999). The overall picture that emerges of recent climate shifts is that North Africa and the Arabian peninsula may be found in either of two states: desert or savannah. The presence of each state creates conditions favorable to its own existence and persistence. The state, therefore, persists for a while, even in the face of astronomical conditions becoming unfavorable, until it finally suddenly gives way to the other state. This relatively sudden shift to today’s arid conditions ca. 6000 to 5000 YBP is known as the ‘Dark Millenium’, when archaeological evidence suggests a broad collapse of early pastoral societies along the UAE’s Arabian Gulf coast and a return to nomadism and a mass migration of people to more benign environmental refugia on the Oman coast (Rose 2022) (see also Chap. 22); this period led to the onset of the arid conditions that persist to this day and continue to shape local ecology.

3 The Shamal and Other Winds

The dominant, large-scale wind over the Arabian Gulf blows from north-west to south-east (Fig. 3.2a,b). When it reaches speeds capable of dominating over the local breezes this wind becomes known as Shamal (Arabic for north). It is convenient to distinguish between a summer and a winter Shamals: even though both kinds of wind blow in the same direction, their origin and nature is distinct.

The summer shamal originates in the Tigris and Euphrates valley, where it blows stronger, and gradually weakens as it moves towards the UAE (Fig. 3.2b). It may be interpreted as the return path of air circulating in the Northern Hadley Cell subsiding over the Mediterranean Levant and flowing towards the summer low pressure area residing over the Rub’ al Khali. The Zagros mountains in Iran help guide this air stream over the Gulf and the east coast of the Arabian peninsula (Membery 1983). The summer Shamal is sometimes called the “wind of 120 days” (Yu et al. 2016) because of its persistent nature. However, individual Shamal events of greater strength than the average are recognizable during the course of a single summer. Because the summer Shamal is linked to the position of the ITCZ, which is itself modulated by several global-scale cycles, in some years the summer Shamal is stronger than in others (Al Senafi and Anis 2015). The summer Shamal is also a main contributor to the regulation of the Arabian Gulf’s sea temperature, because it induces strong evaporative cooling, which, in the Abu Dhabi waters, may reach 400 W/m2 (Paparella et al. 2019; see also Chap. 4). Such shamal-driven cooling can be critically important for marine organisms in summer, as they can lead to temperature declines of up to 2 °C that can persist for over a week (Paparella et al. 2019). In the absence of summer shamals, water temperatures may rise to levels that exceed the lethal limits of marine fauna, such as during the calm summer of 2017 when sea temperatures reached 37 °C and caused the loss of 73% of all coral in Abu Dhabi (Burt et al. 2019). Finally, strong summer Shamal events can cause large-scale dust storms. Iraq and Kuwait are most affected by these, but the dust-loaded air can easily reach the UAE shores, severely deteriorating the air quality for several days in a row (Bou Karam Francis et al. 2017). Such dust storms have been suggested to be important nutrient fertilizers (particularly of iron) that enhance phytoplankton productivity in the Gulf (Moradi and Moradi 2020), although they may also exacerbate harmful algal blooms (Hamza 2021).

During winter, the establishment of a region of high pressure in the center of the Arabian peninsula produces a clockwise (anti-cyclonic) circulation that, over the Gulf, leads to dominant winds blowing from north-west to south-east (Fig. 3.2a). These, however, are generally weak and fluctuating. The actual winter Shamal events are relatively rare, generally occurring two or three times per year, between November to March. They each typically last 3–5 days, during which the wind speed over the Gulf exceeds gale force (more than 50 km/h). A winter Shamal event is triggered by the formation of a low pressure over the Mediterranean Levant. The low pressure, as it moves eastward over the southern Tigris-Euphrates valley, attracts air from the Gulf, which takes the form of a wind from the south east, named the Kaus wind locally, from Farsi for ‘south’ (Reynolds 2002). The strength of the Kaus varies greatly, ranging from nearly zero to gale-force winds, and may be associated with rain or dust storms. The low pressure then moves over the Iranian Zagros mountains, opposite to the high pressure residing over Saudi Arabia. This pushes a cold front over the Gulf: a tongue of cold air which propagates from north-west to south-east, sandwiched between the high and the low pressure. In the wake of the cold front, a jet of gale-force winds, the actual winter Shamal, persists for a few days before weakening and dissipating when the low pressure over the Zagros mountains finally disappears (Perrone 1979). The extreme cold carried by some winter shamals can have rare but significant biological consequences, such as leading to cold-induced coral bleaching and mass mortality on reefs in the southern Gulf (Shinn 1976; Coles and Fadlallah 1991).

Outside of shamal events, winds on the UAE coastline are more typically dominated by daily land-sea breeze cycles. The basic mechanism is well-known: air rising over hot afternoon land draws-in cooler air from the sea, giving a so-called seabreeze each afternoon that persists until sunset. Late at night, when land becomes colder than the sea, the flow reverses, resulting in a slight landbreeze that moves offshore during the early morning hours. In the UAE this basic mechanism is present, but with a literal twist. Air masses pushed in any given direction, because of Earth’s rotation, don’t keep traveling in that same direction, but deviate to their right, so that, if nothing else intervenes, the air mass travels a full circle, performing what is called an inertial oscillation. At the latitudes of the Gulf, the circle closes in about 1 day. There’s thus the potential for resonance between the daily land-sea breeze and the inertial oscillations (Fearon et al. 2020). As a result, the direction of the breeze on the UAE coastline typically performs a full 360° turn every 24 h (Fig. 3.3, top panel). The wind speed still shows the usual modulation of a land-sea breeze: it picks up speed early in the morning and late in the afternoon, and subsides in the middle of the day and of the night (Fig. 3.3, bottom panel) but the wind direction rotates, rather than reverse its course (Paparella et al. 2019). Little is known of the biological importance of the daily land-sea breeze cycles in terrestrial systems, but they can be highly important for turning over waters in lagoon systems, modulating temperatures (Mohamed et al. 2005).

Fig. 3.3
2 charts of wind direction and wind speed from 8 August 2017 to 15 August 2017. Top panel. Wind direction. The wind blows from south-west-north-east-south direction. Bottom panel. Wind speed. The wind speed fluctuates between 0 and 7 meters per second.

Rotating summer coastal breezes off the Abu Dhabi coast (25.75 N, 54.25 E). Top panel: wind direction. Bottom panel: wind speed. Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Convective storms occur when warm, humid air is, within hours, uplifted from near the surface to heights of several kilometers, triggering condensation and rain. Rain-cooled air falls to the surface, replacing the uplifted warm air, producing cold, strong surface winds in front of the storm. Squalls of this sort are commonly observed in association with mid-latitude thunderstorms, where they generally precede by a few tens of minutes the arrival of the rain, but are otherwise inconsequential except to sailors. Desert storms also have cold squalls that precede their arrival, but in a dry, sandy environment they cause dangerous dust storms called ‘haboobs’ (from the Arabic for ‘blowing’) that suddenly reduce visibility and bring unhealthy air. In the UAE haboobs generally are caused by storms originating from warm, humid air coming from the Sea of Oman, uplifted over the slopes of the Hajar mountains, which are able to cross the mountains where they drop rain that results in the cooling front and move into the flatlands overlooking the Gulf coast. More rarely, haboob-forming storms originate from the Empty Quarter, entering the UAE from its southern border (Miller et al. 2008).

Tropical cyclones are the most powerful storms on the planet. Those originating in the Atlantic are known as hurricanes, and those originating in the Asian Pacific are known as typhoons. The Arabian Sea can become warm enough to trigger the formation of tropical storms and tropical cyclones. They generally form near the Laccadive islands, west of the southernmost tip of India, and travel north-westward, with the potential for making landfall anywhere from Somalia to Pakistan. Curiously, tropical cyclones are relatively rare in this region in July, August and September, when the strong Somali jet induces upwelling of cold, deep water along the Somali and Omani coast, thus reducing the ocean water temperature, which, in turn, hampers the formation of tropical storms. Regionally, most tropical cyclones form in May and June, and in October and November. About one third of the storms forming in the Arabian Sea eventually reach the Sea of Oman, making landfall on average once every 3 years. These bring high seas and flooding rain that can easily exceed 200 mm for a single event (Membery 1998). The tropical cyclone Gonu, in June 2007, is so far the strongest tropical cyclone on record in the Arabian Sea. It reached the Sea of Oman, generating waves higher than 9 m, and causing extensive coastal damage in Oman, Iran and on the UAE east coast (Dibajnia et al. 2010). Such storms can cause significant damage to coral reefs, as occurred during Cyclone Gonu when over 50% of branching and table corals were lost from coral reefs on the Fujairah coast (Foster et al. 2011, ch. 11). However, they can also provide unexpected environmental benefits, acting as a significant source of rainfall to recharge groundwater aquifers by bringing precipitation in 24 h that can more than double the amount that typically falls in an entire year (Al Khatry and Helmi 2008; Abdalla and Al-Abri 2011).

4 Can We Glimpse UAE’s Future Climate?

The Eastern Mediterranean and Middle-East region is warming at a rate of 0.45 °C/decade (Zittis et al. 2022). This is 1.66 times higher than the global average (0.27 °C/decade), and slightly more than the average temperature increase on land between ±40° latitude, which is 1.5 higher than the global average (Sutton et al. 2007).

The warming trend is projected by the IPCC to continue with an end-of-century increase over the Gulf region of 1.3 °C from the present-day average in the optimistic SSP1-2.6 scenario, and an increase of 4.7 °C from the present-day average in the pessimistic SSP1-8.5 scenario (Gutierrez et al. 2021). Neither scenario should be considered as particularly likely to occur, and we’ll probably witness temperature increases intermediate between these two extremes (Hausfather and Peters 2020).

The impacts of increasing temperature are likely to be most extreme for ectothermic (cold-blooded) animals that have difficulty in maintaining core body temperatures. This is particularly true of marine fauna, which are unable to escape extreme water temperatures using behavioral strategies that can be used by animals on land (burrowing, shading, etc.; see Chaps. 14 and 16). Recurrent coral bleaching events are unfortunately already common on the UAE’s Gulf coast (Riegl et al. 2018), and have begun to occur on the east coast in recent years (J.Burt, unpubl. data, ch. 11). Currently, ectothermic Hawksbill turtles undertake migration to offshore environments to avoid extreme inshore summer temperatures (Marshall et al. 2020), therefore the projected increased temperatures represent a threat to these important thermal refugia.

For other parameters future trends are not so clear. Some studies find a slight (and weakly significant) decrease in wind speed over the past 40 years (Paparella et al. 2022), while others project a 10% increase by the end of the century (Feron et al. 2021).

Most uncertain of all is the future amount of rainfall. The most recent IPCC models project an increase of rainfall over the Arabian peninsula of 5% in the SSP1-2.6 scenario and of 25% in the SSP5-8.5 scenario. These are, however, multi-model averages. The IPCC models actually disagree among each other, and the individual models project rainfall changes ranging from −21% to +196% for the SSP5-8.5 scenario (−17% to +93% for the SSP1-2.6 scenario). This range is much wider than the natural variability of the climate system and signals an intrinsic inability of current climate models to produce reliable scenarios at regional and local scales, particularly in some locations, such as the Gulf region (Zittis et al. 2022). Future rainfall in the UAE will be determined by a complex interplay between shifts of the ITCZ, interactions of the large-scale atmospheric circulation with the Hajar mountains (Jing et al. 2020), and subtle feedbacks between uplift of dust and cloud formation (Francis et al. 2021). Most of these processes are poorly represented (or not represented at all) in global climate models, which calls for renewed efforts in regional-scale climate modeling.

5 Atlas of UAE Climatology: 1979–2021

Climate is defined by the World Meteorological Organization as the average meteorological conditions across a period of three decades or longer, allowing a smoothing out of short-term variability. We conclude this chapter with a series of maps of key climatic parameters for the period 1979–2021, as captured in the ERA5 reanalysis (Hersbach et al. 2020), in order to provide a comprehensive and current description of the national climate. The ERA5 is the most extensive collection of global meteorological and climate data currently available, assimilated in a state-of-the-art model of the Earth’s atmosphere, and released with a horizontal resolution of about 30 km. From the same dataset we also present the monthly statistics of temperature, wind and rainfall data at the location of 10 UAE cities.

Unsurprisingly for anyone who has spent time in the Emirates, there is great variation in temperature between seasons across the nation, but there is also significant variability across locations (Fig. 3.4). Daily and seasonal temperature fluctuations are larger in the desert interior of the UAE, and are less pronounced along the coastline where coastal waters can dampen temperature fluctuations (Fig. 3.4).

Fig. 3.4
4 maps of United Arab Emirates highlight average temperatures. Top panel. January. The average minimum and maximum temperature is between 11 and 16, and 22 and 25, respectively. Bottom panel. July. The average minimum and maximum temperature is between 29 and 31, and 39 and 45, respectively.

Average daily minimum temperature (left panels) and maximum temperature (right panels) in January (top panels) and July (bottom panels). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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In addition to seasonal temperature fluctuations, we are also experiencing a warming trend, presumably as a localized response to global climate change. The July maxima in the decade between 1979 and 1988 were about one degree cooler at any given location in the UAE than in the most recent decade of 2012 to 2021, except on the Musandam peninsula, where there has not been any significant trend (Fig. 3.5). While a 1 °C temperature increase in a three decade period may not sound significant to many people in the general public, this is faster than is happening on a global average (Zittis et al. 2022), and is rapidly approaching the 1.5 °C that is considered a critical threshold beyond which substantial impacts to ecosystems, human health and economies are projected to occur (Masson-Delmotte et al. 2018). The UAE is already experiencing recurrent marine heat waves that have substantially degraded its coral reefs, for example (see Chap. 11), and the increased temperature is presumably also enhancing evaporation on land, reducing water availability for terrestrial vegetation and exacerbating ongoing desertification (see Chap. 5). The consequences of even further ‘modest’ increases in temperature are dire, particularly given that many organisms in the UAE are already living at the physiological limits of temperature exposure.

Fig. 3.5
2 maps of United Arab Emirates highlight average maximum temperatures in degree Celsius. Left. July 1979 to 1988. The average maximum temperatures are between 36 and 44. Right. July 2012 to 2021. The average maximum temperatures are between 36 and 45.

Average daily maximum temperature in the decade 1979–1988 (left) and in the decade 2012–2021 (right). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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In addition to temperature, precipitation is one of the main factors structuring the occurrence and distribution of organisms in the UAE. Precipitation in the Emirates is extremely variable from year to year, with a few rainy years interspersed between dryer ones. It is highly seasonal, with virtually all rainfall occurring in winter and in early spring (Fig. 3.6). It varies spatially as well, with the majority of precipitation occurring in the vicinity of the Hajar mountain range on the eastern side of the nation Figs. 3.7–3.11. These spatial differences in rainfall, in combination with the local geology and geography, are a primary factor driving variation in vegetation assemblages around the Emirates (Chap. 5). While the absolute amount of rain varies between locations across the UAE, March is typically as rainy as the winter months in most locations (for the monthly statistics of rainfall at selected UAE locations, see below Figs. 3.12–3.21). Very little rainfall occurs from May to October, regardless of location (Fig. 3.6). This seasonal variation in rain drives the ebb-and-flow of annual plant life cycles, with most emerging following the spring rains to rapidly produce seeds and die off with the onset of summer (Chap. 13). Perennial plants, in contrast, have developed unique adaptations to help them persist through the extreme aridity of the summer months (e.g. drought tolerance, drought avoidance and succulence; see Chap. 13).

Fig. 3.6
4 maps of United Arab Emirates highlight precipitation in millimeter. December to February. The precipitation is between 5 and 30. March to May. The precipitation is between 5 and 15. June to August. September to November. The precipitation is at 5. December to May. The precipitation rises from south to northeast.

Maps of seasonal rainfall expressed as mean precipitation (mm) over 1 month. Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.7
2 maps of United Arab Emirates highlight winter average monthly precipitation in millimeter. December to February, 1979 to 1988. The precipitation is between 10 and 30. December to February, 2012 to 2021. The precipitation is between 5 and 25. The precipitation is lowest in west and south.

Winter average monthly precipitation in the decade 1979–1988 (left) and in the decade 2012–2021 (right). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.8
4 maps of United Arab Emirates highlight seasonal mean wind speed in meter per second. December to February. March to May. The speed is between 2 and 4.5. June to August. The speed is between 2 and 4. September to November. The speed is between 2 and 3.5.

Seasonal mean wind speed (m/s). Note that here and in Fig. 3.9 we report the average of the wind speed. This differs from Fig. 3.2, which shows the average of the wind vector (thus in Fig. 3.2 a low average wind may be the result of an alternation of winds blowing in opposite directions, while here low values indicate a persistent lack of wind). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.9
2 maps of United Arab Emirates highlight average summer wind speed in meter per second. June to August, 1979 to 1988. The wind speed is between 1.5 and 4. June to August, 2012 to 2021. The wind speed is between 1.5 and 4. The wind speed is high in the west and south, and low in the north and east.

Average summer wind speed (m/s) in the decade 1979–1988 (left) and in the decade 2012–2021 (right). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.10
4 maps of United Arab Emirates highlight average wet bulb temperature. Top panel. June and July. The average wet bulb temperatures are between 22 and 27, and 24 and 28, respectively. Bottom panel. August and September. The average wet bulb temperatures are between 25 and 29, and 23 and 28, respectively.

Monthly average of the daily maximum wet bulb temperature (centigrade) in June, July, August and September. Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.11
2 maps of United Arab Emirates highlight average of the daily maximum wet-bulb temperature. August, 1979 to 1988. The average of maximum wet-bulb temperatures are between 24 and 28. August, 2012 to 2021. The average of maximum wet-bulb temperatures are between 25 and 29.

Average of the daily maximum wet-bulb temperature in August in the decade 1979–1988 and in the decade 2012–2021. Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.12
6 graphs of Abu Dhabi. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 43 and 31 degree Celsius. Right panels. 4 rose charts of December to February, March to May, June to August, and September to November plot highest wind speed in the northwest.

Climatological statistics of Abu Dhabi. Top left panel: mean monthly maximum and minimum temperature. Bottom left panel: statistics of monthly precipitation. Right panels: seasonal wind roses showing the direction from which the wind blows; the width of the colored bars is proportional to the likelihood of occurrence of a given wind speed (in m/s, as given by the central legend). Data are from the ERA5 reanalysis, copyright 2023 European Centre for Medium-Range Weather Forecasts (ECMWF); this data is published under a Creative Commons Attribution 4.0 International (CC BY 4.0)

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Fig. 3.13
6 graphs of Ajman. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 40 and 31 degree Celsius. Right panels. 4 rose charts. December to February, March to May plot highest wind speed in the north, June to August, September to November in southeast.

Climatological statistics of Ajman. See legend of Fig. 3.12

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Fig. 3.14
6 graphs of Al Ain. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 44 and 31 degree Celsius. Right. 4 rose charts. December to February, March to May, and September to November plots highest wind speed in the northwest, June to August in southeast.

Climatological statistics of Al Ain. See legend of Fig. 3.12

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Fig. 3.15
6 graphs of Al Ruwais. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 42 and 29 degree Celsius. Right panels. 4 rose charts. December to February, March to May, June to August, and September to November plots highest wind speed in the northwest.

Climatological statistics of Al Ruwais. See legend of Fig. 3.12

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Fig. 3.16
6 graphs of Dubai. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 41 and 31 degree Celsius. Right. 4 rose charts. December to February, March to May plots highest wind speed in the north, June to August in southeast, and September to November in south.

Climatological statistics of Dubai. See legend of Fig. 3.12

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Fig. 3.17
6 graphs of Fujairah. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 44 and 31 degree Celsius. Right. 4 rose charts. December to February, March to May plots highest wind speed in the north, June to August in southeast, and September to November in north.

Climatological statistics of Fujairah. See legend of Fig. 3.12

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Fig. 3.18
6 graphs of Liwa. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 44 and 29 degree Celsius. Right. 4 rose charts. December to February, March to May plot highest wind speed in the northwest, June to August, September to November in west.

Climatological statistics of Liwa. See legend of Fig. 3.12

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Fig. 3.19
6 graphs of Ras Al Kaimah. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 37 and 32 degree Celsius. Right. 4 rose charts. December to February, September to November plot highest wind speed in the north, March to May in northeast, June to August in southeast.

Climatological statistics of Ras Al Kaimah. See legend of Fig. 3.12

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Fig. 3.20
6 graphs of Sharjah. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 41 and 31 degree Celsius. Right. 4 rose charts. December to February, March to May plot highest wind speed in the north, June to August, September to November in southeast.

Climatological statistics of Sharjah. See legend of Fig. 3.12

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Fig. 3.21
6 graphs of Umm Al Quwain. Top left. The lines of mean monthly maximum and minimum temperature plot bell curves with peaks at 39 and 32 degree Celsius. Right. 4 rose charts. December to February, March to May plot highest wind speed in the north, June to August, September to November in southeast.

Climatological statistics of Umm Al Quwain. See legend of Fig. 3.12

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The long-term comparison of precipitation in the decade between 1979–1988 versus the more recent decade between 2012–2021 is disheartening: in most of the UAE the amount of rainfall in the winter months has more than halved (Fig. 3.7). On the Hajar mountains and on the Musandam peninsula the rainfall decline has been less extreme, but even there a decrease is quite evident. The biological implications of such radical reductions of an already limited water supply cannot be understated. Most arid region vegetation exists in a fairly fragile balance with local water supply, and such reductions can result in dramatic reductions in vegetation cover, restructuring of community composition, and exacerbation of desertification, potentially causing phase-shifts that lead to degraded stable states where recovery is near impossible. This is particularly important for highly localized and unique vegetation communities as they are unlikely to be able to migrate to other areas due to highly restrictive habitat requirements (see Chap. 5). As this vegetation also serves as food, shelter and habitat for the remainder of the food web, the knock-on implications of such substantial reductions in precipitation since the 1980s to the biodiversity of the UAE are alarming.

Although the occurrence of seasonally extreme winds such as winter ‘shamal’ events and early summer ‘cyclones’ would hint at a high degree of seasonality in the wind climate of the UAE, this is not the case. As shown in Fig. 3.8, the average wind speed is only slightly higher during the spring, and just slightly lower during the autumn, compared to the year as a whole. This is because the extreme events, such as a shamal-driven storm, are typically very short in duration (ca. 3 days), and occur only occasionally, with the wind climate much more typically characterized by near-calm or breeze conditions, resulting in the limited seasonal variability observed in Fig. 3.8. In terms of spatial differences, the complicated topography of the Hajar mountains and of the Musandam peninsula acts to reduce the surface wind speed in that area. Winds are substantially higher in the more unobstructed Empty Quarter, where winds are typically more than double that occurring in the northern Hajar mountains (Fig. 3.8). No appreciable changes in average wind speed have occurred in the last 40 years at any location in the UAE (Fig. 3.9).

Wet-bulb temperature is the temperature of a thermometer whose heat-sensing device is wrapped in a water-soaked cloth and exposed to strong wind. As a result of the evaporative cooling, the wet-bulb temperature is always lower than the actual air temperature. The difference between the two temperatures is a measure of the humidity content of the air. For the same air temperature, dry conditions yield a lower wet-bulb temperature than when the air is very humid. This seemingly arcane quantity is of great usefulness for quantifying the discomfort experienced by humans exposed to a hot environment. The more the wet bulb temperature increases, the less a human body is capable of losing heat by sweating. The physiological limit of the wet bulb temperature for humans is considered to be 35 °C. An environment where the wet bulb temperature is above that threshold is considered life-threatening.

Recently it has been suggested that some regions near the Arabian Sea may experience wet-bulb temperatures close to 35 °C as a result of climate change (Pal and Eltahir 2016; Raymond et al. 2020). Our analyses show that in the UAE the wet-bulb temperature has historically been substantially higher along the coastline than in the dryer interior, reflecting the strong influence of evaporation from the Arabian Gulf and Gulf of Oman for coastal humidity. In Abu Dhabi, during August, it is on average higher than 29 °C (Fig. 3.10). While this is still sufficiently below the physiological threshold to avoid human health alarm, the trend does not bode well: in the last 40 years the wet bulb temperature in the UAE coastal regions has increased by one degree (Fig. 3.11), suggesting that the dire human health impact projections of Pal and Eltahir (2016) are reasonable. What implications this has for the native fauna (or flora) of the UAE remains largely unknown, although heat stress impacts on domesticated livestock appear to be a significant cause for concern (Carnovale and Phillips 2020), and are likely equally an issue for natural populations.

6 Conclusions

In summary, the climate of the UAE is a highly peculiar one. It is a harsh desert, but it is strongly affected by the Indian monsoon, which in a past age made this a green land. It lies on the southern shores of an internal sea, the Arabian Gulf, that channels the dominant winds and through which it feels the influence of the Mediterranean Levant. The large-scale dynamics that broadly determine UAE’s climate are adjusted and corrected by local features, mainly the presence of the Hajar Mountains, which trigger most of the scarce rainfall descending on this land. Along with the underlying geology, these climate features are a primary features driving vegetation patterns across the Emirates, and indirectly structuring the animal food web upon which they subsist.

In spite of its interesting features, many details of the UAE climate are still poorly understood. The mutual interaction between dust and rainfall, how the UAE climate will evolve in a globally warming world, and the feed-backs of local climate changes to the UAE ecosystems are examples of topics where more research is needed.

7 Recommended Readings

A good introduction to the meteorology and climatology of deserts and arid regions is given by Warner (2009). A technical, but still accessible, review of the climate of the Middle East, with a special focus on the projected climate change may be found in Zittis et al. (2022). Pal and Eltahir (2016) focus on the overlooked threat constituted by the combination of rising temperatures and high humidity, which may lead to unbearable outdoor conditions. Finally, deMenocal et al. (2000) and Lüning and Vahrenholt (2019) give detailed reviews on how climate shaped the civilizations of the earliest inhabitants of the Arabian Peninsula.