Description :This study investigates the climatic dynamics and meteorological variability of a coastal zone located in an arid environment and strongly influenced by the African monsoon and large-scale atmospheric circulations from the Indian and Pacific Oceans. Climate reanalysis datasets from ERA5 (ECMWF/C3S), CFSR (NCEP), and MERRA2 (NASA/GMAO) are employed to characterize long-term trends and climate extremes over the period 2000–2021. The analysis focuses on multiple parameters, including precipitation, temperature, humidity, wind, evapotranspiration, cloud cover, and sea level rise, with a spatio-temporal resolution tailored to strategic coastal environments.
The findings reveal pronounced interannual irregularities in precipitation, a significant intensification of hydrometeorological extremes (droughts and flash floods), contrasting seasonal wind regimes associated with monsoonal dynamics and the Khamsin phenomenon, as well as a progressive rise in sea level consistent with broader trends observed in the Indian Ocean. These insights contribute to a better understanding of regional climate risks and provide critical knowledge to support the resilience of coastal and port infrastructures in the area.
Umbrothermal diagram showing the monthly variation of precipitation (mm) and temperature (°C) from 2017 to 2021 of Djibouti airport station is presented in Figure 1.
The study area is marked by low rainfall. The variation in precipitation from 2017 to 2021 shows an irregular pattern. For example, the cumulative rainfall in 2019 was 427 mm. It is almost 3 times that of 2018 (138 mm), which in turn is almost 2 times that of 2017 (59mm). The total monthly rainfall in last year, recorded by the meteorological station at Djibouti-Ambouli airport, was 143.6 mm (Figure 1). The interannual variation (in-line curve) shows an irregular rainfall pattern with occasional heavy downpours that can cause flooding. In addition, the study area receives most of its rainfall in two seasons, March-May (also called long rains) and October-December (also called short rains). In particular, two consecutive years (2020 and 2021) show that April is wettest than the other months of the year.
Figure 1. Umbrothermal diagram showing monthly variation in rainfall (mm) and temperature (°C) from 2017 to 2021 for the Djibouti airport station which is the closest to the study area.
As far as temperature (°C) is concerned, the study area shows a more regular annual variation from 2017 to the 2021. The average temperature rises during the season from April to September (warm season) and falls from October to March (cool season). The peak is usually reached in July. The 2017 hot season was the warmest with an average temperature of 35.51°C. Similarly, the coolest season, with an average temperature of 27.35°C, was observed in the same year. Overall, it can be seen that precipitation is more abundant during the cool season than during the hot season (Figure 1).
Due to the unavailability of relative humidity data for the airport station, we use simulated data from ERA5. The yearly variation in relative humidity (%) from 2017 to 2021 is presented in Figure 2. Over the study period, the interannual variation in relative humidity follows a regular pattern from year to year. In particular, we can see in the figure 2, that the relative humidity is high in 2019 during the months of October, November and December with 77.79%, 78.68% and 81.70% respectively. During this season, the Djibouti airport station recorded a cumulative rainfall of 401mm, which caused human and material damage. In addition, in April 2020, the relative humidity reached 80.37% (figure 2). This coincides with a rainfall of 80mm with data from the airport station. On the other hand, the relative humidity is below 70% from June to September, which corresponds to a season of low rainfall with an increase in average temperature (Figures 1 et 2).
Figure 2. Monthly variation in relative humidity (%) between 2017 and 2021 in the study area.
Due to the unavailability of evapotranspiration data for the airport station, we use simulated data from ERA5. The yearly variation of potential evapotranspiration obtained with the Hargreaves method is presented in Figure 3. There is an almost regular annual variation from 2017 to 2021. During the warm season, the monthly evapotranspiration exceeds 90 mm. This indicates that the increase in temperature influences the evapotranspiration potential and leads to its highest value. On the other hand, during the cool season, which is also the season of dense rainfall, the evapotranspiration is at its lowest level of the year (Figure 3).
Figure 3. Monthly variation in potential evapotranspiration (mm) between 2017 and 2021 for the study area.
Due to the unavailability of cloud cover data for the airport station, we use simulated data from ERA5. The yearly variation in cloud cover over the study area during the period 2017 to 2021 is presented in Figure 4. An irregular pattern can be observed from year to year. In particular, during the months of February, March, April, July and November, the cloud cover exceeds 50% over the study area. Furthermore, three peaks (exceeding 55%) can be observed, namely in February 2017, November 2019 and April 2020. During these periods, heavy rainfall of 12 mm, 338 mm and 80 mm was recorded at the airport station. However, this is not always the case, for example, in February 2018, the airport station recorded no rainfall while the cloud cover was 52.48%.
Figure 4. Monthly variation in cloud cover between 2017 and 2021 for the study area.
Due to the unavailability of wind speed data for the airport station, we use simulated data from ERA5. Figure 5 shows the average variation in monthly wind speed occurred between 2017 and 2021. It can be seen that the wind speeds exceed 4 m/s in February, July and August. These are the windiest months of the year. In contrast, the wind speed is lower during the months of May, June, September and October.
Figure 5. Monthly variation of average wind speeds between 2017 and 2021 for the study area.
Due to the influence of the African monsoon, the wind comes from the east (maritime wind) during the month of September to May and from the west and southwest (Khamsin wind) between June and August. The wind rose of the study area at an altitude of 10 meters is presented in figure 6a. During the period 2007 to 2021, wind speeds are dominant in the east, northeast and southeast directions (from 45° to 135°) with a frequency of 15 to 25%. The highest wind speed class (which is also the least frequent) in this area is between 6 to 7 m/s. In contrast, to the north and south of the study area, the wind speed frequencies are less than 10% and vary between 1 and 7 m/s. In addition, the west and south-west direction (225° to 290°C) represents the area where wind speeds exceed 8 m/s. Overall, this region is particularly marked by the highest wind speeds in the study area, but with a frequency of less than 10%. On the other hand, it can be seen that hourly wind speeds above 5m/s were particularly active in 2020 and 2021. It can be said that this is the windiest year. Moreover, between 10 am and 3 pm, winds reach speeds above 3m/s between 2017 and 2021. In contrat, from 5 pm to 5 am, the wind speeds can be seen below than 3 m/s.
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Figure 6. (a) Wind rose diagram and (b, c) Hourly and daily variation of wind speeds (m/s) for the year 2021.
Figure 7 shows the average annual change in global and Indian Ocean sea level rise over the period 1992 to 2021. During this period, global sea level has risen significantly by 3,013 mm/year. This trend is slightly higher in the Indian Ocean, which shows a sea level rise rate of about 3,115 mm/year. According to the IPCC climate panel prediction scenarios, with the RCP8.5 model, the global mean sea level rise in the years 2046 to 2065 is likely to be in the range of 0.22 to 0.38 m and for the years 2081 to 2100 it would be in the range of 0.45 to 0.82 m. These projected increases in sea level will not affect the current project in the sense that the Damerjogue oil terminal project is at an elevation of more than 3 meters. However, a possible sunnami cannot be excluded.
Figure 7. Change in sea level rise from 1992 to 2021.
Figure S1. Monthly variation of specific humidity (g/kg) and evapotranspiration (mm) during the year 2021.
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Figure S2. Seasonal distribution of wind directions and frequencies (%) from January to December (2017–2021).
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