1- General features of AT, RH and VW (1961-1990 and 1991-2020)
Figure 1 shows the mean values of summertime air temperature (AT; contour lines), relative humidity (RH; shadings) and vector wind (VW; arrows) for the period of 1961-1990. The highest values of seasonal air temperature (greater than 30°C) are seen over more populous parts of the study area including Iran, Saudi Arabia, Iraq, Kuwait, United Arab Emirate, Bahrain, Qatar, Sudan, Egypt, Turkmenistan and Afghanistan. These areas that mostly locate between the subtropical-high pressure belt (25°-35° N), can be characterized as the region with the highest values of AT and lowest amounts of RH (contour lines and yellow shading in Figure 1, respectively). For most parts of the study area, maximum values of RH are harmonized with low amounts of air temperature. However, the low values of AT and the near saturation humidity are dominant in some high latitude areas of ME such as the western coasts of the Black Sea in Georgia and northeast of Turkey. For the marine environments and coastal areas, regardless of air temperature, RH is generally near 100% (Figure 1).
According to Figure 1, southern parts of Europe and the Mediterranean Sea are the main sources of airflows that blow over the ME and North Africa. These northwesterly airflows could be considered as the foremost respiratory system or nose of these geographical expanses. If the ME is considered as a room, its fresh air is supplied by these northwesterly circulations. In their over-sea travel, these westerly winds are divided into two distinct parts, namely southern and northern flanks. While the southern flank transfers (drain) moisture into the northern parts of Egypt, Libya and Saudi Arabia (the southern coasts of Turkey), the northern flank propagates eastward or northeastward. The southern airflow becomes warmer and dryer as travel toward the lower latitudes of Africa and Saudi Arabia. As indicated in Figure 1, the main parts of Sudan are the ultimate destination of the warm and dry air-flows that are passed over Egypt and Saudi Arabia.
Although blowing the southern flank enhances humidity over North Africa and Saudi Arabia, this enhancement is highlighted only over the southern coasts of the Mediterranean Sea (blue shadings in southern parts of the Sea in Figure 1). Compared to coastal regions of Egypt and Libya, the magnitudes of RH are less in Turkey's southern coasts; where wind has the land to Sea direction.
Those geographical expanses that are affected by the northerly flank include some areas in Syria, Jordan, Iraq, north of Iran, the Caspian Sea, Azerbaijan, Armenian, Georgia, Turkmenistan, the eastern half of Iran and western half of Afghanistan. In contrast to the southerly flank which is generally blowing southward, the direction of northerly flank changes to southwesterly or southerly over some areas including east of Turkey, northwest of Iran, the Caspian Sea, Azerbaijan, Georgia and Armenia (Figure 1). This northward circulation generally increases RH and decrease AT over its passages.
As is indicated in Figure 1, a part of the Caspian Sea-based southerly wind changes its direction toward Turkmenistan to blows as the westerly wind. These westerly airflows turn into the northerly wind in the middle of the road to enter Iran and Afghanistan (Figure 1). While a part of this northerly circulation reaches the Indian Ocean in southeastern parts of Iran, the other part blows toward tropical areas of Africa via the Persian Gulf, Oman Sea and the Arabian Peninsula. Therefore, the Oman Sea in the northwestern parts of the Indian Ocean and tropical parts of North Africa are the ultimate destinations of the northern flank of the Mediterranean-based circulations (Figure 1).
The magnitudes of AT and RH are mostly modulated by the velocity and direction of the mentioned atmospheric circulations. For instance, for eastern half of Iran, the magnitude of these variables depends on the strength of northerly winds that come from Turkmenistan. Moreover, the amount of these variables over Eritrea and Sudan are mostly controlled by the characteristics of the warm and dry winds that come from Saudi Arabia.
Figure 2 is similar to Figure 1 except that the former is related to the period of 1991 to 2020. Although the geographical distributions of AT, RH and VW are similar in these two Figures, the magnitudes of AT (RH) have predominantly increased (decreased) during the second 30 years period. For instance, for some areas such as western parts of Afghanistan and Pakistan, decrease in relative humidity is obvious for the second period. The easterly and northerly (westerly and southerly) winds are also generally stronger for the second (first) 30 years period.
2- Differential values
For demonstrating the signals of climate change more clearly, differential values between Figures 1 and 2 are depicted in Figure 3.The positive (negative) values of isotherms imply that weather is warmer (colder) during 1991-2020 as compared to its earlier period (1961-1990). Since all the isotherm's labels are either positive or zero, it can be concluded that warming is the dominant feature of the Middle Eastern climate (contour lines in Figure 3). According to the presented statistics in Table 1 (first line), compared to the first 30 years period, for the whole of Middle East, AT (RH) has been warmed up (dried) by 0.83oC (1.93%) in the second period. The highest values of warming associate with some areas including northeastern parts of Iran and their nearby areas in Afghanistan and Turkmenistan, the south Cuscuses regions, western areas of Saudi Arabia along the Red Sea, the central parts of Turkey, north part of Egypt, south of Sudan and eastern parts of Eritrea (Table 1).
Except for a small area of ME that is denoted as blue shading in Figure 3, RH is either unchanged or decreased over the other areas. The magnitudes of this decrease vary from about 1% in central south of Iran to around 10% in some parts of Sudan and Eritrea; almost along the latitude 15o N. The decrease (increase) in RH (AT) is also remarkable in the high latitude areas of the ME such as the northwestern and northeastern districts of Iran, most of Caucasus region and the eastern parts of Turkey (Figure 3 and Table 1). The regions with increase in the RH data include a small area in the southeast of Iran, total territory of Oman, northern parts of Yemen, southwestern coasts of Pakistan, Cyprus and a part of Turkey's southern coasts. Climate change is also particularly investigated over a part of Oman (region H in Figure 4).
For the central and northeastern parts of Iran; where temperature rise is one of the highest in the Middle East, the moisturized westerly (dry easterly) winds during the recent period are weaker (stronger) than that of the first period (Figure 3). Similarly, compared to the period of 1961-1990, blowing the dry northerly winds is generally stronger over Turkmenistan and the northeastern parts of Iran for the recent 30 years. These southward wind increases air temperature over the northeastern parts of Iran and central districts of Turkmenistan. Also, the observed temperature rise in the western parts of Saudi Arabia and Cuscuses region mostly associates with the strength in the dry northerly winds over these areas during the recent period. Comparing Figures 1, 2 and 3 implies that the southerly winds over the Caspian Sea are decreasing. This reduction partially justifies the observed decrease in the RH data over the neighboring countries such as Turkmenistan and Azerbaijan.
In contrast to other regions, for southeastern corner of the ME that includes Oman, the northern parts of Yemen and southeastern extremity of Iran, the northerly winds have been recently either weakened or replaced by the moisturized southerly and easterly winds that lessen the adverse consequences of climate change over these regions (blue color in Figure 3 and region H in Table 1). However, the situation is slightly different for Cyprus and southeastern coasts of Turkey for which a marginal-improvement in the RH data coincides with a considerable increase in the AT. This improvement is mostly due to the recent weakness (strength) in the northerly (southerly) winds.
The suppressed northerly and westerly wind over the Mediterranean Sea decreases cold advection from the high latitude areas of Europe and the Atlantic Ocean into the ME during recent periods. This suppression increases (decreases) AT (RH) over this area by reducing the volume of the circulated air.
Table 2 summarizes the results of the Kolmogorov–Smirnov (K-S) test that were conducted to assess significant changes in AT, RH, UW, and VW between the first and second 30-year periods. Besides the whole of ME, the results are also shown for ten sub-regions of this expanse. According to the given statistics, in addition to the whole of ME, increase in air temperature is also significant for the sub-regions B, E and J. Although decrease in RH is almost overwhelming, it is only significant for region B and c. The significant change in wind data associates with Turkey, west of Turkey, north of Egypt and the East Mediterranean Sea (regions F, G, I and J, respectively). For these areas, northward (southward) component of the vector wind is enhanced (suppressed) during the recent 30 years period. These changes lead to the improvement (suppression) of wetness over southwestern parts of Turkey (northern parts of Egypt and eastern parts of the Mediterranean Sea).
3- Decadal analysis
Figures 5, 6 and 7 are, correspondingly, similar to Figures 1, 2 and 3 except those three formers are related to the first and last decades of the study period; namely 1961-1970 and 2011-2020. In other words, instead of two consecutive 30-year periods, the data are analyzed for two discrete decades. Comparison between Figures 3 and 7 and Tables 2 and 3 implies that differences are more significant if instead of 30 years, changes are evaluated between the first and last decades. For instance, while maximum difference in AT data over the Iran-Turkmenistan regions is about 1.5oC in Figure 3, it varies between 2.0oC and 3.0oC in Figure 7. Similarly, for Sudan, air temperature in Figure 7 is nearly twice of that in Figure 3. For some parts of Sudan and Eritrea that locate around 15o N, AT has increased from 3oC to 4.0oC which is the highest value over the study area. Consistent with the increase in AT, decrease in RH data is more evident in Figure 7 as compared to Figure 3. Tropical parts of Africa, east of Turkey, Azerbaijan, most of Afghanistan and northern parts of Egypt are the areas that are subjected to the greatest decrease in RH (Figure 7). The decrease is acute for the mentioned areas in Sudan and Eritrea; where the RH reduction reaches to about 27%. These areas are upstream of the Nile River; the main water resources of Egypt. Unfortunately, the second sharpest decrease in RH data lies over the central and eastern parts of Turkey; where the Euphrates and Tigris Rivers are originated. The mountainous area in the western and northwestern parts of Iran that is the origin of Iran's main rivers have also been subjected to about 1.5oC to 2.0oC increase in AT and up to 5% decrease in the RH data.
Comparing Figures 3 and 7 and the presented statistics in Table 1 to 3 suggest that, compared to some areas such as Sudan or east of Turkey, Iran's relative humidity particularly over its eastern half tends to be stable. This stability could mostly be attributed to the pouring of the Indian Ocean moisture into Iran via the northeastern districts. Besides eastern districts, a white patch in the southeastern coasts of the Caspian Sea (Figure 7) implies that, due to the recent enhancement in the over-sea northerly winds, RH data have also slightly improved over this region. Noshadi and Ahani (2015) have also reported a positive trend in the RH data of this area. The presented statistics in Table 3 imply that for some areas such as west of Turkey, north of Egypt, east of the Mediterranean Sea, Oman (Azerbaijan, Armenia, the Caspian Sea, the central parts of Turkmenistan, most of Iran, Afghanistan, Saudi Arabia and Sudan). The northward (southward) component of the vector-wind has recently enhanced.
Comparison between Figures 3 and 7 and the provided statistics in Tables 1 to 3 supports our previous discussion about improvement or stability in the RH or AT data over Oman, some parts of Yemen, southwestern coasts of Pakistan and the southeastern corner of Iran, respectively. While air temperature over these areas has increased from 0.5 to 1.0oC during the last six decades, RH data are improving over time. This improvement is mostly associates with the enhanced southeasterly wind over the northwestern parts of the Indian Ocean which improves the natural ventilation system of the ME. In the other words, the enhanced southeasterly winds over this part of the Indian Ocean have been marginally compensated the adverse effects of the suppressed northwesterly winds over the Mediterranean Sea. These findings are consistent with one of the latest IPCC's reports that anticipates increase precipitation over land in most of East Asia at the end of the 21st century due to the strengthened summer monsoon circulation (IPCC 2021). For Cyprus and the southeastern coasts of Turkey AT or RH has increased by about 2.5oC or 4%, respectively.
The increase in the RH data is mostly due to the recent amplification (weakness) in the southerly and easterly (northerly and westerly) winds over the Mediterranean Sea (Figure 7). As is discussed earlier, the weakening of northwesterly winds over the Mediterranean Sea straitens the respiratory system of the ME and increases its warming. Northward component of the vector-wind has also strengthened in west of Turkey and north of Egypt.
4- 5 years analysis
For consolidating the obtained results, significant differences in the considered variables were investigated between the first and last five years of the study period (1961-1965 and 2016-2020). The spatial distribution of AT, RH and WV (not shown) were mostly found to be similar to those shown in Figures 5 and 6. Figure 8 and Table 4 are, correspondingly, similar to Figure 7 and Table 3 except those formers are related to these two five years analyses. The number of significant results in Table 4 as well as differential values in Figure 8 are greater than corresponding values in Table 3 and Figure 7, respectively. For example, the given information in Figure 8 suggests that, for some parts of Sudan and western sides of Eritrea, the difference in AT (RH) escalates to 5.0ºC (30%) which are greater than the corresponding values in Figures 3 or 8. These two statistics are, respectively, about 2.5ºC and 13% for the middle and eastern parts of Turkey; where the Euphrates and Tigris Rives are originated (Figure 8 and Table 4). Meanwhile, the increase (decrease) in AT (RH) data for the Caucasus region and northwestern parts of Iran varies from 1.5ºC in south to 3ºC in the north (5% to 13%). Besides these, differences values in the AT data are worrying over some areas including northern parts of Egypt (3.0ºC), the middle parts of Turkmenistan, (3.0ºC) western side of Saudi Arabia (2.5ºC) and northeastern parts of Iran (2.5 ºC).
For some areas mostly situated in the southeastern parts of the ME, differential values in the AT (RH) data have been almost stabilized (increased) over time. For instance, increase in temperature (RH) data of Oman, north of Yemen and the southeastern coasts of Iran varies from zero to about 0.5oC (2% to 8%). Although difference in the AT data suggests a considerable warming over some areas including United Arab Emirate, Qatar, the eastern parts of Saudi Arabia and central south of Iran, RH data exhibit a marginal improvement over these areas. This improvement could be mostly attributed to the the enhanced southerly wind over these parts of the ME.
A comparison between Figures 3, 7 and 8 suggests that, for most parts of Iran, RH data are improving during the recent decade. For instance, the areas that are shown by white color (yellow color) in Figure 3 have been substantially increased (decreased) in Figures 7 and 8 suggesting that RH is improving over the country. Besides this, the regions with a sharp reduction in the RH data are getting distance from Iran's territory. As an example, according to the given information in Figure 3, the northwestern parts of Iran and most of the Caucasus region, have experienced a sharp reduction in the RH data during the recent 30-year periods (brown and yellow colors in Figure 3 for the latitudes above 35o N). However, as indicated in Figures 7 and 8, the areas with such characteristics have been gradually moved toward the west or north, to be centered over eastern and central parts of Turkey or northern parts of the Caucasus region.
The increase in RH data over a considerable part of Iran and the Arabian Peninsula could mostly be attributed to the strengthened southeasterly and easterly circulations over the northwestern parts of the Indian Ocean which stabilize AT and increase RH over these parts of the ME. Overall, weakness in the northwesterly winds over the Mediterranean Sea is partially compensated by the strengthened southeasterly airflows over the northwestern parts of the Indian Ocean. Furthermore, climate condition in the northern parts of Iran is also influenced by the atmospheric circulation above the Caspian Sea. As an example, compared to Figures 3 and 7, Figure 8 shows less difference in the air temperature and humidity over the southern parts of the Sea. Therefore, for this part of Iran, the magnitudes of AT and RH during 2016-2020 are similar to the corresponding values during 1961-1965. This stability in climate condition is partially due to the recent weakness (strength) in the southerly (northerly) winds over the Caspian Sea.
Comparison between Figures 3, 7 and 8 infers that, for most parts of the Black Sea, Turkey and the East Mediterranean Sea, the westerly and northerly (easterly and southerly) winds have been recently weakened (enhanced). Moreover, wind direction over the eastern coasts of the Black Sea is mostly northwesterly or (westerly) during the first (resent) periods, respectively. This means that moisture transport from the Black Sea into Turkey is decreasing. Blowing the dry southeasterly winds over Turkey are increasing. These winds that come from northern parts of Iraq and Syria induce dryness over central to eastern parts of this country. Although, moisture transport has been increased from the Mediterranean Sea into the southern coasts of Turkey with, RH data are decreasing over most parts of the country and particularly its eastern half.