Current Research on Geography, Earth Science and Environment Vol. 8

The mixed layer depth (MLD) is a critical feature of the upper ocean that controls physical heat transfer and marine biological processes. This chapter presents a detailed investigation of the monthly climatology of MLD in the Gulf of Aden and reveals a strong seasonal cycle across the basin. During winter (December to February), severe surface cooling drives deep convective mixing and creates a deeply mixed upper ocean, particularly in the western and northern regions. In spring, rapid surface warming substantially increases the static stability of the water column. Consequently, the mixed layer shoals rapidly and remains exceptionally shallow throughout summer (June to August). Although the Gulf experiences net heat loss in summer because of strong evaporative winds, the calculated energy required to break the existing stratification is too high to permit deep vertical mixing. The basin also exhibits a sharp north-south MLD asymmetry that reverses between winter and summer. Furthermore, a uniquely deep mixed-layer core forms in the central Gulf during summer due to the persistent presence of mesoscale ocean eddies. Long-term trends are evaluated using ocean model reanalysis data spanning from 1993 to the present. A clear annual deepening trend is observed across the entire basin, with the fastest rate of 0.19 m/y occurring in the western Gulf. This long-term deepening is driven mainly by winter mixing, with rates reaching 0.53 m/y in the west, whereas summer trends remain weak. This climatology and trend analysis provide a baseline for future ocean and climate research in this dynamic marginal sea.