Circulation and thermal structure on the northern Omani shelf

We study thermocline dynamics on the northern Omani shelf and how local circulation is linked to basin scale drivers. The Sea of Oman is characterized by a field of energetic eddies, water exchange between the Arabian Sea and the Persian Gulf and circulation driven by seasonally variable wind. The cape of Ras al Hamra near Muscat marks the transition from a broader shallower shelf towards the west to steep continental slopes with canyons towards the opening to the Arabian Sea in the southeast. We combine in situ measurements of stationary ADCPs and arrays of vertical temperature loggers on both sides of the cape with remotely sensed data on sea surface temperature, sea surface height and wind to integrate inner shelf processes into a broader context.

On the shallow shelf west of the cape a strong thermocline develops during summer. Irregular flow pulses of hours to days induce vertical displacement and mixing of the thermocline. Easterly pulses frequently depress the thermocline. This downwelling is driven by pronounced geostrophic off shore circulation occasionally reinforced by wind. Events of strong westward wind trigger upwelling in the lee of the cape and can break up the easterly pulses. Due to the short periods of upwelling favorable wind (1-10 days) the thermocline often does not reach the surface of the inner shelf preventing detection by satellite imagery.

At the steep and narrow shelf east of the cape upwelling is still often incomplete although surface SST signatures do appear more often than in the west. Upwelling events east of the cape are sometimes connected to wind but a lifting of the thermocline can also be induced by the influence of strong anticyclonic eddies impinging on the coast. Pronounced eddy geostrophy can drive strong near-shore flow in both directions over periods of days to weeks.

We find that thermocline variability on the northern Omani shelf appears mostly in the form of irregular events driven by wind and eddies. Upwelling alternates with periods of thermocline depression. Coupling mechanisms between basin scale drivers and shelf dynamics include coastal trapped waves induced by eddy topography interactions and submesoscale structures reaching across the shelf slope.