The role of mesoscale processes controlling physical and biological variability in the oligotrophic Central Red Sea

Abstract

The existing observations and model simulations indicate that mesoscale eddies and the Eastern Boundary Current (EBC) have a significant role in the complex circulation of the Red Sea. However, a full understanding of the processes that contribute to the physics and biological responses of the central Red Sea (CRS) has been limited due to the lack of sustained in-situ observations. In this dissertation study, in-situ observations extending over a thirty-three month period from spring 2013 through winter 2015 include an intensive ship-based and glider monitoring program to understand the key dynamic features of the CRS circulation. Nine glider missions and five ship-based surveys provide concrete resolution of both spatial and temporal variability in the CRS. The quasicontinuous glider observations resolve the influence of distinct water masses with a different origin that is present in the study area. Our results show that mesoscale eddies and the an intrusion of Gulf of Aden water governs the physical and biochemical characteristics of the CRS during the winter to summer transition period in 2013. During this period, an anticyclonic eddy appears to redirect the northward flow along the eastern boundary. Ship-based observations in fall 2013 indicate that the EBC can periodically transport patches of less salty and warmer water containing higher chlorophyll concentrations from south into the CRS. During spring 2014, ship observations show the presence of a cyclonic/anticyclonic eddy pair. The cyclonic eddy contribute an upward nutrient flux, resulting in an increase integrated chlorophyll concentration within the eddy. Higher chlorophyll and CDOM concentrations and lower N:P ratios characterized the inflow of lower salinity Gulf of Aden water from the south. To understand better how the mesoscale eddy activity, stratification, and the EBC modulate the nutrient availability and planktonic food web architecture in Red Sea two addition hydrographic surveys with plankton sampling were conducted in fall 2014 and spring 2015. The seasonal availability of Gulf of Aden water, stratification and eddies exerted a demonstrable effect on the plankton community by modulating the availability and utilization of allochthonous vs. autochthonous macronutrients by phytoplankton. Strong stratification, higher temperatures and depletion of nutrients by phytoplankton, subjected the plankton community to an overall nitrogen and phosphorus deficit in fall. To evaluate the role of the winter mixing, mesoscale eddies, and EBC within CRS during the winter to spring period, a sustained glider study (~91 days) was initiated from December 2014 to March 2015. Glider observations show the seasonal contrasts and transitions from strong summer stratification to winter mixing, with a corresponding transition from a well defined deep chlorophyll maximum to phytoplankton population intrusions of lower salinity water from the Gulf of Aden contributed to both the physical and biochemical variability within the region. Both GASW and GAIW can be entrained and diverted across the basin by larger eddies. Eddies play a role in the mixing between warmer, fresher water from the Gulf of Aden, and cooler, saltier water from the northern Red Sea.