Abstract
The Central Red Sea (CRS) has been characterized by significant eddy activity throughout the year. Weakened wintertime stratification contributes to enhanced vertical exchange. In winter 2014–2015, an extended glider time series in the CRS captured this variability. Surface cooling and stronger winds resulted in deepening of the mixed layer (ML) to nearly 90 m. The vertical distributions of density and oxygen suggest that the ML did not penetrate the nutricline. However, mixing events dispersed phytoplankton from the deep CHL maximum (DCM) throughout the ML, increasing near‐surface chlorophyll. Following the mixing events, a mesoscale cyclonic eddy (CE) grew and intensified, weakening stratification and decreasing the ML depth within the eddy. Where the CE interfaced with an adjacent anticyclonic eddy (AE), the CE DCM subducted beneath the shallower AE DCM leading to a local integrated chlorophyll maximum. Low salinity water containing relatively high chlorophyll and CDOM concentrations, originating from the Gulf of Aden, appeared in late winter. Mesoscale eddy activity resulted in a 160 m upward displacement of the nutricline to ∼60 m, well within the euphotic layer. Remote sensing imagery indicates that these eddies contribute to horizontal dispersion, including exchange between the open sea and coastal coral reefs. When the phytoplankton is distributed through the ML, clear diel variability was evident in the temporal CHL distribution. Because not all of the biogeochemical responses were apparent at the surface, sustained glider observations were essential to understand the temporal and spatial scales and their impact on these processes.
Plain Language Summary
An extended glider time series captured the physical and biogeochemical variability in the Central Red Sea (CRS) during the wintertime. Weakening of the stratification enhances vertical exchange surface cooling and stronger winds resulted in deepening of the mixed layer (ML) up to 90 m. Vertical distributions of density and oxygen suggests that the ML did not penetrate the nutricline. However, mixing events dispersed phytoplankton from the deep CHL maximum (DCM) throughout the ML, increasing near‐surface chlorophyll that was detected by satellites. However, an increase in the surface chlorophyll does not necessarily represent an increase in the water column's integrated chlorophyll. Following the mixing events, a mesoscale cyclonic eddy (CE) grew and intensified causing weakening of stratification and a decrease in the ML depth within the eddy. Where the CE interfaced with an adjacent anticyclonic eddy (AE), the CE DCM subducted beneath the shallower AE DCM leading to a local integrated chlorophyll maximum. Low salinity water containing relatively high chlorophyll and CDOM concentrations, originating from the Gulf of Aden, appeared in late winter. Clear diel variability was evident in the temporal CHL distribution when the phytoplankton is distributed through the ML.
Key Points
In situ and remote sensing observations shows for first time the persistence of a cyclonic eddy in the central Red Sea (CRS) for 4 months
An enhancement of the surface chlorophyll does not always indicate an increase in water column phytoplankton biomass and/or productivity
Subduction, as indicated by the deep CHL maximum (DCM), occurred at the interface between an anticyclonic eddy and cyclonic eddy in the Central Red Sea