SEASONAL AND INTERANNUAL VARIABILITY OF MIXED LAYER DYNAMICS IN THE ARABIAN GULF AND ITS IMPLICATIONS ON NUTRIENTS AND CHLOROPHYLL-A

Abstract

This study employs the climatological analysis of winds, currents, sea water temperature, and mixed layer depth (MLD) for the period 1993-2022 utilizing reanalysis data retrieved from Copernicus Marine Services (CMEMS). In addition, the hydrographic parameters (temperature, salinity, density, dissolved oxygen (DO), and pH), chlorophyll-a, and nutrients off the east coast of Qatar have been investigated using the measured data from 6 stations during November 2024. Sea surface temperature (SST) climatology indicates that the highest monthly mean temperature was estimated in August (35°C) and the lowest in February (15°C). The highest monthly mean wind speed was observed during February (~7m/s), followed by January (6.5 m/s) whereas the lowest wind speed was observed in August (3-4 m/s). Climatological analysis revealed distinct seasonal patterns in MLD, with the deepest mixed layer occurring in winter (41.4 m) and the lowest depth in summer (15.3 m). MLD variability is primarily driven by wind-induced mixing and surface cooling in winter, while increased temperature enhances stratification in summer. Measured hydrographic data revealed vertical homogeneity in far offshore regions (stations A3 and B3), while thermocline was observed in the nearshore regions. The presence of a halocline at station B3, at 20 m depth, indicates the existence of two water masses. The horizontal variability of the salinity range was <1, while density variability is confined to 1.0 kg/m³. DO is within the healthy limits, while nutrient concentrations varied: nitrite (0.03–0.66 μM), nitrate (0.02–1.82 μM), phosphate (0.17–0.54 μM), ammonia (0.04–1.56 μM), and silicate (1.78–7.67 μM). Validation of reanalysis data against in situ measurements showed a temperature bias of -0.86°C, Root Mean Square Error (RMSE) of 0.92°C, and a correlation coefficient of 0.79. Long-term trend analysis of MLD reveals a deepening of the ML during 1993-2022 in all the select locations, except P1 (northern Gulf) and P5 (Strait of Hormuz). Although the trends showed contrasting patterns, the deepening rate of MLD along the east coast of Qatar was observed as 1m over 30 years, which is considered insignificant. The results highlight the importance of understanding the interactions between physical oceanographic processes, biogeochemical cycling, and other anthropogenic influences in regulating the Gulf's marine environment.