Seasonal to Intraseasonal Variability of the Upper Ocean Mixed Layer in the Gulf of Oman

• Published in: Journal of geophysical research. Oceans Vol. 127; no. 3
• Main Authors: Font, Estel, Queste, Bastien Y., Swart, Sebastiaan
• Format: Journal Article
• Language: English
• Published: 01.03.2022
• Subjects: latent heat flux; mixed layer depth; restratification; Shamals; submesoscale fluxes
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC018045

High‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone. Plain Language Summary Atmospheric forcings, such as wind and solar heating, and small‐scale ocean processes (1–10 km; e.g., eddies, fronts, filaments) modify the properties and the structure of the water column near the surface. These processes regulate the surface layer, creating a well‐mixed surface layer. The variation in these processes determine how the depth of this surface mixed layer changes through both time and space. This study investigates the variability of this layer during winter and spring in the Gulf of Oman using in situ observations and atmospheric data derived from models and observations. Episodic strong and dry winds from the northwest (Shamals) increase mixing and cause shorter‐term variability of the surface mixed layer. Concurrently, we find that the small‐scale ocean processes mainly shoal the mixed layer depth during winter. These processes are also important in determining the timing of the change from the deeper winter mixed layer to the shallower spring mixed layer, as we find a three‐week difference between the two observed years. The observations illustrate previously unquantified processes in the region that can impact coupling between the atmosphere, surface ocean, and deep ocean, with consequences for regional marine ecosystems. Key Points Ocean glider observations reveal mixed layer variability that cannot be explained by seasonal warming alone Shamal winds dominate intraseasonal variability of the mixed layer in the Gulf of Oman Submesoscale mixed layer eddies are responsible for 68% of the restratifying buoyancy flux in winter