Winter mixing, mesoscale eddies and eastern boundary current: Engines for biogeochemical variability of the central Red Sea during winter/early spring period

Abstract. The central Red Sea (CRS) has been shown to be characterized by significant eddy activity throughout the year. Weakened stratification in winter may lead to enhanced vertical exchange contributing to physical and biogeochemical processes. In the winter of 2014–2015, we began an extended glider time series to monitor the CRS where eddy activity is significant. Remote sensing and glider observations that include temperature, salinity, oxygen, carbon dissolved organic matter (CDOM), chlorophyll fluorescence (CHL) and multi-wavelength optical backscatter have been used to characterize the effects of winter mixing, eddy activity and lateral advection. During winter and early spring, mixing up to 90m driven by surface cooling and strong winds combined with eddy features was insufficient to penetrate the nutricline and supply nutrients into the upper layer. However, the mixing events did disperse the phytoplankton from the deep chlorophyll maximum throughout the upper mixed layer (ML) increasing the chlorophyll signature detected by ocean colour imagery. In early spring, the eastern boundary current (EBC) is evident in CRS. The EBC brings relative high concentrations of CHL and CDOM along with lower oxygen concentrations indicative of previous nutrient availability. In addition to the vertical mixing, mesoscale eddy activity cause 160 m vertical displacement of the 180 µM isopleth of oxygen, proxy of the nutricline interface. Within the cyclonic feature, this oxygen isopleth shallowed to 60 m, well within the euphotic layer. Remote sensing analyses indicate that these eddies also contribute to significant horizontal dispersion, including the exchange between the open sea and coastal coral reef ecosystems. When the phytoplankton is distributed through the mixed layer clear diel variability was evident in CHL concentration. The biogeochemical responses provide a sensitive indicator of the mixing and eddy processes that may not be detectable via remote sensing. Sustained in situ autonomous observations were essential to understand these processes.