JKAU: Mar. Sci., Vol. 31 No. 2 pp: 31- 43 (1442 A.H. / 2021 A.D.) DOI :10.4197/Mar.31-2.3 31 Factors Controlling Seasonal Variability of Sea Level in the Western Gulf of Aden Abdullah M. Al-Subhi Department of Marine Physics, Faculty of Marine Sciences, King Abdulaziz University, B.O. Box 80207 Jeddah 21589, Saudi Arabia amalsubhi@kau.edu.sa Abstract. Daily means sea level has been computed from hourly observations at Aden and Djibouti for the period of 2011–2014 to study seasonal variability in sea level in the western Gulf of Aden. This variability is investigated against the following controlling factors: sea level pressure, wind, and steric sea level. Variability in sea level at Aden and Djibouti shows strong seasonality with higher sea level in winter and lower sea level in summer with a range up to about 35 cm. The high-frequency variability in sea level agrees with variability in pressure, especially during winter following normal inverse barometric relation. Cross-shore wind affects sea level variability in Djibouti more than that in Aden, while along-shore wind plays a significant role in sea level variability at Aden. Daily steric sea level for both stations play a significant role in seasonal sea level anomaly (SLA) variability. Both stations shows that strong signals are the annual and semiannual, while small frequencies are negligible in comparison. For Aden and Djibouti stations, steric sea level is the dominant factor with a determination coefficient (DC) of 0.73, and 0.71, respectively. Along-shore wind has the second higher contribution for Aden with (DC) of about 0.3. The cross-shore wind component has the second highest contribution on Djibouti SLA with a DC of about 0.3, while for Aden, this factor has no effect on SLA. Sea level pressure (SLP) contribution is clearly seen in short-period variability for both stations. Keywords: Gulf of Aden, Mean Sea Level, Steric Sea Level, Wavelet, Atmospheric forcing. 1. Introduction The Gulf of Aden is a strategic water body that connects the Red Sea with the Indian Ocean, and it is distinguished by the complexity and variability of its climate and oceanography. As part of the northwestern Indian Ocean, it falls under the control of the monsoon reversal wind regime, which contributes to the formation of various water circulation patterns such as water exchange with the Red Sea and the Arabian Sea, eddy formation, and coastal upwelling. In the winter NE monsoon, winds push the gulf’s surface water westward along the axis of the gulf and continue to flow NNW into the Red Sea through the strait of Bab-el- Mandab. During the summer SW monsoon, winds are relatively stronger and reach 20–40 km/h (Sultan and Ahmed, 1997). The surface water of the gulf is pushed eastward under the effect of the SW wind along the Arabian coast and deflected SE; this type of steering causes some eddies to form along the center of the gulf (Alsaafani et al., 2007). Other eddies propagate to the gulf from the Arabian Sea (Alsaafani et al., 2007; Fratantoni, et al., 2006). Due to these eddies, some weak and scattered upwellings occur in a few spots along the Somali coast. Along the Arabian coast, strong upwelling occurs due to the Ekman transport, which pushes surface water offshore and is replaced with subsurface waters; in this season, upwelled waters come from much greater depth (Currie et al., 1973). The Gulf of Aden experiences strong evaporation; Ahmad and Sultan (1989)